identification of nutritional profiles associated with...
TRANSCRIPT
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Identification of nutritional profiles associated with
lower predicted glycemic load among potato cultivars
Aydin Sarang
School of Dietetics and Human Nutrition
McGill University, Montreal
September, 2011
A thesis submitted to McGill University
In partial fulfillment of the requirements of the degree of Master of Science
© Aydin Sarang, 2011
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Table of contents
Page
Table of contents ............................................................................................................. ii
List of tables .................................................................................................................... iv
List of figures ................................................................................................................... v
List of abbreviations ..................................................................................................... vii
Acknowledgement ........................................................................................................... x
Abstract .......................................................................................................................... xii
Résumé .......................................................................................................................... xiii
Contribution of authors ............................................................................................... xiv
I. Introduction .............................................................................................................. 1
1.1. Statement of the problem .................................................................................... 1
1.2. Rationale ............................................................................................................. 3
1.3. Hypothesis .......................................................................................................... 4
1.4. Objectives ........................................................................................................... 5
II. Literature review ..................................................................................................... 8
2.1. Potato: nutritional benefits and controversial associations with diabetes risk.... 8
2.2. The association between glycemic impact of foods and diabetes risk and
management ...................................................................................................... 10
2.3. The glycemic impact of potatoes ...................................................................... 13
2.4. Factors affecting the glycemic impact of potatoes ........................................... 16
2.4.1. Moisture content ................................................................................... 16
2.4.2. Protein content ...................................................................................... 18
2.4.3. Phenolic content .................................................................................... 18
2.4.4. Starch characteristics related to digestibility: resistant starch and
percentage amylose in starch ............................................................... 20
2.4.5. Phosphorylated starch content .............................................................. 24
2.4.6. Sugar content ........................................................................................ 26
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Page
2.4.7. Extrinsic factors affecting the glycemic impact of potatoes ................. 26
III. Nutritional profiles associated with predicted glycemic load among potato
cultivars ................................................................................................................... 33
3.1. Abstract ............................................................................................................. 34
3.2. Introduction ....................................................................................................... 35
3.3. Material and methods........................................................................................ 36
3.3.1. Samples ................................................................................................. 36
3.3.2. Compositional analysis ......................................................................... 37
3.3.3. Digestibility analysis ............................................................................. 41
3.4. Statistical analysis ............................................................................................ 43
3.5. Results ............................................................................................................... 44
3.6. Discussion ......................................................................................................... 47
IV. Summary and concluding remarks ...................................................................... 60
4.1. General discussion and conclusion ................................................................... 60
4.2. Contribution to knowledge ............................................................................... 62
4.3. Limitations and suggestions for future studies ................................................. 62
Literature cited .............................................................................................................. 64
Appendix ........................................................................................................................ 78
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List of tables
Page
1.1. The average glycemic index (GI) values of 13 common foods adapted
from the ―International Tables of Glycemic Index and Glycemic Load
Values‖ by Atkinson et al. (2008).............................................................................. 6
2.1. Selected studies examining the effect of low glycemic index (GI)
vs. high GI foods on glycemic control ..................................................................... 29
2.2. Glycemic index (GI) and glycemic load (GL) values reported for fresh or
cooked potato cultivars in 11 studies selected from the review of Lynch
et al. (2007) .............................................................................................................. 31
3.1. Content of percent moisture, total soluble protein (TSP), chlorogenic acid
(CGA), percent amylose and phosphorylated starch of a serving size (150
g FW) of 12 Canadian potato cultivars .................................................................... 53
3.2. Resistant starch and available carbohydrate content in one serving (150 g
FW) of six selected Canadian cultivars ................................................................... 54
3.3. t-test significance of the predicted glycemic index (GI) and glycemic
load (GL) between warm vs. refrigerated samples of each selected
cultivars .................................................................................................................... 57
3.4. Pearson correlation coefficient (r) between predicted glycemic index
(GI), glycemic load (GL), and potato phytonutrients .............................................. 58
3.5. Independent predictors of the predicted glycemic load (GL) of
refrigerated and warm potatoes after cooking ......................................................... 59
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A.1. Table of glycemic index (GI) and glycemic load (GL) values of a typical
serving size (150 g) of potatoes, adapted from the International Table of
Glycemic Index and Load by Foster-Powell et al. (2002) .............................................. 78
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List of figures
Page
1.1. Possible factors affecting starch digestibility in potatoes. ......................................... 7
2.1. Overview of starch granule in potato (adapted from illustration made by
Pilling and Smith 2003 and from http://archaeobotany.dept.shef.ac.uk
/wiki) ........................................................................................................................ 32
3.1 (a). Predicted glycemic index (GI) values in selected warm samples ...................... 55
3.1 (b). Predicted glycemic index (GI) values in selected refrigerated samples ............. 55
3.2 (a). Predicted glycemic load (GL) values in selected warm samples ....................... 56
3.2 (b). Predicted glycemic load (GL) values in selected refrigerated samples ............. 56
A.1. Field-grown tubers of the 12 Canadian cultivars used in this study: (A)
Atlantic, (B) Green Mountain, (C) Goldrush, (D) Kennebec, (E) Norland, (F)
Onaway, (G) Russet Burbank, (H) Red Pontiac, (I) Sebago, (J) Shepody, (K)
Superior, and (L) Yukon Gold (from CFIA, 2011 and Vunnam, 2011) ......................... 80
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List of abbreviations
% Percentage
µg Microgram
AACC American Association of Cereal Chemists
AAS Atomic absorption spectroscopy
ANOVA Analysis of variance
AMG Amyloglucosidase
BG Blood glucose
BMI Body mass index
C-3 Carbon number 3
C-6 Carbon number 6
CGA Chlorogenic acid
CRP C-reactive protein
Cv. Cultivar(s)
Da Dalton
DDW Double distilled water
DF Dietary fibre
DMSO Dimethylsulfoxide
DS Digestible starch
DW Dry weight
EDTA Ethylenediaminetetraacetic acid
FAO (UN) Food and Agriculture Organization (of the United Nations)
FBS Fasting blood sugar
FW Fresh weight
g Gram
GBSS Granule-bound starch synthase
GI Glycemic index
GL Glycemic load
GOPOD Glucose oxidase-peroxidase
GWD α-Glucan water dikinase
Gy Gray: the SI unit of energy absorbed from ionizing radiation
HbA1c Haemoglobin A1C
HPLC High performance liquid chromatography
HPS High phosphorus starch
h Hour(s)
iAUC Incremental area under curve
IMS Industrial methylated spirits (denatured ethanol)
ITT Insulin tolerance test
LMWC Low molecular weight carbohydrate
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LPS Low phosphorus starch
mg Milligram
ml Milliliter
µl Microliter
mM Millimole
MPS Medium phosphorus starch
nm Nanometer
NMR Nuclear magnetic resonance
NSP Non-starch polysacharides
ppm Parts per million
RAG Rapidly available glucose
RCA Re-crystallized amylose
RDC Rapid digestible carbohydrate
RDS Rapidly digestible starch
RS Resistant starch
SE Standard error
s Second
SDC Slowly digestible carbohydrate
SDS Slowly digestible starch
TSP Total soluble protein
U Unit
WCS Waxy cornstarch diet
WHO World Health Organization
wk Week (s)
yr Year (s)
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Dedication
To the best family in the world:
My lovely mom, super dad, and my adorable siblings
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Acknowledgment
My thanks are due primarily to my supervisors Dr. Stan Kubow, Dr. Danielle J.
Donnelly, and my advisory committee member Dr. Alfred Aziz. They helped me learn
and develop my knowledge. I am particularly grateful to Dr. Kubow. He guided me to
expand my knowledge and learn a lot, through his numerous expert comments,
corrections, and suggestions. His kind support is what I will cherish throughout my life.
I acknowledge the great support and assistance which Dr. Kebba Sabally gave me
to learn and to run the high performance liquid chromatography (HPLC) equipment.
Thanks are due to Dr. Atef Nassar for his kind support during development of the
amylose: amylopectin assay. My thanks are due to Mr. Behnam Azadi for his comments
and help with the digestibility assay. I am also grateful to Ms. Hélène Lalande for her
kind support and help in running the phosphorus assay. I would also like to thank the
staff of the McGill School of Dietetics and Human Nutrition, particularly Ms. Lise Grant
and Ms. Francine Tardif. Thanks are also due to the Macdonald Library staff for their
guidance and support. I am grateful to the Natural Sciences and Engineering Research
Council of Canada (NSERC) for their financial support.
I would like to thank my dear friends Ms. Negar Tabatabaei, Ms. Niloofar Hariri,
Ms. Shima Sadeghi Ekbatan, Ms. Elham Azarpazhooh, Ms. Shirin Munshi, and Mr.
Rakesh Vunnam for their help in sharing their knowledge, for their kindness, and for
supporting me during the difficult times of my life.
My sincere thanks are due to my parents who showed me how to be a good human
being by their example. It’s my honour to dedicate this work to the best mother in the
world, Mrs. Zahra Nematollahi, for her kindness, love, and support throughout my life
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which motivates my courage, progress, and achievements and to my father, Mr. Bahram
Sarang, for teaching me how to be a better person and not to give up easily. I also
dedicate this work to my sisters, Elnaz and Naghme, and to my brother, Soroush, for
making me feel so proud and lucky to have them in my life. Their love is what I live for.
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Abstract
Potato (Solanum tubersum L.) is classified as a high glycemic index (GI) food.
Depending on cultivar, storage conditions, and cooking methods, potatoes can contain a
wide range of components including water, protein, polyphenolic compounds,
amylose:amylopectin ratio, and phosphorylated starch that might modify their glycemic
impact. This study tested the impact of compositional differences of the above parameters
among 12 potato cultivars grown in Canada on starch digestibility and predicted GI and
glycemic load (GL). A wide range of phytonutrients was found among these cultivars.
The predicted GI and GL were associated with resistant starch (RS) content when
samples were either warm or refrigerated, whereas GL was associated with both the RS
and phosphorylated starch content only when samples were refrigerated. We conclude
that RS and phosphorylated starch are important modifiers of the glycemic impact of
potatoes.
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Résumé
Les pommes de terre sont classées comme des aliments avec un indice
glycémique (IG) élevé. Selon le cultivar, les conditions de stockage et les méthodes de
cuisson, les pommes de terre peuvent contenir un large éventail de composants, y compris
l'eau, les protéines, les composés polyphénoliques, le rapport de
l’amylose :l’amylopectine, et l’amidon phosphorylé qui pourraient modifier leur potentiel
glycémique. Cette étude a testé l'impact de ces différences de composition entre 12
cultivars de pomme de terre cultivées au Canada sur la digestibilité de l'amidon, ainsi que
l’indice glycémique et la charge glycémique (CG) prédits. Un large éventail a été
constaté entre les cultivars pour leur phytonutriments. L’indice glyémique prédit des
pommes de terre était élevé mais leur CG, basée sur la taille de portion typique, était
modérée. Les IG et CG prédits ont été associés au contenu de l'amidon résistant (AR)
quand des échantillons étaient chauds ou froids, tandis que le CG a été associé à l’AR et
la teneur en amidon phosphorylé uniquement lorsque les échantillons ont été réfrigérés.
Nous concluons que l’AR à la digestion et l’amidon phosphorylé sont des modulateurs
importants du potentiel glycémique des pommes de terre.
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Contribution of authors
This thesis is prepared according to ―Thesis preparation and submission
guidelines‖ recommended and approved by Graduate and Postdoctoral Studies (GPS).
This thesis consists of four chapters with Introduction and Literature Review in Chapters
I and II, respectively. Chapter III describes a laboratory study on potato tubers and is
written in the form of a manuscript to be submitted to Journal of Nutrition. Chapter IV
contains the Summary, Conclusions and Suggestions for Future Research in this field of
study.
Chapter III of this thesis is part of an on-going investigation of potato nutrients in
the laboratories of Dr. Stan Kubow, School of Dietetics and Human Nutrition, and Dr.
Danielle J. Donnelly, Plant Science Department. All the experiments for this study were
conducted by me under the plan and supervision of Dr. Stan Kubow and Dr. Danielle J.
Donnelly. Dr. Kebba Sabally and Mr. Behnam Azadi, School of Dietetics and Human
Nutrition, and Ms. Hélène Lalande, Natural Resources Department, helped me in
conducting the HPLC analyses, starch digestion assays, and determining the starch
phosphorus contents respectively for this study. Dr. Atef Nassar, Plant Science
Department, helped with the amylose:amylopectin pre-analysis. The statistical analyses
were conducted by me with the supervision of Dr. Kubow and guidance from Dr. Atef
Nassar. All the chapters in this thesis were prepared by me with extensive editorial help
from Dr. Stan Kubow, Dr. Danielle J. Donnelly, and my advisory committee member, Dr.
Alfred Aziz, Nutrition Research Division, Health Canada.
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I. Introduction
1.1. Statement of the problem
Diet is one of the most cost-effective strategies for preventing and managing
obesity, diabetes, and cardiovascular diseases. During the last decade, the role of dietary
carbohydrates in the etiology and management of these chronic diseases has been
extensively debated in the scientific community and the media. Proponents of low
carbohydrate diets argue that high carbohydrates in the diet promote an obesogenic,
diabetogenic, and atherogenic state through elevated postprandial glucose concentrations
(Atkins, 2001; Atkins et al., 2004). However, the glycemic impact of carbohydrates
varies not only with the quantity but also with the source and form (Reaven, 1979).
The glycemic index (GI) is an experimentally derived value that classifies
carbohydrates and carbohydrate-containing foods according to their blood glucose-raising
potential (Jenkins et al., 1981). The GI is expressed as a percentage that refers to the
incremental glucose area under the curve (iAUC) of a test food relative to a reference
food (white bread or glucose) containing the same amount of available carbohydrates (25
or 50 g) (Wolever et al., 1991). For most people, consumption of foods with low GI
values instead of those with high GI values (more than 70) is associated with better health
outcomes (Brand-Miller et al., 2010). For example, hyperglycemia caused by a high
glycemic diet is believed to be associated with the cause of diabetes complications
(Sheard et al., 2004). Therefore, low GI diets have generally been associated with
reduced risk and better management for diabetes (Brand-Miller, 2003). However, the GI
is a poor predictor of the glycemic response if used alone because it ranks the glycemic
impact of foods on an equi-carbohydrate basis and so GI may not reflect typical serving
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size. To overcome this limitation, the glycemic load (GL) concept was introduced. The
GL predicts the glycemic impact of foods based on both their GI and their available
carbohydrate content (GI/100 x amount of available carbohydrate (g) per portion)
(Salmeron et al., 1997). Each unit of GL equals the glycemic effect of 1 g carbohydrate
from a reference food (white bread or glucose) (Willet et al., 2002). High GL values are
considered to be >20, medium GL values range from 11 to 19, and low GL values are <
10 (Brennan, 2005).
Potato is classified as a high GI food as shown in Table 1.1. Potato (instant
mashed) has a high GI, which is higher than the GI of spaghetti (white), ice-cream or
apple juice that are ranked with low GI values (Atkinson et al., 2008). The high GI of
potatoes imparts a negative characteristic that ranks them among the less desirable
sources of carbohydrate. However, potatoes possess positive nutritional qualities that
contribute to a healthy diet. For example, potatoes are rich in vitamin C and a good
source of dietary fibre (Beals and Kraus, 2005), and minerals such as potassium, and
phosphorus (Prokop and Albert, 2008). Potatoes are also a significant source of
polyphenolic compounds (Reddivari et al., 2007). Despite their high GI, potatoes contain
moderate amounts of available carbohydrates in a typical serving size of 148 g (Lynch et
al., 2007). So, their GL is expected to be moderate or low, as Lynch et al. demonstrated.
Additionally, as illustrated in Figure 1.1, extrinsic factors such as cultivation and storage
conditions can impact upon the intrinsic factors such as the content of amylose,
polyphenolics, and phosphorylated starch that could influence the GI and/or GL of
potatoes (Anderson et al., 1981; Thompson et al., 1984; Friedman, 1997; Lynch et al.,
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2007; Absar et al., 2009). The above phytonutrients could affect the glycemic impact of
potatoes through affecting the digestibility of starch.
The goal of this thesis study was to determine whether major differences exist in
the phytonutrient and phytochemical concentrations of 12 common Canadian-grown
cultivars and whether such differences in composition could be related to the GI and GL
of cooked potatoes. Our study could enable a screening approach to identify cultivars that
might have a lower GI and GL. Such studies may have future applications towards
identifying potato cultivars and methods of preparation and storage that are associated
with a lower glycemic impact.
1.2. Rationale
Based on the GL concept, the glycemic impact of potatoes is low to medium
rather than high (Lynch et al., 2007). However, there is a lack of comprehensive studies
examining the combination of possible factors affecting GL. Such factors could include
potato cultivar differences that can affect components such as moisture, protein,
polyphenolic compounds, amylose, and phosphorylated starch, which can influence
glycemic impact through effects on starch digestibility.
Blood glucose response to carbohydrates is influenced by their quality and
quantity. Starch is the main form of carbohydrate in potato. Resistant starch is a starch
that is not digested in the small intestine and is fermented in the large intestine. The
relative amount of the resistant starch (RS) for digestion could affect the glycemic
response to potatoes. Starch digestibility could be influenced by different intrinsic
(genetic) and extrinsic (environmental) factors relative to a potato cultivar. Intake of
starch containing higher amounts of amylose resulted in increased resistance to digestion
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and reduced glucose and insulin responses (Behall et al., 1989; Karlsson et al., 2007).
Apart from the effect of amylose content, higher content of protein (Anderson et al.,
1981), phosphorylated starch (Absar et al., 2009) and polyphenols (Thompson et al.,
1984; Friedman, 1997) could contribute to decreased digestibility of starch in potatoes.
On the other hand, higher content of moisture could lower the glycemic impact by simply
decreasing the available carbohydrates on a per serving basis (Lynch et al., 2007).
Digestibility of potato starch could also be affected by extrinsic factors such as the
cooking method (Garcia-Alonso and Goñi, 2000), maturity of potato at the time of
harvest, and storage duration and temperature (Haase and Plate, 1996), which could
indirectly affect the above mentioned intrinsic factors. These factors vary significantly
among different potato cultivars (Jansen et al., 2001).
Based on differences in compositional profiles of the above mentioned
components among potato cultivars, starch digestibility, and consequently the glycemic
impact of cooked potatoes could vary with cultivar. To our knowledge this study is the
first that tests the effect of the combination of possible compositional factors that vary
among cultivars on the GI and GL values of cooked potatoes. Such studies could be used
to develop a screening technique to facilitate identification and development of low GI or
GL potato cultivars. Defining the GI and GL for individual potato cultivars could avoid
the misclassification of all potato cultivars collectively as an unhealthy carbohydrate
source, which masks their nutritional value.
1.3. Hypothesis
The tested hypotheses were: (1) potato cultivars grown and stored under the same
conditions vary significantly in their nutritional content including: moisture, total soluble
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protein (TSP), chlorogenic acid, resistant starch, the percentage of amylose, and
phosphorylated starch; (2) cooked potato cultivars vary significantly in their predicted GL
measured either when warm or after refrigeration due to differences in their phytonutrient
and phytochemical profiles.
1.4. Objectives
In order to test our hypothesis, our objectives were:
1. To measure moisture, total soluble protein (TSP), chlorogenic acid, resistant
starch, the percentage of amylose, and phosphorylated starch content of 12 raw
potato cultivars.
2. To determine the predicted GI and GL of cooked potato in six selected cultivars
with comparatively minimum, medium and maximum content of phytonutrients
mentioned in Objective #1.
3. To correlate the predicted GI and GL values of selected cooked potato cultivars
with their raw compositional profiles.
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Table 1.1 The average glycemic index (GI) values of 13 common foods adapted
from the ―International Tables of Glycemic Index and Glycemic Load Values‖ by
Atkinson et al. (2008)
Low GI* (≤55)
Medium GI (56-69)
High GI (≥70)
Ice cream 51±3(1) Brown rice 68±4
Potato,
instant mashed 87±3
Apple juice 41±2 Couscous 65±4
Potato, boiled 78±4
Mung bean 39±8 Potato,
French fried 63±5
White wheat bread 75±2
Chocolate 40±3 Honey 61±3
Whole wheat bread 74±2
Chickpeas 28±9 Pineapple 59±8
White rice, boiled 73±4
Spaghetti,
white 49±2
Sweet Potato
boiled 63±6
White Yam
Peeled, boiled 75±6
(1)
Data are mean ± SE
*GI values are relative to glucose as reference. The average GI values are derived
from multiple studies by different laboratories as reported by Atkinson et al. (2008).
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Figure 1.1 Possible factors affecting starch digestibility in potatoes.
Digestibility of starch affects glycemic impact. Potential factors that can affect the
digestibility of starch could be intrinsic or extrinsic to cultivar. Intrinsic factors,
include the quality of starch in terms of the amylose:amylopectin ratio and
phosphorylated starch, and other components such as polyphenolics. Extrinsic
factors include storage, cultivation conditions, cooking and cooling. Cooling after
cooking not only might affect the nutrient contents such as moisture, it will affect
the starch structure which can affect the starch digestibility consequently.
Cultivar
- Moisture
- Protein
- Polyphenolic compounds
- Amylose: Amylopectin ratio
- Phosphorylated starch
- Storage
- Cultivation
Starch
Digestibility
Cooking/ Cooling
Glycemic
Impact
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II. Literature Review
2.1. Potato: nutritional benefits and controversial associations with diabetes
risk
The potato (Solanum tuberosum L.), an herbaceous annual that grows up to 100
cm, is the world's fourth most important food crop, after maize, wheat, and rice. The stem
tubers (potato) come in thousands of cultivated varieties (cultivars) with great variation in
size, shape, colour, texture, cooking characteristics, and taste (FAO, 2008). Potatoes are a
valuable source of dietary vitamins, minerals, and phytonutrients because of their per
capita consumption (Singh and Kaur, 2009). Potatoes are a good source of vitamin C, and
minerals such as potassium (Prokop and Albert, 2008), and are an important source of
antioxidants (Vunnam, 2011). Potatoes and processed potato foods are an important part
of the Canadian diet (McLaughlin, 2005). Potato consumption in Canada is about 30% of
the 113 kg average total vegetable intake per year. Potato production is important in the
Canadian agricultural sector as the industry was valued at $902 million in 2004.
Additionally, potatoes are popularized for their relatively easy preparation, and for
contributing carbohydrate energy (Singh and Kaur, 2009). Despite all mentioned benefits,
due in part to the categorization of potato as a high GI food, the potato has not typically
been termed as a ―health promoting food‖.
There is an existing controversy regarding epidemiological associations regarding
the consumption of potato, as a high GI food, and diabetes risk. Liu et al. (2004) studied
the association of vegetable and fruit consumption with risk of Type 2 diabetes with an
average follow up of 8.8 years in 39,876 females (aged ≥ 45 years), who showed no
evidence of heart disease, stroke, or cancer at baseline. Overall, 1,614 individuals (4%)
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developed Type 2 diabetes during the study time period. Dietary assessment showed a
significant positive association between potato consumption and the risk of developing
Type 2 diabetes although the association did not remain significant after adjustment for
known diabetes risk factors such as age, smoking status, total energy intake, and other
coronary disease risk factors.
In agreement with the study of Liu et al. (2004), Halton et al. (2006) also
observed a positive association of consumption of both potatoes and French fries with
Type 2 diabetes incidence after adjustment for age and both dietary and non-dietary
factors. Halton et al. (2006) investigated 84,555 women in the Nurse’s Health Study who
had no history of chronic disease at baseline. A total of 4,496 diabetes cases (5.3 %) were
reported during the 20-yr follow up.
An inverse association between potato consumption and diabetes risk has also
been reported. In a cohort study with a 4- to 6-yr follow up, Villegas et al. (2007) studied
the effect of dietary carbohydrate, GI and GL on the incidence of Type 2 diabetes in
64,227 Chinese females (aged 40 to 70 years) who had no chronic disease history. They
showed a reduction in relative risk of developing Type 2 diabetes with potato tuber
consumption. They explained the apparent inconsistency with previous studies as being
due to different dietary patterns in the Chinese diet vs. the typical western diet. Among
the top 10 food items, the main carbohydrate food consumed in the Chinese diet was rice
with a 73.9% contribution to total carbohydrate intake and an average intake of 250 g/d,
while potato was consumed as a vegetable with only a 0.6% contribution to total
carbohydrate intake with average intake of 8.1 g/d. Moreover, potato in the Chinese diet
was consumed with less fat compared to the western diet.
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The proposed relationship between the glycemic impact of potato consumption
and diabetes risk is still debatable partly due to the existing uncertainties between the
glycemic impact of foods in relation to diabetes risk and management.
2.2. The association between glycemic impact of foods and diabetes risk and
management
The greatest influence on blood glucose is from dietary carbohydrates. Two
methods for classification of carbohydrate related to their effect on postprandial glucose
are the glycemic index (GI) and glycemic load (GL). GI ranks carbohydrates according to
their blood glucose response relative to a reference food over a 2 h period. However, this
ratio does not change with increasing or decreasing serving sizes. To overcome this
limitation, the GL concept was introduced (Salmeron et al., 1997; Willet et al., 2002). GL
combines the GI value and the quantity of carbohydrates to quantify the overall estimated
glycemic impact of a typical portion size of a food.
Glycemic index is a value introduced by Jenkins et al. (1981) to find out which
food is best suited for diabetics. This concept was applied to be useful for most of the
general population, under most circumstances (Brand-Miller et al. 2010). The Food and
Agriculture Organization (FAO) of the United Nations (UN) and the World Health
Organization (WHO) have recommended the use of GI values as a tool for promoting a
healthier carbohydrate choice (FAO/WHO, 1997). Although the American Diabetes
Association (ADA) does not fully accept the use of GI in diabetes prevention and
management due to inconsistent results that occurred in several randomized trials, the
ADA noted that GI and GL may provide greater health benefits than when total
carbohydrate intake is considered alone (Sheard et al., 2004; Brand-Miller et al., 2010).
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Some epidemiological studies have shown that high GI diets are positively
associated with diabetes risk whereas other studies showed that diabetes risk only relates
to a negative association between low intake of low glycemic diets and diabetes risk.
Salmeron et al. (1997) examined dietary risk factors associated with Type 1 diabetes
development in a cohort study involving 65,173 healthy US women from the Nurse’s
Health Study. They concluded that dietary GI was positively associated with the risk of
diabetes after adjustment for age, body mass index, smoking, physical activity, family
history of diabetes, alcohol, and intake of cereal fibre intake and total energy. Schulze et
al. (2004) also found that high GI and GL diets were significantly positively associated
with an increased risk of diabetes in 91,249 women tracked over 8 years after data was
adjusted for age, body mass index, and family history of diabetes.
The above results have not been confirmed in elderly populations. During a 6-yr
follow-up study by Meyer et al. (2000), that involved 35,988 older Iowa women who
were initially free of diabetes, the quintiles of GI and GL values were not correlated with
diabetes incidence. The study results of Sahyoun et al. (2008) involving males and
females in their 70s were in agreement with Meyer’s findings. Sahyoun et al. (2008)
studied 662 randomly selected elderly subjects and assessed their risk of Type 2 diabetes
over a 4-yr period according to their dietary GI and GL. No significant association was
observed between dietary GI and GL and the risk of Type 2 diabetes in this population.
As age is a risk factor for diabetes, the associations between dietary GI and GL and
diabetes risk could be changing in the aged population.
In terms of diabetes management, many human studies have confirmed the effect
of low glycemic foods on glycemic control parameters such as blood glucose, glycated
12
haemoglobin A1C (HbA1c), serum fructosamine, and inflammation markers such as C-
reactive protein (CRP), although studies are inconsistent in terms of which parameters are
affected (Table 2.1). For example, Jenkins et al. (1988) observed a significant reduction
from baseline in fasting blood glucose, HbA1c, and serum fructosamine with the low GI
diet in eight noninsulin-dependent diabetic patients over a 2 wk period. However, blood
fructosamine was the only parameter that showed a significant reduction with low GI
feeding in comparison to the high GI diet.
In an attempt to clarify the effects of low GI or low GL diets on glycemic control in
diabetics, Thomas and Elliot (2009) conducted a Cochrane Review of assessing the
glycemic impact observed in 11 randomised controlled trials of ≥ 4 wk involving 402
participants. These studies tested the effect of low or high GI or GL in subjects with no
optimally controlled Type 1 or 2 diabetes. When comparing low with high GI diet, there
was a significant decrease in blood HbA1c, with a weighted mean difference (WMD) of -
0.5% with a 95% confidence interval (CI) of - 0.9 to -0.1, p = 0.02 in trials using a
parallel study design and a WMD of -0.5% with a 95% CI of -1.0 to -0.1, p = 0.03 in the
cross-over design trials. Thomas et al. concluded that low GI diets improve glycemic
control in diabetic subjects. The study results supported a previous meta-analysis
regarding the effect of low GI diets on management of Type 1 and 2 diabetes (Brand-
Miller, 2003). The meta-analysis done by Brand-Miller (2003) indicated that
consumption of a low GI diet lowered blood HbA1c values by 0.43% (CI 0.72–
0.13) compared to a high GI diet. After adjusting for baseline differences, the reduction
of glycated proteins (both HbA1c and fructosamine) was 7.4% (CI 8.8–6.0) more when
consuming a low-GI diet as compared to a high-GI diet. Inconsistent results in some
13
studies could be due to lack of GI values among food databases, study biases in terms of
over- or under-reporting of glycemic foods by subjects, and possible confounding factors
such as family history for diabetes.
In summary, the effect of low GI/GL diets in the prevention of diabetes is still
under investigation and controversial. Intervention studies performed regarding the
impact of low GI/GL diets on diabetes management suggest the usefulness of this dietary
approach (Brand-Miller et al., 2010). A confounding aspect in interventional trials is that
a reduction in dietary GL does not necessarily follow a reduction in dietary GI, as
reductions in dietary GI are sometimes accompanied by higher intakes of available
carbohydrate (Livesey et al., 2008). Therefore, it appears that GL, which takes into
account the GI level and a typical portion size of the specific food, should ideally be used
together with GI for dietary management of glucose control. The use of GI alone for
diabetes management could exclude some nutritious foods such as potatoes that contain a
medium range of available carbohydrate per serving (GL) (Lynch et al., 2007). Potatoes
have been categorized as high glycemic foods due to the high GI values measured in
initial studies that did not take into account the concept of GL (Singh and Kaur, 2009).
2.3. The glycemic impact of potatoes
Based on clinical trial evidence, Soh and Brand-Miller (1999) suggested that
potatoes have a high GI that is unaffected by cultivar or cooking method but can be
impacted by stage of maturity. They evaluated the in vivo GI effect of three potato
cultivars (Sebago, Desirée, and Pontiac), four cooking methods on cv. Pontiac
(microwaved, boiled, mashed, and oven baked), and stage of maturity (two cultivars of
canned new potatoes compared with three cultivars of mature boiled potatoes). After an
14
overnight 10-h fast, 10 healthy individuals were fed 8 test meals containing a 50 g portion
size of available carbohydrate. They compared different potato cultivars, cooking
methods, and maturity stages vs. two reference meals of white bread. There was no
significant difference in the GI values among the different cultivars and cooking methods.
Significantly lower GI values were obtained for canned new potatoes compared with
boiled mature potatoes of cv. Desirée. Moreover, the GI of the three potato cultivars and
the fresh and canned boiled new potatoes correlated with average tuber weight, which
suggests that potatoes with greater maturity have greater GI values. The author
generalized potatoes as a high GI food, although they examined the effect of cooking
method on GI of only one cultivar. The general classification of potato as a high glycemic
food might be inaccurate as potatoes generally have a low to medium glycemic impact
based on their GL (Lynch et al., 2007).
In contrast to the findings of Soh and Brand-Miller (1999), McNab et al. (2004)
showed that the GI values of two different cooked potato cultivars varied significantly.
They studied the effect of cv. Russet Stampede and Russet Burbank on the GI in twelve
diet-controlled Type 2 diabetic individuals in a cross-over trial. Subjects were fed the
same breakfast meal, which included 225 g of boiled potatoes from either cultivar. The
mean GI after consuming the breakfast containing cv. Russet Stampede was 7.4% less
than with cv. Russet Burbank. The difference in glycemic response was attributed to
greater water content in cv. Russet Stampede (pers. com., Dr. Stan Kubow, 2011).
As mentioned above, Lynch et al. (2007) indicated that GL of different potato
cultivars that are cooked in different ways can vary from low to medium (Table 2.2).
They reviewed 38 studies, which reported the GI for a range of potato cultivars prepared
15
by different cooking methods. Among these studies, the GI varied from 23 in an non-
specified potato cultivar that was boiled, refrigerated, and reheated (Kanan et al. 1998) to
111 in baked cv. Russet Burbank (Crapo et al. 1977). An important consideration is that
the above mentioned value of 111 described by Crapo et al. (1977) is the value of the
glycemic response attributed to a portion of the potato equivalent to 50 g glucose, which
precedes the GI terminology introduced in 1981 by Jenkins et al. (1981). Thus, Lynch et
al. (2007) developed the range of GL values of 4.5 to 24.1 for potatoes by utilizing the
potato GI and GL values from the databases established by Foster-Powell et al. (2002)
and Denyer and Dickinson (2005). The reported variation in GI values of potatoes might
be due to a variety of factors (Lynch et al., 2007). These factors could include: 1)
different cultivars might have different GI values when prepared and cooked differently;
2) tested subjects were different in sex, health (diabetic or non-diabetic) which caused the
variation. Standard procedures, preferably on healthy subjects, are needed to achieve a
reliable comparison of GI of potato cultivars; and 3) the glucose values on which the GI
was derived were obtained via different methods, i.e., either from capillary or venous
blood glucose tests. Although glucose values from these two tests are highly correlated,
the capillary blood test is preferable. Changes in blood glucose concentrations in capillary
blood samples are a more relevant indicator of the physiological consequences of high GI
foods than blood glucose alterations obtained from venous blood samples (Foster-Powell
et al., 2002; Wolever, 2003). Moreover, the available carbohydrate used to calculate the
GI and GL values was not directly measured. Indirect calculation of available
carbohydrate content was done by subtracting the dietary fibre content from the total
carbohydrate content of the food as obtained from the food composition tables. This latter
16
procedure could limit conclusions because different cultivars grown under different
conditions could have different content of starch, resistant starch, fibre or sugars.
Considering these limitations, more research is needed regarding the glycemic impact of
different potato cultivars.
In conclusion, differences in GI and GL values among different potato cultivars
has been indicated, and the suggested cause of these differences was related to the way
potatoes were prepared and consumed. Despite the available studies performed to date
regarding variations in GI and GL values of potatoes, the key intrinsic compositional
factors within potatoes that affect GI and GL have not been clarified. As potato
phytonutrient content has been shown to vary widely among cultivars (Jansen et al.,
2001), it is possible that cultivars can have different GL values based on differences in
their phytonutrient composition. Thus, the phytonutrient content among different
cultivars need to be investigated in order to choose the cultivars with the lowest glycemic
impact together with consideration of the optimal method for cooking and preparation.
This would provide a better nutritional approach as opposed to the removal of potatoes
from the staple food list.
2.4. Factors affecting the glycemic impact of potatoes
2.4.1. Moisture content
Different potato cultivars ranged in moisture content from 70 to 87% (Kadam et
al., 1991), with an average of approximately 76% (Lister and Munro, 2000), depending
on storage duration. Longer storage duration at higher temperature results in more loss of
water. Jansen et al. (2001) analyzed the dry matter content of 205 different potato
cultivars and 1220 genotypes and accessions of wild potato using oven-drying at 60ºC
17
until the tissue reached a constant weight. They found that moisture content ranged from
61.9 to 81.9% fresh weight (FW) and 52.3 to 89.4% FW, in potato cultivars and wild
species, respectively.
The water content of different potato cultivars may exert a significant effect on
their glycemic impact (Lynch et al., 2007). The wide range of GL values reported among
potato cultivars ranged from a low value of 4.5 in a boiled non-specified cultivar (Kanan
et al., 1998) to a very high value of 24.1 in baked cv. Russet Burbank (Crapo et al.,
1977). The water content, along with sugar content and starch digestibility of potatoes,
affects the rapidly available glucose (RAG) value. Kingman and Englyst (1994)
demonstrated lower RAG values in potato cultivars and commercially produced products
using potatoes with greater water content. RAG values were examined in cv. Marfona
cooked with different methods. The RAG values varied greatly according to cooking
method with values of 14 g/100 g in tubers ―slow-cooked‖ over night to 32 g/100 g in
boiled potatoes refried in oil. Additionally, comparison of digestibility among three
potato cultivars that were cooled after the same boiling treatment showed a range of RAG
values from 14 in the cold boiled cv. Belle de Fontenay with 78.8% moisture content to a
RAG value of 18 in cold boiled cv. Maris Piper with 74.9% moisture content.
Commercially available potato products showed RAG values ranging from 7 associated
with potato salad and with cold canned potato (75% and 84% moisture content,
respectively) to RAG values of 51 in potato crisps (2% moisture content).
A wide range of moisture content in potatoes has been detected that depends on
cultivar, cooking, or storage conditions. Variation in moisture content could be one of the
18
factors resulting in the high range of GL values observed among different potato
cultivars.
2.4.2. Protein content
Starch digestibility can be affected by the protein content of the food. For
example, removing protein in wheat can improve the absorption of carbohydrates by 10-
20%, which might be due to a protein-starch complex that reduces starch digestibility
(Anderson et al., 1981). Lentil flour had the least GI compared to pea and chickpea flour,
which was attributed to its greater protein content (Chung et al., 2008).
Ortiz-Medina et al. (2009) reported a range of 6.8-8.6% (FW basis) in total
soluble protein (TSP) content of fresh field-grown tubers of 20 North American- and
European- grown cultivars, determined by Brandford method using bovine serum
albumin. A range of 4.5 - 13.6% of dry matter (washed and unpeeled) for total protein
content was found in 205 different potato cultivars using the Kjeldahl method (Jansen et
al., 2001). Therefore, with such variation in TSP content, this factor is likely to affect
starch digestibility among different potato cultivars.
2.4.3. Phenolic content
Phenolics constitute one of the most common types of dietary antioxidants that
contribute to human and plant health (Friedman, 1997; Niggeweg et al., 2004). These
compounds are the plant metabolites and oxidation products of polyphenols, which are
used as part of plant’s protective mechanisms. In humans, intake of polyphenols is
implicated in health benefits including anti-carcinogenic, hypoglycaemic, and anti-
oxidative effects.
19
Potatoes contain significant amounts of polyphenolic compounds (Reddivari et al.
2007). Chlorogenic acid constitutes up to 90% of the total phenolic content of potato
tubers (Friedman, 1997). The range of total phenolic content varies significantly with
cultivar. Lewis et al. (1998) analyzed 8 wild Solanum species and reported a wide range
of total phenolic acids ranging from 90-405 mg/150 g FW in skin and 15-90 mg/ 150 g
FW in flesh. The same group showed that flavonoid content varied between 3-25.5
mg/150 g FW in skin and 0- 3.75 mg/ 150 g FW in flesh. Anthocyanin content varied
between 0-30 mg/ 150 g FW in skin with no anthocyanins found in flesh. Im et al. (2008)
analyzed the polyphenolic content, chlorogenic acid, a chlorogenic isomer (5-
caffeoylquinic acid), and caffeic acid, of 5 potato cultivars grown in Korea and 25 from
U.S.A. They found a wide range of total phenolic content ranging from 9.75 to 63.1
g/150 g FW in peels and 0.75 to 24.75 g/150 g FW in flesh of 5 potato Korean cultivars
with peel:flesh ratios from 2.6 to 21.1. The total phenolic content of 25 American
cultivars ranged from 1.5 to 258 g/150 g FW with highest concentrations in red and
purple cultivars. Major polyphenols in early potatoes are chlorogenic acid and catechins
followed by moderate amounts of caffeic and ferulic acids (Leo et al., 2008). Chlorogenic
acid in potatoes can represent up to 90% of the total polyphenolic content (Prohens et al.,
2007), and was found in amounts up to 28.5 mg/150 g FW (Dao and Friedman, 1992).
Based on in vitro studies, phenolic compounds make insoluble complexes with
macronutrients including starches, which could inhibit starch digestibility (Griffiths,
1986). Additionally, polyphenols inhibit the action of digestive enzymes such as α-
amylase, trypsin, and lipase probably by making insoluble complexes with
macronutrients. The two main theories by which polyphenols can lower GI are: (1) the
20
direct inhibition of amylases by polyphenols; and (2) the formation of polyphenol
complexes with starch. However, more studies are needed to clarify the mechanism of
action by which polyphenols may inhibit digestive enzyme action (Friedman, 1997).
Polyphenol-rich extracts from soft fruit, with a 10-fold difference range inhibited
the in vitro action of α-glucosidase and α-amylase (McDougall and Stewart, 2005).
Polyphenolic compounds significantly reduced the activity of digestive enzymes such as
α-amylase depending on the polyphenolic concentration. The greatest magnitude of
inhibition of α-glucosidase was associated with extracts containing greater anthocyanin
content, while a greater content of soluble tannins in the extracts was related to more
inhibition of α-amylase. type of polyphenols based on number of hydroxyl groups (Rohn
et al., 2002). Lower α-amylase activity was observed in compounds with greater
capability of forming quinones (i.e., caffeic acid, chlorogenic acid, and gallic acid) as
compared to polyphenols not capable of forming quinones (i.e., ferulic acid). Since
chlorogenic acid constitutes up to 90% of the total phenolic content of potato tubers
(Friedman, 1997), the wide range of chlorogenic acid content could affect the glycemic
impact of potatoes, through their effect on starch digestibility.
2.4.4. Starch characteristics related to digestibility: resistant starch and
percentage amylose in starch
Carbohydrates can be classified based on their digestibility: rapidly digestible
carbohydrates (RDC), slowly digestible carbohydrates (SDC), RS, and non-starch
polysaccharides (NSPs) (Brennan, 2005). Starch is the most common storage form of
carbohydrate in plants, and is synthesized and stored in amyloplasts within cells. Starch is
a polysaccharide containing two types of glucose polymers: amylose and amylopectin
21
(Kuipers et al., 1994). Amylose makes up approx. 15 to 35% of the starch molecules in
plants. Amylose is a long, virtually un-branched, glucose polymer linked by α (1-4)
bonds with a molecular mass of 1,000 to 100,000 dalton (Da). Amylopectin is a larger
and highly branched glucose polymer. Amylopectin has a molecular mass of about
1,000,000 Da with α (1-4) bonds between the glucose units and α (1-6) bonds within the
branches. Starch has a water-insoluble granule form with a para-crystalline structure due
to amylopectin. Amylose is located in amorphous (non-crystalline) layers within the
starch grain (Figure 2.1) that makes this starch component resistant to amylase hydrolysis
(Nugent, 2005). Following high temperature heating of starch in the presence of water,
starch crystals begin to gelatinize. Gelatinized starch is easily accessible to amylase-
mediated hydrolysis allowing digestion to occur in the small intestine. The starch
structure affects the swelling of starch during gelatinization, which affects the
accessibility of the starch to hydrolytic digestive action (Brennan, 2005).
Resistant starch is not digested in the small intestine and is fermented in the large
intestine (Kumari and Thayumanavan, 1997). These two characteristics categorize RS as
a dietary soluble fibre (Berry, 1986). While there is no global agreement on the definition
of dietary fibre, the American Association of Cereal Chemists (AACC) defines dietary
fibre (DF) as ―any edible part of a plant or analogous carbohydrate that is resistant to both
digestion and absorption in the small intestine with partial or complete fermentation in
the large intestine‖ (Nugent, 2005).
Resistant starch is classified into four groups based on the structures of the starch
molecules:
22
1. RS1 refers to starch granules that are physically inaccessible to the enzyme and
are found in whole or partially milled grains and seeds.
2. RS2 are starch granules with low digestible starch content due to high
compactness of the amylose molecule in contrast to amylopectin. RS2 can mainly
be found in raw potato, green banana, and high amylose maize starch.
3. RS3 are retrograded starches found in starchy food that has been cooked and
cooled. Following the cooling of a cooked gelatinized starch, starch granules re-
crystallize to form complex structures that are not readily accessible to digestive
enzymes (Brennan, 2005). The cooled gelatinized starch granules that re-form
into para-crystalline shapes are considered to be retrograded and resistant to
hydrolysis (Nugent, 2005).
4. RS4 are chemically modified starches such as starch esters, which are cross-
bonded during heating and chemical processing. This cross-linking of the
polymer chains causes starch to depolymerize, which causes pre-gelatinization
and decreases the accessibility of the starch molecule to enzymatic hydrolysis
(Brennan, 2005).
Jansen et al. (2001) showed a wide range of amylose content ranging from 23-
37% of total starch among wild and cultivated potato genotypes. This wide range of
amylose content could cause a wide range in the digestibility of the starch among
different potato cultivars (Kuipers et al., 1994). A key factor affecting amylose content of
potatoes is the granule-bound starch synthase (GBSS) enzyme, which increases the length
of amylose and amylopectin chains by adding adenosine diphosphate glucose to the non-
reducing end. Karlsson et al. (2007) studied genetically modified potatoes that were
23
inhibited in terms of GBSS and observed how this genetic modification affected their
starch granules, starch gelatinization, and re-crystallization properties. Granule size was
smaller in potato with greater amylose content relative to potato with greater amylopectin
content. Potato with higher amylose content had more RS content and the RS content in
high amylose potatoes was positively associated with lower glycemic impact measured
using enzymatic in vitro procedures. The lower glycemic impact of potatoes with greater
amylose content was due to decreased starch digestibility. The authors suggested the
possibility that when GBSS was inhibited, granule size was smaller and amylose content
was greater resulting in a greater RS content.
Many studies showed the lower glycemic impact of diets with higher amylose
content. For example, Behall et al. (1989) found that blood fasting glucose concentrations
and insulin response to a standard glucose tolerance test were significantly less in men
fed the amylose compared to the amylopectin–based meals after a 5 wk but not a 4 wk
diet period. They fed healthy men diets of which 66% of the carbohydrate obtained from
corn starch, with either 70% amylose starch (diet 1) or 70% amylopectin starch (diet 2).
The subjects were randomly fed one of the two starch diets for 5 wk and then were
crossed over to the other diet for another 5 wk.
Similar results were observed in the study done by Behall and Scholfield (2005)
in 24 women and men aged 25-57 years old. Average glucose, insulin, and glucagon
concentrations were significantly lower in subjects after consuming high amylose test
food (containing high amylose corn starch with 70% amylose) than low amylose test food
(containing standard corn starch with 30% amylose and 70% amylopectin). As described
above, it appears that the RS2 and RS3 content of potatoes as affected by the percent
24
composition of amylose in the starch as well as cooking and cooling are important factors
that affect the glycemic impact of potatoes.
2.4.5. Phosphorylated starch content
The degree of starch phosphorylation influences starch texture as the phosphorus
content of starch affects its viscosity (Noda et al., 2006). Phosphate groups are found in
small amounts in starch and glycogen and reduce the crystallization of the starch
molecule (Blennow et al., 2002). The phosphate group in potato starch is covalently
attached to carbon number 3 (C-3) or 6 (C-6) in the glucose unit of amylopectin but not
amylose. During starch biosynthesis, a protein called α-glucan water dikinase (GWD) is
responsible for the starch phosphorylation on C-3 and C-6 glucosidyl units of
amylopectin. Muhrbeck and Svensson (1996) studied the amount of phosphorus bonding
to the C-3 and C-6 portions of glucosidyl units of amylopectin in eight potato cultivars.
Using nuclear magnetic resonance (NMR) spectroscopy, they found that the degree of
phosphorylation on C-3 is almost constant regardless of potato cultivar. However,
phosphorylation on C-6 was closely and linearly correlated to the total phosphorus
content of the potato.
Phosphorylated starches from different sources (corn, rice, and potato) was less
digestible compared with non-phosphorylated starches as phosphorus groups caused
greater inhibition of enzymatic hydrolysis in vitro (Sitohy and Ramadan, 2001). The
mechanism for this inhibition was postulated to be due to: (1) negatively charged
phosphate groups scavenging protons and preventing the hydrolytic action of α-amylase
on glycosidic bonds; (2) phosphate groups reducing the digestibility of starch by
competing with chloride anions binding to basic amino acid residues at the active centre
25
of α-amylase; and (3) phosphorus on the C-3 and C-6 of amylopectin acting as a barrier
against α-amylase.
Starches with phosphorus content < 400 ppm were considered to be low
phosphorus starch (LPS) (Absar et al., 2009). Starch with phosphorus content between
400-800 ppm was considered medium phosphorus starch (MPS), whereas starch with
phosphorus content > 800 ppm was considered high phosphorus starch (HPS). Greater
phosphorus content in starch resulted in lower starch digestibility in a study by Absar et
al. (2009) involving 36 cultivars of potato, sweet potato, cassava, and yams. The
correlation was examined between enzymatic hydrolysis (mediated by different types of
amylase activities including bacterial liquefying α-amylase, Bacillus licheniformis α-
amylase and glucoamylase) in relation to the content of phosphorus, amylose, and median
granule size. Starch samples were gradually heated to 100ºC for 3-4 min after being
incubated with termomyl 120 L (high temperature bacterial alpha amylase). HPS was
more resistant to termomyl 120 L hydrolysis than MPS, suggesting that greater
phosphorus content reduced the digestibility of starch. Similar findings were reported by
Noda et al. (2008). They studied the correlation between starch hydrolysis by amylase
and other starch quality parameters in 26 cultivars of potato, sweet potato, cassava, and
yam. Greater phosphorus content among the various cultivars resulted in decreased raw
starch hydrolysis in vitro, from the released glucose after 2 h of incubation with digestive
enzyme solution of pancreatine, amyloglucosidase, and invertase.
In summary, starch phosphorylation affects digestibility of both raw and
gelatinized starches. Considering the wide range of phosphorylated starch content
26
observed among certain potato cultivars, this could be a factor that affects the glycemic
impact of potatoes.
2.4.6. Sugar content
Glucose, fructose, and sucrose are the main sugars in potato and the content of
these sugars varies among cultivars and is affected by storage conditions (Singh and
Kaur, 2009). Karlsson et al. (2007) studied transgenic potatoes with different
amylose:amylopectin ratios. Low molecular weight carbohydrates (LMWC) such as
glucose, fructose, and sucrose were measured in boiled and peeled tubers. The expected
GI was calculated according to the LMWC content that was added to the free sugar
content, which was generated via the in vitro enzymatic hydrolysis rate of the starch.
Samples with a greater amylose content showed lesser digestibility despite a concurrently
higher content of LMWC. The free sugar content does not appear to impact significantly
on the GI of potatoes. However, the amount of sugar in different cultivars varies
depending on cultivation and storage conditions. Due to such ranges among cultivars, the
sugar content should be measured directly to use in calculating the total GL of different
potato cultivars.
2.4.7. Extrinsic factors affecting the glycemic impact of potatoes
Environmental factors involved with potato growing can affect potato starch
quality which can indirectly affect the digestibility of potato starch and consequently their
glycemic impact. In the comparison of 363 potato samples from 8 cultivars grown in
different sites in Germany, there was little variety in amylose content but a significant
variation in phosphorus and starch content depending on growing conditions (Haase and
Plate, 1996).
27
Different cooking and processing methods are other extrinsic factors affecting the
digestibility and possibly GI and GL of potatoes. The digestibility of the potato starch
was increased depending on the cooking method as boiled and mashed potato had the
highest starch digestibility compared to raw, boiled, oven-baked, crisped, French fried
and retrograded potato (García-Alonso et al., 2000). Resistant starch content varied from
1.18% in boiled potato to 10.38% in retrograded flour indicating that cooking and
processing methods could affect the RS and digestible starch content in addition to the
digestibility of the potato starch. In another study, Gahlawat and Sehgal (1998) also
confirmed that different processing methods affect the digestibility of potato starch. They
tested food products that were processed primarily by baking and roasting, which
contained potato flour, defatted soy flour, or corn flour. Products containing potato flour
had the same protein, ash, and fat content as raw potato but the protein and starch
digestibility was significantly greater in processed potato flour product. The authors
concluded that processed starch products, which are mainly processed by baking and
roasting, are more easily digested than raw products with the same proximate analysis
composition. Processed starch products are considered to contain lower concentrations of
phytate, tannins, and amylase inhibitors that can reduce starch digestibility.
Gamma irradiation together with storage time could be another factor that
conceivably affect the digestibility of potato starch through affecting polyphenolic
content (Blessington et al., 2007). The total phenolic compound content of cv. Atlantic
exposed to 0, 75, and 200 Gy gamma irradiation doses, increased after 0, 10, 20, 75, and
110 days in storage at 20ºC. However, certain phenolic compounds, such as quercetin
dehydrate, reduced as storage duration increased.
28
The meal context is another factor that should be considered when potato is not
consumed alone (Singh and Kaur, 2009). Leeman et al. (2005) showed that adding
vinegar to boiled potato with a GI value of 168 (bread as a reference) could reduce the
postprandial response to 96 (31% reduction). Some other additives such as fats also delay
the digestion rate and result in lower glycemic response to the food, (Garcia-Alonso and
Goñi, 2000). Such extrinsic factors should be considered while testing potato cultivars for
their nutritional composition and glycemic impact. As shown above, the same cultivar
grown and/or stored under different conditions could have different composition leading
to variation in glycemic impact.
In conclusion, the glycemic impact of cooked potatoes could vary in different
cultivars due to one or a combination of the intrinsic and extrinsic factors. Based on this
literature review, our study was performed to investigate the correlation of phytonutrient
content with glycemic impact for several cultivars grown and stored under the same
conditions.
29
Table 2.1 Selected studies examining the effect of low vs. high glycemic index (GI) foods on glycemic control.
Author Diet Outcome in
Low GI vs. High GI Subjects (n)
Study
Design Duration
(Weeks)
Jenkins et al.
(1988) Low vs. high GI
Significant decrease in
serum fructosamine
Noninsulin-dependent Type
2 diabetic (NIDDM) (8) Crossover 5
Fontvieille et
al. (1992)
Low vs. high GI (53 vs.
90)
Decreased fructosamine,
FBS*, and mean daily BG*
No change in HbA1c*
Well-controlled Type 1 and
Type 2 non-insulin-treated
(12 and 6)
Crossover 5
Frost et al.
(1996)
Low vs. high GI (77 vs.
82)
Decreased
fructosamine Type 2 diabetic (51) Parallel 12
Jarvi et al.
(1999)
Low vs. High GI (57 vs.
83)
Increased peripheral insulin
sensitivity
30% Reduction for iAUC*
of BG and plasma insulin
Type 2 diabetic (5 females,
15 males) Crossover 3.5
Luscombe et
al. (1999)
Low vs. high GI and
high-mono high GI (43
vs. 63 and 59)
No significant differences
in metabolic control
NIDDM (14 male, 7
female) Crossover 4
Giacco et al.
(2000)
Low GI vs. high GI (90
vs. 70) Decreased mean of FBS Type 1 diabetic (63)
Randomized
Parallel 24
29
30
Table 2.1 Continued
Author Diet Outcome in
Low GI vs. High GI Subjects (n)
Study
Design Duration
(Weeks)
Komindr et al.
(2001)
Low GI vs. high GI (70
vs. 106)
No change in BG
Type 2 diabetic females
(10) Crossover 4
Heilbronn et
al. (2002)
Low vs. high GI diet (43
vs. 75)
Not significant decrease in
HbA1c
Significant decrease in
LDL concentrations
Type 2 diabetic female and
male (45) Randomized 48
Nansel et al.
(2008)
Low vs. high GI (40 vs.
64)
Decreased BG, 2h after
meal Type 1 diabetic youths (20) Crossover 0.28
Wolever et al.
(2008)
Low vs. high GI and low
carbohydrate
No change in HbA1c but
sustained reduction in CRP
and postprandial glucose
Type 2 diabetic (162) Randomized 48
*FBS= fasting blood sugar, BG= blood glucose, HbA1c= glycated haemoglobin A1c, iAUC= Incremental area under curve,
C-reactive protein= CRP
30
31
Table 2.2 Glycemic index (GI) and glycemic load (GL) values reported for fresh or
cooked potato cultivars in 11 studies selected from the review of Lynch et al. (2007).
Cultivar GI* GL Cooking
Method
Carbohydrate
(g/serving) Authors
Russet
Burbank
111 24.1 Fresh 24.1 Crapo et al.
(1977)
Desiree 101 19.7 Boiled 19.5 Soh and
Brand-Miller
(1999)
Sebago 87 17.0 Boiled 19.5 Soh and
Brand-Miller
(1999)
Pontiac 88 17.2 Boiled 19.5 Soh and
Brand-Miller
(1999)
Pontiac 79 15.4 Microwaved 19.5 Soh and
Brand-Miller
(1999)
Not specified 75 22 French-fried 29 Wolever et
al. (1994)
Not specified 74 19.6
Instant
mashed
22.8 Brand et al.
(1985)
Nardine 70 13.7 Boiled 19.5 Perry et al.
(2002)
Ontario 58 11.3 Boiled 19.5 Wolever et
al. (1994)
Not specified 24 4.7 Boiled 19.5 Ayuo and
Ettyang
(1996)
Not specified 23 4.5
Boiled,
refrigerated,
and
reheated
19.5 Kanan et al.
(1998)
*GI values are relative to glucose as reference
32
Figure 2.1 Overview of starch granule in potato (adapted from illustration made
by Pilling and Smith, 2003 and from http://archaeobotany.dept.shef.ac.uk/wiki)
Starch has a water-insoluble granule form with para-crystalline layers mainly due to
amylopectin and amorphous (non-crystalline) layers due to amylose. This structure
makes raw starch grains resistant to amylase hydrolysis.
Amylose
Amylopectin Crystalline
layer
Non-crystalline
layer
Potato Starch granule
33
III. Nutritional profiles associated with predicted glycemic load
among potato cultivars
Aydin Sarang 1, Danielle J. Donnelly2, Alfred Aziz3, and Stan Kubow1,4
Keywords starch · cultivar · glycemic load · glycemic index · potato ·
phosphorylated starch· resistant starch
1School of Dietetics and Human Nutrition,
2Plant Science Department, Macdonald
Campus of McGill University, 21,111 Lakeshore, Ste. Anne de Bellevue, QC, H9X
3V9, Canada. 3Nutrition Research Division, Food Directorate, Health Canada, 251
Sir Frederick Banting Driveway, PL 2203E, Ottawa, ON, K1A 0K9
4Corresponding Author: Tel: 514-398-7754; Fax: 514-398-7739; E-mail:
34
3.1. Abstract
Although potatoes have been indicated to have a high glycemic index (GI)
and medium glycemic load (GL), the impact of variations in nutrient content among
cultivars on GI and GL values has not been comprehensively investigated.
Depending on cultivar, storage conditions, and cooking method, potatoes can have a
wide range of water, protein, phosphorus content, amylose:amylopectin ratio, and
polyphenolic content that affect starch digestibility, and impact the glycemic
response. The objectives of this study were to: (1) investigate nutritional components
(flesh content of moisture, chlorogenic acid, total soluble protein, amylose, and
phosphorylated starch) that may impact on starch digestibility among 12 Canadian-
grown potato cultivars; and (2) examine a subset of 6 cultivars for effect of cooking
(30 min at 100°C, examined warm) and cooling (refrigerated for 24 h) on starch
digestibility, to predict GI and GL values. Multivariate analysis showed that RS
content was significantly associated with the predicted GI, whereas both RS and
phosphorylated starch contents were significantly associated with the predicted GL
values. The predicted GI and GL values were significantly less (p < 0.05) when
samples were refrigerated compared to warm samples in only the two cultivars
(Kennebec and Russet Burbank); which had the greatest raw flesh content of
phosphorylated and resistant starch (RS), respectively. The present findings indicate
that glycemic impact of potatoes varies primarily with the RS and phosphorylated
starch content of potatoes. Our data suggest a screening tool to identify and develop
potato cultivars with lower glycemic impact a far better strategy than excluding this
nutritive vegetable from our diet.
35
3.2. Introduction
Potatoes are usually considered a high GI food irrespective of cultivar (Soh
and Brand-Miller, 1999), which has led to recommendations that potato consumption
be limited, particularly due to reported positive associations with the development of
diabetes (Halton et al., 2006). However, potatoes are rich in vitamin C (Beals and
Kraus, 2005), minerals such as potassium (Prokop and Albert, 2008), and are a
significant source of polyphenolic compounds (Reddivari et al., 2007). Potato
cultivars can have major compositional differences in water, protein, and phenolic
acid content affected by growing and storage conditions, which might significantly
impact their starch digestibility and glycemic impact (Liu et al., 2007). Such
variation in glycemic impact was suggested by McNab et al. (2004) who showed a
lower glycemic response after a potato-based breakfast meal containing cv. Russet
Burbank vs. the cv. AC Stampede Russet. Lynch et al. (2007) suggested that certain
potato cultivars could have either a moderate or low GL based on their carbohydrate
and high moisture content per serving. Moreover, the GL of potatoes could range
from low to medium based on cooking method (Lynch et al., 2007).
To date, no study has systematically investigated whether compositional
differences among potato cultivars could significantly impact their GI and GL
values. Variations in phytonutrient components among potato cultivars might affect
starch digestibility and consequently glycemic impact. For example, a wide range of
phytonutrients such as total phenolic content (chrologenic acid), phosphorylated
starch, and protein exists among wild potatoes and potato cultivars (Jansen et al.,
2001) which could affect glycemic impact. Fruits with higher polyphenol content are
36
associated with reduced α-glucosidase and α-amylase digestive action (Friedman,
1997). Starch resistance to the digestive action of α-amylase and α-glucosidase due
to polyphenolic effects, mainly of the chlorogenic acid content as the predominant
compound, could lower the glycemic impact of potatoes. Additionally, greater
phosphorylated starch content in certain potato cultivars has been associated with
lower starch digestibility (Absar et al., 2009). Wheat starch showed up to 20%
reduction in resistance of starch to digestion when the protein was removed
(Anderson et al., 1981), which suggests that differences in protein content among
potato cultivars may lead to differences in their glycemic impact.
The objective of the present study was to determine whether compositional
differences in moisture, protein, polyphenols, amylose, resistant and phosphorylated
starch among potato cultivars obtained from the same growing and storage
conditions could lead to differences in their GI and GL as assessed by in vitro starch
digestibility assays.
3.3. Material and methods
3.3.1. Samples
Twelve cultivars grown in Canada were used in this study, including
Atlantic, Goldrush, Green Mountain, Kennebec, Norland, Onaway, Red Pontiac,
Russet Burbank, Sebago, Shepody, Superior, and Yukon Gold. These cultivars were
grown at the Bon Accord Elite Seed Potato Centre, NB, Canada under conventional
field practices for New Brunswick. The tubers were received during the first week of
October 2009.
37
Following 1 mo storage at 10oC, five representative tubers from each
cultivar, with weights within the same confidence interval were washed with tap
water and air-dried. Skin and flesh were separated, diced, and, weighed, and
collected into 50 ml plastic vials, and then freeze-dried (FTS Systems, NY, USA) for
2-3 days. Lyophilized samples were weighed, ground, and stored at -80ºC until
analysis. The % moisture lost in the freeze-drying process was calculated.
Lyophilized tuber samples were analyzed for their total soluble protein
(TSP), chlorogenic acid, amylose, and phosphorylated starch conent. In subsequent
analyses for estimation of GI and GL values, six representative cultivars containing
minimum, medium, and maximum ranges of each nutrient in relation to all cultivars
tested were examined for their in vitro starch digestibility, RS, and available
carbohydrate content after cooking in either a warm state or after cold storage for 24
h.
3.3.2. Compositional Analysis
3.3.2.1. Percentage of moisture
Samples were weighed before and after lyophilization on an analytical
balance (APX-200, Denver Instrument, NY, USA). Moisture content was calculated
as percentage of the weight loss of the fresh sample after freeze-drying, using the
following equation: percentage moisture content = (fresh weight (FW) minus dry
weight (DW)/FW) x 100.
3.3.2.2. Total soluble protein content
Total soluble protein (TSP) content was evaluated using the method of Jones
et al. (1989). In brief, 30 mg of freeze-dried sample was weighed into each 2 ml
38
glass vial. Then, 1.5 ml of phosphate buffer (pH 7.5) was added and the vials were
vortexed for 30 s. Vials were then incubated at 4ºC for 2 h and centrifuged at 3,000 x
g at 4ºC for 40 min. Supernatant was collected for TSP analysis. The analysis was
based on the dye-binding method of Bradford (1976) using bovine serum albumin (2
mg/ml) (Bio-Rad Laboratories, ON, Canada) as a standard and read at 595 nm in a
spectrophotometer (Beckman DU 640, Beckman Instruments, Fullerton, CA).
3.3.2.3. Chlorogenic acid content
Chlorogenic acid was measured by high-performance liquid chromatography
(HPLC) (Varian 9012, Varian Chromatography Systems, Walnut Creek, CA),
equipped with a tertiary pump, refrigerated auto-sampler and single variable
wavelength detector. The chlorogenic acid content was identified and quantified
compared to a pure standard (Sigma-Aldrich Canada Ltd., ON, CA), based on the
method of Shakya and Navarre (2006), using a reverse phase HPLC Gemini-NX (5
m, 100 mm × 4.6 mm) column (Phenomenex) and a 4.6 mm x 2.0 mm guard
column. Briefly, 50 mg of freeze-dried sample (5 samples per cultivar) was weighed
into 1.5 ml Eppendorf tubes then 0.9 ml of extraction buffer (50% methanol, 1 mM
ethylenediaminetetraacetic acid (EDTA), and 2.5% metaphosphoric acid) were
added. Duplicate samples were then vortexed (Fisher Genie Vortex, Scientific
Industries, NY, USA) for 60 s and centrifuged (Accuspin 3R centrifuge, Fisher
Scientific, CA, USA) at 1,500 x g for 15 min at 4ºC. Supernatant was transferred
into a 1.5 ml glass vials (Fisher Scientific, Ottawa, ON) and the extraction procedure
was repeated another time with 0.6 ml of the extraction buffer until the total
supernatant collected reached approximately 1.5 ml. The glass vials were then placed
39
in a speed vacuum (Savant SpeedVac SC210A, Thermo Scientific, CA, USA) for 6-
8 h to evaporate the extraction buffer. The precipitate was re-solubilized in 0.5 ml
100% methanol and vortexed for 60 sec. The solution was filtered through a 0.2 µm
Whatman nylon filter using a 1-ml syringe (Fisher Scientific, Ottawa, ON). The
filtered solution was analyzed for chlorogenic acid via HPLC using two buffers as
mobile phases (buffer A: 10 mM formic acid and buffer B: 5 mM ammonium
formate).
3.3.2.4. Resistant starch and available carbohydrate content
The RS content was measured in six selected cultivars by kit assay (K-
RSTAR, Megazyme International, Wicklow, Ireland). In brief, the kit assay
procedure involves incubation of freeze-dried samples with pancreatic -amylase
(10 mg/ml) containing amyloglucosidase (AMG) (3,300 U/ml) for 16 h to hydrolyze
digestible starch (DS). The digests were then washed with ethanol (99 % v/v) or
industrial methylated spirits (denatured ethanol) (IMS) (99 % v/v) to separate free
glucose and digested starch from the pellet. The supernatant was collected to
measure the available carbohydrate and to measure RS content, while the pellet was
re-suspended in 2 ml 2 M KOH. The digested pellet and supernatant were separately
incubated with AMG (3,300 U/ml) and 1.2 M sodium acetate buffer (pH 3.8). The
glucose released in both solutions was measured using a glucose oxidase-peroxidase
(GOPOD) reagent based on the absorbance of each solution at 510 nm against the
reagent blank. The glucose content of the collected supernatant and digested pellet
were multiplied by 0.9 to calculate the available carbohydrate and RS content,
respectively.
40
3.3.2.5. Isolation of starch
Starch was extracted according to the method of Nielsen et al. (1994).
Freeze-dried samples were weighed (2 g) into 15 ml Eppendorf centrifuge tubes and
4 ml of double-distilled water (DDW) was added. Samples were mixed on a
magnetic stirrer for 2-3 min then filtered into centrifuge vials through two layers of
cheese cloth using a glass funnel. The filtrate was washed four times with 10 ml
DDW and then centrifuged at 3,000 x g for 10 min at room temperature. The
supernatant was discarded and the pellet was washed three times with 10 ml acetone.
The final pellet was left to dry overnight in the fume hood at room temperature then
stored at -20ºC until analysis.
3.3.2.6. Percentage amylose
Percentage amylose in starch was measured according to the method of
Hoover and Ratnayake (2001), which takes into account the iodine affinity of
amylopectin. Isolated starch (20 mg) was weighed into 15 ml Eppendorf centrifuge
tubes to which 8 ml of 90% dimethylsulfoxide (DMSO) was added, then vortexed
for 2 min. The resulting solution was incubated in a shaking water bath at 85 ºC for
15 min. Tubes were cooled at room temperature for 45 min and samples were diluted
to 25 ml with DDW. A 0.1 ml aliquot of the diluted solution was added to 40 ml of
DDW in a 50-ml volumetric flask and 5 ml of iodine solution (0.0025 M I2/0.0065
M KI mixture) was added and mixed. The solution was subsequently diluted with 50
ml of DDW and incubated for 15 min at room temperature. After mixing, the sample
41
absorbance was measured spectrophotometrically at 600 nm. The percentage of
amylose was calculated from an equation obtained from the standard curve.
3.3.2.7. Percentage of phosphorylated starch
Percent phosphorylated starch was measured by flame atomic absorption
spectroscopy (AAS) after digestion of isolated starch, using the method of Parkinson
and Allen (1975) described by Lachat Instruments QuickChem method number 13-
115-01-1-B. Freeze-dried samples (0.160 g) were digested at 340ºC for 3 h in 4.4 ml
of a digestion mixture (420 ml sulfuric acid and 350 ml peroxide (30%) with the
addition of 14 g of lithium and 0.42 g of selenium). The digest was diluted to 100
ml and analysed calorimetrically for phosphorus content at a wavelength of 880 nm
on a flow injector analyzer instrument (QuickChem series 8000, Lachat Instruments,
CO, USA).
3.3.3. Digestibility analysis
3.3.3.1. Rehydration of lyophilized samples
For digestibility analysis, samples from selected cultivars were first
rehydrated to the exact amount of moisture lost during freeze-drying by leaving them
in the refrigerator at 4°C until the water was fully absorbed. Rehydration allows
lyophilized samples to be accurately assessed for digestibility as Mishra et al. (2008)
have concluded that ―freeze-drying raw samples does not have a major impact on the
proportions of starch fractions of differing digestibility when the potato powder is
subsequently cooked and cooled‖. They also showed that freeze-drying does not
significantly affect the RS content.
42
3.3.3.2. Cooking method
To test the impact of cooking on digestibility, duplicate samples were
rehydrated, then 1 ml of DDW was added to each and these were cooked in a heating
block in their tubes, which were placed into a heating block (100°C) for 30 min. One
set of samples was cooled at room temperature to a temperature of 40°C, and then
used for in vitro digestion. The second sample group was cooled in the refrigerator
and processed via in vitro digestion after 24 h.
3.3.3.3. In-vitro Digestibility of Starch, predicted Glycemic index, and
Glycemic load
A modified in vitro method based on the procedure of Goñi et al. (1997) was
followed to measure the digestibility of starch and predict the GI and GL values.
Briefly, 10 ml HCl-KCl buffer (pH 1.5) was added to the samples followed by 0.2
ml of pepsin solution (1g in 10 ml HCl-KCl buffer) (pH 1.5) and incubated at 37°C
for 1 h in a shaking water bath. Subsequently, phosphate buffer (pH 7.8) was added
to raise the pH to 7.8 and samples were then treated with 200 l of pancreatin in
phosphate buffer (0.15 mg enzyme/ ml buffer) and incubated at 40°C for 45 min.
The enzyme reaction was stopped with the addition of 70 l Na2CO3 and diluted to
25 ml with tris-maleate buffer (pH 6.9). A 5 ml -amylase solution (2.6 U -
amylase in tris-maleate buffer) was added to the samples and incubated at 37°C in a
shaking water bath for 90 min. Aliquots of 1 ml were taken from the samples and
placed into boiling water for 5 min with vigorous shaking at intervals. According to
Goñi et al. (1997), a 90 min interval after -amylase treatment provides the most
accurate hydrolysis value for estimation of the in vivo glycemic response. Samples
43
were kept at 4°C to deactivate the enzyme. Aliquots were treated with 3 ml of 0.4 M
sodium acetate buffer (pH 4.75) and 60 l of AMG (3,300 U/ml) then incubated at
60°C for 45 min in a shaking water bath. The glucose released was measured with
GOPOD reagent and converted into starch by multiplying the amount of glucose
released by 0.9, which converts the determined free D-glucose value to anhydro-D-
glucose as occurs in starch (Megazyme, 2002).
Predicted GI (reference food white bread) was calculated as follows: GI =
39.21 + 0.803 x percent of starch hydrolyzed at 90 min (Goñi et al., 1997). Predicted
GL was then calculated using the following equation: GL = GI/100 x (available
carbohydrate (g) in food portion taken) (Salmeron et al., 1997).
3.4. Statistical analysis
Tuber nutrient content was assessed on the basis of a whole fresh tuber of
150 g fresh weight (FW) (1 serving size of a virtual tuber) using the method of Ortiz-
Medina et al. (2009). Ortiz-Medina et al. (2009) introduced a method for inter-
cultivar comparison of potato tuber nutrient content using specific tissue weight
proportions. The percentage contribution of each tissue (skin, cortex and pith) to the
total tuber volume or weight is calculated by using conversion tables for volume or
weight of each tissue. The nutrient content determined for each tissue per g DW was
converted to FW and summed to make the virtual tuber (150 g FW).
The results were analyzed using SAS version 9.2 (2010) and reported as
mean ± SE. ANOVA (one-way) was used for comparing the means among the tested
cultivars for content of water TSP, chlorogenic acid, RS, percentage amylose and
phosphorylated starch. Duncan’s Multiple Comparison test was used to compare
44
these components among the 12 cultivars and the means of the predicted GI and GL
among the selected 6 cultivars. A paired t-test was used to compare the GL and GI
mean values of refrigerated vs. warm samples within each cultivar. Pearson’s
correlation was used to relate nutrient variables with starch digestibility. Multiple
regression analysis was used to investigate the association between the predicted GI,
GL, and the measured phytonutrients. In all cases, significance level was p < 0.05.
3.5. Results
3.5.1. Percentage moisture
Percentage of moisture content in flesh was significantly different among
cultivars and ranged from a minimum content of 57±0.70% (85.5 g/150 g FW)
(Atlantic) to a maximum value of 62±0.77% (93 g/150 g FW) (Kennebec) (Table
3.1). The cv. Kennebec had significantly greater moisture content in comparison to
the cv. Atlantic, Norland, Shepody, and Yukon Gold. The cv. Green Mountain and
Sebago had significantly greater moisture content relative to the cv. Atlantic.
3.5.2. Total soluble protein content
Most cultivars had similar TSP content of the cv. Atlantic, Goldrush, Kennebec,
Sebago, Shepody, and Superior were greater than other cultivars as low as 3.9± 0.25
g/150 g FW in cv. Russet Burbank (Table 3.1). TSP content of Russet Burbank was
significantly less than Onaway, Yukon Gold, and Red Pontiac.
3.5.3. Chlorogenic acid content
Large significant variation occurred in the chlorogenic acid content of the 12
cultivars (Table 3.1). The chlorogenic acid content of cv. Onaway (9.25± 0.52
mg/150 g FW) was significantly greater than all other cultivars except for Russet
45
Burbank (7.35± 0.81 mg/150 g FW) which was similar to cv. Superior (6.39± 1.2
mg/150 g FW). Most other cultivars had relatively low chlorogenic acid content.
3.5.4. Resistant starch and available carbohydrate in raw samples
Resistant starch and available carbohydrate content did not differ
significantly among the six select cultivars with values ranging from 12.27± 1.55
g/150 g FW in Sebago to 15.04± 1.88 g/150 g FW in Superior.
3.5.5. Percentage amylose in isolated starch
Cultivars varied significantly in percent amylose in starch ranging from 18.6±
3.46 % in cv. Russet Burbank to 33.2± 0.28 % in cv. Sebago (Table 3.1). Percent
amylose in starch was significantly greater in cv. Sebago than cv. Russet Burbank
and Norland.
3.5.6. Phosphorylated starch
Kennebec and Sebago showed similar percentage phosphorylated starch that
was significantly greater (0.08± 0.001%) than the other cultivars (Table 3.1).
Superior had a percentage phosphorylated starch (0.05± 0.003%) that was
significantly less than other cultivars.
3.5.7. Predicted GI and GL
The predicted GI and GL of six selected cultivars are shown in Figure 3.1.
No significant difference in predicted GI values occurred between warm samples.
All cultivars showed high predicted GI values ranging from 102.70± 1.08 in Norland
to 127.50±14.49 in Superior (Figure 3.1a). The predicted GI value of refrigerated
samples of cv. Superior (118.10± 11.80) was significantly greater than that of cv.
Kennebec (100± 2.83) (Figure 3.1b). The GI values decreased significantly only in
46
cv. Kennebec and Russet Burbank when the GI values of refrigerated were compared
with warm samples (Table 3.3).
Predicted GL values of selected cultivars were in the medium range for GL
when the samples were either warm or refrigerated (Figure 3.2a and b, respectively).
When samples were warm, the GL of Superior (18.7± 2.50) was similar to that of
Onaway (15.4± 0.72) and significantly greater than cv. Kennebec, Russet Burbank,
Norland, and Sebago (Figure 3.2a). Predicted GL values were significantly lesser in
refrigerated compared with warm samples only in cv. Kennebec and Russet Burbank
(Table 3.3).
The GI of warm samples had a significant negative correlation with RS (r= -
0.40, p < 0.05) as did the GI of refrigerated samples (r = -0.46, p < 0.01) (Table 3.4).
The GL of warm samples had a significant negative correlation with both RS (r = -
0.56, p < 0.005), and % phosphorylated starch (r = -0.48, p < 0.01), and a positive
correlation with available carbohydrate (r = 0.65, p < 0.0001). Similarly, the GL of
refrigerated samples had a significant negative correlation with both RS (r = -0.46, p
< 0.01), and % phosphorylated starch (r = -0.63, p < 0.001), and a significant
positive correlation with available carbohydrate (r = 0.64, p < 0.0001). A significant
inverse correlation was observed between the predicted GL of refrigerated samples
and % phosphorylated starch (r= -0.63, p < 0.0005) while a significant positive
correlation was found between predicted GL of warm samples and total of RS (r=-
0.56, p < 0.01).
Multiple regression analysis (Table 3.5) showed that RS and phosphorylated
starch content of raw tubers of the six selected cultivars had the greatest influence on
47
the variability of the predicted GL values with a predictability of 78.02 and 75.13 %
(R2*100) for warm and refrigerated samples, respectively.
3.6. Discussion
The present findings are supportive of earlier studies by Lynch et al. (2007)
that indicated that the glycemic impact of potatoes is generally in the medium range
based on GL values and that GL values of potatoes can vary significantly with
cultivar. Our results provide supportive evidence to the conclusions of the Lynch et
al. (2007) review in terms of possible factors associated with variations in GL values
among cultivars. A key finding based on multivariate analysis indicated that a high
amount of the variation of the GI and GL values of cooked potatoes could be
accounted for by the RS and phosphorylated starch content of raw potatoes. We
realize that the RS changes after cooking and thus no assumptions can be made
regarding how RS content prior to cooking relates to RS content after cooking. This
is a limitation to the present study in terms of identifying possible mechanisms
involved in the effect of RS on GI and GL variations. However, the approach taken
can be assessed for use as a screening technique to identify cultivars that might have
a lesser GI and GL.
As showed by Mishra et al. (2008), there was a complete transformation of
RS to rapidly digestible starch (RDS) when potatoes were cooked, and then cooling
promoted the formation of slowly digestible starch (SDS) from RDS. Based on these
findings, it seems that the RS content in raw potato could predict the lowered GL in
warm cooked samples. This could readily be confirmed if RS content of cooked
potatoes were measured.
48
Refrigeration, which increases RS content of RS3 due to retrogradation,
causes the starch to be less digestible due to re-crystallization after cooling, which
forms a less digestible structure than gelatinized starch (Brennan, 2005). Both RS
and phosphorylated starch content of raw potatoes were significantly associated with
predicted GL of cooked refrigerated samples probably because more phosphorylated
starch induces further RS formation after cooling. Phosphorylated starch refers to the
attachment of phosphorus to the amylopectin portion of potato starch, which can
inhibit the digestive action of α-amylase and α-glucosidase enzymes (Absar et al.,
2009; Sitohy and Ramadan, 2001). After cooling, the of amylopectin takes longer
time (by several days) to recrystallize due to its crystalline shape, which allows
amylopectin to be more amenable to enzymatic digestion (Van Soest et al., 1994).
This might be the reason why the phosphorus attached to amylopection
(phosphorylated starch) is more effective in making the starch resistant to digestion
when samples were refrigerated.
The phosphorylated starch content of the tested potatoes of 0.05-0.08%
(Table 3.1) was well within the range of 0.036-0.1158% and 0.038-0.1244%
reported by Noda et al. (2007) and Absar et al. (2009), respectively. According to the
classification of Absar et al. (2009), the average phosphorylated starch content of the
cultivars tested in our study was in the medium phosphorus starch (MPS) (0.0400-
0.0800%) category. Since phosphorylated starch content varies significantly with
cultivar (Noda et al., 2007), a wider range of phosphorylated starch content might be
expected with a greater number of tested potato cultivars. So, based on the present
findings showing that phosphorylated starch content affected GL values of
49
refrigerated cooked potatoes, it is conceivable that significantly lower GL values of
cooked potatoes might be observed with higher phosphorylated potato starch content
up to 0.1158% and 0.1244% were reported as high phosphorus starch (HPS) for
some potato cultivars (Absar et al., 2009; Noda et al., 2007).
Interestingly, refrigeration of the cooked potato samples was associated with
lowered GI and GL values only in cv. Russet Burbank and Kennebec which also had
the greatest content of phosphorylated starch and RS. The present study results
therefore suggest that the conformational changes after retrogradation of potato
starch may be dependent on the presence of relatively greater RS and phosphorylated
starch content present in some potato cultivars.
Direct comparison of GI and GL values among studies can be confounded by
the different in vitro and in vivo methodologies used to assess the GI of potatoes.
However, the predicted GI ranges for warm and refrigerated cooked potatoe samples
(Fig. 3.1) were similar to those reported in previous in vitro digestibility studies of
mashed potatoes (Goñi et al., 1997), which used white bread as a reference food for
assessing the GI of cooked potatoes. Likewise, predicted GI values seen in the
present work are similar to previously reported in vivo GI mean values for mashed
potatoes of the cv. Pontiac (Soh and Brand-Miller, 1999) as well as the mean GI
value of 105 obtained from three other in vivo studies carried out on mashed potato
using non-specified cultivars, which is available in the International Table of
Glycemic Index and Glycemic Load values (Foster-Powell et al., 2002). The
predicted GL observed in the present work is also in agreement with the range of GL
50
values of 13.1 to 17.7 for mashed potato of unspecified cultivars (Lynch et al.,
2007).
In terms of compositional analysis, the TSP content among tested potato
cultivars (2.6-3.9% FW) was similar to previously reported values (0.7-4% FW)
(Kadam et al., 1991). The percent amylase content in starch among tested cultivars
(18.6- 33.2%) was also in accordance with previous data (23-37%) (Jansen et al.,
2001). The range of chlorogenic acid content (1.75-9.25 mg/150 g FW) among 12
tested cultivars is in agreement with the range of chlorogenic acid content among
potato cultivars (3.3-10.65 mg/150g FW) reported in a systematic review of previous
studies (Lachman and Hamouz, 2005).
In contrast to observations from previous studies, except for RS and
phosphorylated starch content, other measured components were not correlated with
GI and GL values. However, it is possible that the wider ranges in content of those
components might lead to differences in GL values. For example, the tested potato
cultivars showed a range of 57-62% moisture content in flesh, which shows a lower
range of moisture content relative to previous work that demonstrated a wide range
of 52.3-81.9% FW moisture content in wild potatoes and potato cultivars (Jansen et
al., 2001). Also, there might not have been sufficient range in polyphenol content
among the tested potato cultivars to result in differences in starch digestibility. In our
study there was only a six-fold range in polyphenol content (Table 3.1), as opposed
to the tested polyphenol content in fruit extracts, with a 10-fold range, which was
associated with differences in starch digestibility in the study by McDougall and
Stewart (2005). Additionally, higher anthocyanin content was specifically associated
51
with lower activity of α-glucosidase (McDougall and Stewart, 2005). As the 12
tested cultivars in the present study did not include any anthocyanin-rich cultivars, it
is conceivable that inclusion of coloured-flesh potatoes with a higher range of
polyphenol content might have demonstrated a greater effect of polyphenols on
starch digestibility, GI, and GL values. The present study could not also confirm a
previous association of amylose content in potatoes with predicted GI and GL
values. Karlsson et al. (2007) showed that the RS content and hydrolysis rate of
starch was significantly different between genetically modified potatoes with 64-
78% amylose (w/w of total starch) content compared with non-genetically modified
potatoes that contained 22-23% amylose. The range of amylose content of 18-33%
(w/w of total starch) of potato cultivars observed in the present study may not have
been sufficiently large to demonstrate an impact of amylose content in reducing GL.
In conclusion, the present comprehensive study shows that different potato
cultivars differ significantly in their predicted GL values and it is not necessary or
advisable to exclude or avoid all potatoes from the diet. Among the several potato
components examined, RS and phosphorylated starch content are the major factors
in predicting GI and GL values among the selected potato cultivars. Our results took
a major step toward identification of factors influencing the variation of glycemic
impact of potatoes, and can help in the development of a screening method to
facilitate the prediction of GI and GL of different potato cultivars. Identification of
potato cultivars with a higher content of RS and phosphorylated starch could lead to
the discovery of cultivars that could be classified as low GL. Our results, however,
are limited in terms of identifying possible mechanisms involved in the GI and GL
52
variations as the compositional factors in cooked potatoes were not measured.
Further studies on a wider range of potato cultivars are needed to assess our results.
If these are confirmed, human trials can be performed to investigate whether RS and
phosphorylated starch content in different cooked potato cultivars could affect their
GL values.
53
Table 3.1 Content of percent moisture, total soluble protein (TSP), chlorogenic acid (CGA), percent amylose and
phosphorylated starch of a serving size (150 g FW) of 12 Canadian potato cultivars.
Cultivars
Moisture
(%)
TSP
(g)
CGA
(mg)
Amylose
(%)
Phosphorylated Starch
(%)
Atlantic 57.0 ± 0.70 c(1,2)
5.50 ± 0.28 abc
2.64 ± 0.52 d 31.1 ± 1.37
ab 0.07 ± 0.003
b
Green Mountain 60.6 ± 2.00 ab
4.62 ± 0.27 de
4.23 ± 0.21 cd
30.9 ± 1.39 ab
0.06 ± 0.002 bcd
Goldrush 59.4 ± 0.62 abc
5.10 ± 0.33 abcd
2.17 ± 1.26 d 32.1 ± 1.16
ab 0.06 ± 0.003
bcd
Kennebec 62.0 ± 0.77 a 5.86 ± 0.27
a 3.69 ± 0.97
d 28.1± 0.88
bcd 0.08 ± 0.001
a
Norland 57.6 ± 1.08 bc
4.45 ± 0.16 de
1.75 ± 0.16 d 24.7± 1.88
d 0.07 ± 0.002
bc
Onaway 59.8 ± 1.23 abc
5.02 ± 0.12 bcd
9.25 ± 0.52 a 29.2± 1.01
abcd 0.07 ± 0.002
bcd
Russet Burbank 59.2 ± 1.12 abc
3.90 ± 0.25 e 7.35 ± 0.81
ab 18.6± 3.46
e 0.06 ± 0.002
cd
Red Pontiac 60.3 ± 1.82 abc
4.81 ± 0.16 cd
1.87 ± 0.49 d 32.4± 1.37
ab 0.06 ± 0.002
bcd
Sebago 60.6 ± 0.40 ab
5.58 ± 0.43 abc
4.23 ± 1.27 cd
33.2± 0.28 a 0.08 ± 0.003
a
Shepody 58.6 ± 0.64 bc
5.82 ± 0.31 ab
3.18 ± 1.00 d 31.1± 0.87
ab 0.06 ± 0.002
bcd
Superior 59.2 ± 0.78 abc
5.16 ± 0.04 abcd
6.39 ± 1.20 bc
26.1± 0.75 cd
0.05 ± 0.003 e
Yukon Gold 58.2 ± 0.52 bc
4.95 ± 0.10 cd
3.78 ± 0.49 d 30.2± 1.00
abc 0.06 ± 0.003
d
(1) Values expressed as means ± SE (n=5). Data arranged based on alphabetical order of cultivars. (2) Means of each phytonutrient amongs12 cultivars were
compared using Duncan’s New Multiple Range Test (p < 0.05). Means with same superscript in the same column are not significantly different.
53
54
Table 3.2 Resistant starch and available carbohydrate content in one serving (150 g FW)
of six selected Canadian cultivars.
(1) Values expressed as means ± SE (n=5). Data arranged based on alphabetical order of cultivars.
(2) Means of resistant starch and available carbohydrate of six cultivars were compared using
Duncan’s New Multiple Range Test (p < 0.05). Means with same superscript in the same column
are not significantly different.
Cultivars Resistant Starch
(g)
Available
Carbohydrate
(g)
Kennebec 13.5 ± 0.78 ab (1,2)
13.45 ± 0.53 a
Norland 14.86 ± 0.54 a 13.77 ± 0.45
a
Onaway 12.15 ± 2.59 ab
14.56 ± 0.79 a
Russet Burbank 15.09 ± 0.46 a 14.26 ± 0.61
a
Sebago 14.19 ± 1.59 ab
12.27 ± 1.55 a
Superior 9.84 ± 1.92 b 15.04 ± 1.88
a
55
Figure 3.1 (a). Predicted GI
values in selected warm
samples*
Figure 3.1 (b). Predicted GI
values in selected refrigerated
samples*
*Means of the predicted GI values were compared among six cultivars using Duncan’s New Multiple Range Test (p< 0.05). Means with same alphabetic
label are not significantly different.
55
56
Figure 3.2 (a). Predicted GL
values in selected warm samples*
Figure 3.2 (b). Predicted GL
values in selected refrigerated
samples*
*Means of the predicted GL values were compared among six cultivars using Duncan’s New Multiple Range Test (p< 0.05). Means with same alphabetic
label are not significantly different.
56
57
Table 3.3 t-test significance of the predicted glycemic index (GI) and glycemic load (GL) between warm vs. refrigerated
samples of each selected cultivars
Cultivars Mean Predicted GI
(Warm)
Mean Predicted GI
(Refrigerated)
Mean Predicted GL
(Warm)
Mean Predicted GL
(Refrigerated)
Kennebec* 108.8 ± 3.03 100.0 ± 2.83 14.5± 0.38 13.2 ± 0.43
Norland 102.7 ± 1.08 100.4 ± 0.11 14.1± 1.88 13.8 ± 0.00
Onaway 105.6 ± 3.03 101.4 ± 3.82 15.4± 0.72 14.8 ± 0.58
Russet Burbank* 104.7 ± 1.34 99.0 ± 1.60 14.8± 0.40 14.0 ± 0.36
Sebago 121.1 ± 10.79 106.6 ± 4.40 13.9± 1.01 12.1 ± 0.75
Superior 127.5 ± 14.49 118.1 ± 11.80 18.7± 2.50 17.2 ± 2.15
Values expressed as means ± SE (n=5). Data arranged based on alphabetical order of cultivars Means of predicted GI/GL of warm vs. refrigerated
samples in each cv. were compared using paired t-test ( p< 0.05). Reference food for GI and GL values is white bread.
*Significantly different in predicted GI/GL of warm vs. refrigerated samples
57
58
Table 3.4 Pearson Correlation Coefficients (r) between predicted glycemic index (GI), glycemic load (GL), and potato
phytonutrients
(1) |r| = Pearson Correlation Coefficient
* Significance at p< 0.05
Moisture Total Soluble
Protein
Chlorogenic
Acid
Resistant
Starch
Amylose
in Starch
Phosphorylated
Starch
Predicted GI
(refrigerated)
|r|(1)
P
-0.17351
0.3592
-0.13446
0.4787
0.00112
0.9953
-0.46647
0.0094*
-0.00697
0.9709
-0.31463
0.0904
Predicted GI
(warm)
|r|
P
0.13727
0.4695
0.22562
0.2306
-0.14839
0.4339
-0.57650
0.0009*
0.21551
0.2527
-0.12515
0.5099
Predicted GL
(refrigerated)
|r|
p
-0.16601
0.3806
-0.17599
0.3522
0.22583
0.2302
-0.46264
0.0100*
-0.08930
0.6389
-0.63004
0.0002*
Predicted GL
(warm)
|r|
P
0.01776
0.9258
0.03907
0.8376
0.16657
0.3790
-0.56066
0.0013*
0.02484
0.8964
-0.48866
0.0061*
58
59
Table 3.5 Independent predictors of the predicted glycemic load (GL) of refrigerated and warm potatoes after cooking
Variable
Predicted GL
(refrigerated samples)
Predicted GL
(warm samples)
β±SEM P β±SEM P
% Moisture 0.02±0.14 0.8870 0.13±0.14 0.3768
Total Soluble Protein -0.001±0.002 0.3703 -0.0002±0.002 0.9231
Chlorogenic Acid -0.14±0.30 0.6374 -0.29±0.31 0.3673
Resistant Starch -0.19±0.06 0.0045* -0.25±0.06 0.0006*
% Amylose 0.06±0.06 0.3339 0.05±0.07 0.5016
Phosphorylated Starch (%) -75.44±30.86 0.0234* -64.19±32.41 0.0609
Coefficient Variance 10.8549 10.59336
R2*100 75.13 78.02
F Value (p<0.0001) 7.93 9.32
* Significance at p< 0.05
59
60
IV. Summary and concluding remarks
4.1. General discussion and conclusion
Although potato is a high glycemic index (GI) food (Foster-Powell et al., 2002;
Soh and Brand-Miller, 1999) some studies have shown that depending on cultivar and
growing conditions, they may contain a wide range of components that could conceivably
decrease their glycemic impact, including moisture (Lynch et al., 2007), protein
(Anderson et al., 1981), polyphenolics (McDougall and Stewart, 2005), amylose
(Karlsson et al., 2007), and phosphorylated starch (Absar et al., 2009). Also, a few
investigations have suggested that certain cultivars could have a medium to low glycemic
load (GL), which has been ascribed to greater moisture content. However, a
comprehensive examination of possible components that could modify potato starch
digestibility has not been carried out. In addition to their inhibitory effects on starch
digestion, higher amount of protein and polyphenolic compounds could add to the
nutritive value of potatoes.
This thesis had two main objectives. The first objective was to investigate whether
the 12 selected cultivars grown in Canada (cultivated and stored under the same
conditions) vary in moisture, total soluble protein (TSP), chlorogenic acid, resistant
starch (RS), the percentage of amylose, and phosphorylated starch. Results showed a
wide range of nutrient components among the tested potato cultivars.
The above-mentioned nutrients were compared among the 12 cultivars as virtual
whole tubers by converting the concentration data on a per g DW of flesh basis into a
virtual whole tuber data on a per 150 g FW basis using conversion factors for the specific
61
tissues of the selected cultivars reported by Ortiz-Medina et al. (2009). The findings from
the study examining the first objective were used to identify six cultivars (Kennebec,
Superior, Russet Burbank, Onaway, Norland, and Sebago) with relatively little, medium,
and maximum amount of phytonutrients.
The results showed the major impact of RS on predicted GI and of RS and
phosphorylated starch on predicted GL, whereas other independent variables that can
modify starch digestion such as moisture, TSP, chlorogenic acid, and the percentage of
amylose did not appear to be significantly involved. Predicted GI and GL values were
significantly different between warm and refrigerated samples only in the cv. Kennebec
and Russet Burbank which also had the greatest content of phosphorylated starch and RS,
respectively. This latter result could be indicative of a cultivar-dependent effect of
retrogradation that leads to an increased resistance of starch to digestion and lower
predicted GI and GL in refrigerated cooked potato samples. However, a major limitation
was that the RS content in cooked potatoes was not measured, which limits interpretation
as cooking and cooling treatment of potatoes can affect the RS content (Mishra et al.,
2008). Measuring the RS content of cooked potatoes would give a better idea regarding
the latter effect because cooking will change the resistant starch to rapidly digestible
starch as heating gelatinizes the starch and disrupts its structure. Consequently, the
cooked starch will be more resistant to digestion due to retrogradation. Also, studying a
greater variety of potato cultivars, including ones with coloured flesh, could give a better
idea regarding how major variation in other phytonutrient components (such as
anthocyanins) might impact the GL of potatoes. The predicted GL and GI values of the
cooked potato cultivars were in the range of previously reported GL and GI values of
62
mashed potato. Other cooking methods could be used on whole fresh potatoes including
microwaving, baking or boiling with skin to test the impact of these cooking methods on
GL values as affected by variation in potato components among cultivars.
Potato with lower GL values could be recommended for incorporation as part of a
healthy diet. Other potato components such as polyphenols should also be considered for
their health and nutritional values in selecting potato cultivars for their nutritional
benefits. In that regard, Russet Burbank and Onaway were among the cultivars with the
greatest chlorogenic acid content among the 12 selected cultivars and also had GL values
that would be considered in the medium range.
4.2. Contribution to knowledge
To the best of our knowledge, this is the first comprehensive research study to
determine how the predicted GL of potatoes could be affected by factors related to
phytonutrient and starch composition; including moisture, protein, polyphenolic,
phosphorylated starch, and amylose content.
4.3. Limitations and suggestions for future studies
4.3.1. RS content should be measured in cooked potatoes to determine its
relationship to RS in fresh and freeze-dried potatoes and their effects on
predicted GL.
4.3.2. This study should be repeated on all of the major cultivars grown in Canada,
with larger and more representative sample sizes, to identify cultivars with
lower predicted GL values.
63
4.3.3. Similarly, further studies should examine GI and GL of potato cultivars
subjected to additional methods of cooking such as baking or boiling in skin
or chopped refrigerated potatoes.
4.3.4. Potato cultivars with high anthocyanin content should be studied to
investigate the relationship between anthocyanins and starch digestibility as
suggested by McDougall and Stewart (2005).
4.3.5. Future human feeding trials should carried out to determine whether major
differences in resistant and phosphorylated starch content among potato
cultivars could significantly impact their GL values, preferably starting with
cv. Russet Burbank and Kennebec.
64
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Appendix
Table A.1 Table of glycemic index (GI) and glycemic load (GL) values of a typical
serving size (150 g) of potatoes, adapted from the International Table of Glycemic
Index and Load by Foster-Powell et al. (2002)
Food item GI
(Ref.=Glucose)
GI
(Ref.=Bread)
Available
carbohydrate
GL
(per
serving)
Cv. Ontario, white, baked
in skin
60 85 ± 4 30 18
Baked, cv. Russet Burbank
potatoes
85 ± 12 121 ± 16 27 16
Cv. Desiree, peeled, boiled
35 min
101 ± 15 144 ± 22 17 17
Cv. Ontario, white, peeled,
cut into cubes, boiled in
salted water 15 min
58 83 ± 5 27 16
Cv. Pontiac, peeled, boiled
whole for 30 min
56 80 26 14
Cv. Pontiac, peeled, boiled
35 min
88 ± 9 125 ± 13 18 16
Cv. Sebago, peeled, boiled
35 min
87 ± 7 124 ± 10 17 14
Type not specified, boiled
in salted water
23 33 34 8
New, canned, heated in
microwave 3 min (Mint
Tiny Taters; Edgell’s,
Cheltenham, Australia)
65 ± 9 93 ± 13 18 12
80
Appendix A. Continued
Food item GI
(Ref.=Glucose)
GI
(Ref.=Bread)
Available
carbohydrate
GL
(per
serving)
French fries, frozen,
reheated in microwave
(Cavendish Farms, New
Annan, Canada)
75 107 ± 6 29 22
Instant mashed potato
(mean of 6 studies)
85 ± 3 122 ± 5 20 17
Cv. Pontiac, peeled, cubed,
boiled 15 min, mashed
91 ± 9 130 ± 13 20 18
Cv. Pontiac, peeled and
microwave on high power
for 6–7.5 min
79 ± 9 112 ± 13 18 14
Potato, peeled, steamed 1 h 65 ± 11 93 27 18
Potato dumplings (white-
wheat flour, white potatoes,
boiled in salted water
52 74 ± 12 45 24
81
Figure A.1 Field-grown tubers of the 12 Canadian cultivars used in this study:
(A) Atlantic, (B) Green Mountain, (C) Goldrush, (D) Kennebec, (E) Norland, (F)
Onaway, (G) Russet Burbank, (H) Red Pontiac, (I) Sebago, (J) Shepody, (K)
Superior, and (L) Yukon Gold (from CFIA, 2011 and Vunnam, 2011).
A B
C D
82
Figure A.1 Continued
C D F A B
E F
G H
83
Figure A.1 Continued
I J
J
K L