mcardle energy value food ch4 connection

7/29/2019 Mcardle Energy Value Food Ch4 Connection

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Energy Valueof Food

C h a p t e r O b j e c t i v e s Describe the method for directly determining the en-

ergy content of the macronutrients

Discuss various factors that influence the differencebetween a foods gross energy value and its netphysiologic energy value

Define the following: (1) heat of combustion, (2) di-gestive efficiency, and (3) Atwater factors

Compute the energy content of a meal from itsmacronutrient composition

4C H A P T E R

108


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CHAPTER 4 Energy Value of Food 109

MEASUREMENT OF FOOD ENERGY

The Calorie As a Measurement Unit

In terms of food energy, one calorie expresses the quantity

of heat needed to raise the temperature of 1 kg (1 L) of wa-

ter 1C (specifically, from 14.5 to 15.5C). Thus, kilogram

calorie or kilocalorie (kcal) more accurately defines calo-

rie. For example, if a particular food contains 300 kcal, thenreleasing the potential energy trapped within this foods

chemical structure increases the temperature of 300 L of wa-

ter 1C. Different foods contain different amounts of poten-

tial energy. One-half cup of peanut butter with a caloric

value of 759 kcal contains the equivalent heat energy to in-

crease the temperature of 759 L of water 1C.

A corresponding unit of heat using Fahrenheit degrees is

the British thermal unit, or BTU. One BTU represents the

quantity of heat necessary to raise the temperature of 1 lb

(weight) of water 1F from 63 to 64F. A clear distinction ex-

ists between temperature and heat. Temperature reflects a

quantitative measure of an objects hotness or coldness. Heat

describes energy transfer or exchange from one body or sys-tem to another. (The following conversions apply: 1 cal

4.184 J; 1 kcal 1,000 cal 4,184 J or 4.184 kJ; 1 BTU

778 ft-lb 252 cal 1,055 J.)

The joule, or kilojoule (kJ), reflects the standard interna-

tional unit for expressing food energy. To convert kilocalories to

kilojoules, multiply the kilocalorie value by 4.184. The kilojoule

value for one-half cup of peanut butter, for example, would equal

759 kcal 4.184 or 3,176 kJ. The megajoule (MJ) equals 1,000

kJ; its use avoids unmanageably large numbers. Appendix Apre-

sents a listing of metric system transpositions and conversion

constants commonly used in exercise physiology.

Gross Energy Value of Foods

Laboratories use bomb calorimeters similar to the one illus-

trated in Figure 4.1 to measure the total or gross energy value

of various food macronutrients. Bomb calorimeters operate

on the principle of direct calorimetry, measuring the heat

liberated as the food burns completely.

Figure 4.1 shows food within a sealed chamber charged

with oxygen at high pressure. An electrical current moving

through the fuse at the tip ignites the foodoxygen mixture.

As the food burns, a water jacket surrounding the bomb ab-

sorbs the heat (energy) liberated. Because the calorimeter re-

mains fully insulated from the ambient environment, the in-

crease in water temperature directly reflects the heat releasedduring a foods oxidation (burning).

Heat of combustion refers to the heat liberated by oxi-

dizing a specific food; it represents the foods total energy

value. For example, a teaspoon of margarine releases 100 kcal

of heat energy when burned completely in a bomb calorime-

ter. This equals the energy required to raise 1.0 kg (2.2 lb) of

ice water to the boiling point. Although the oxidation path-

ways of the intact organism and the bomb calorimeter differ,

the quantity of energy liberated in the complete breakdown of

a food remains the same.

Heat of Combustion: Lipid

The heat of combustion for lipid varies with the structural

composition of the triglyceride molecules fatty acids. For ex-ample, 1 g of either beef or pork fat yields 9.50 kcal, whereas

oxidizing 1 g of butterfat liberates 9.27 kcal. The average

caloric value for 1 g of lipid in meat, fish, and eggs equals

9.50 kcal. In dairy products, the calorific equivalent amounts

to 9.25 kcal per gram and in vegetables and fruits, 9.30 kcal.

The average heat of combustion for lipid equals 9.4 kcal per

gram.

Heat of Combustion: Carbohydrate

The heat of combustion for carbohydrate also varies, depend-

ing upon the arrangement of atoms in the particular carbohy-

drate molecule. The heat of combustion for glucose equals 3.74kcal per gram, whereas larger values result for glycogen (4.19

kcal) and starch (4.20 kcal).A value of 4.2 kcal generally rep-

resents the heat of combustion for a gram of carbohydrate.

Heat of Combustion: Protein

Two factors affect energy release during combustion of a

foods protein component: (1) the type of protein in the food

and (2) the relative nitrogen content of the protein. Common

proteins in eggs, meat, corn (maize), and beans (jack, Lima,

Electricalignition

Thermometer

Oxygen inlet

Water bathmixer

Food sample

Pressurizedoxygen

Bomb

Electricfuse

Air space

Insulatingcontainer

Water bath

Oxygentank

FIGURE 4.1 A bomb calorimeter directly measures the en-ergy value of food.


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navy, soy) contain approximately 16% nitrogen and have cor-

responding heats of combustion that average 5.75 kcal per

gram. Proteins in other foods have a somewhat higher nitro-

gen content (e.g., most nuts and seeds [18.9%] and whole-

kernel wheat, rye, millets, and barley [17.2%]). Other foods

contain a slightly lower nitrogen percentage, for example,

whole milk (15.7%) and bran (15.8%). The heat of combus-

tion for protein averages 5.65 kcal per gram.

Comparing the Energy Value of Nutrients

The average heats of combustion for the three macronutri-

ents (carbohydrate, 4.2 kcal g1; lipid, 9.4 kcal g1; protein,

5.65 kcal g1) demonstrate that the complete oxidation of

lipid in the bomb calorimeter liberates about 65% more en-

ergy per gram than protein oxidation and 120% more energy

than the oxidation of carbohydrate. Recall from Chapter 1 that

lipid molecules contains more hydrogen atoms than either

carbohydrate or protein molecules. The common fatty acid

palmitic acid, for example, has the structural formula

C16H32O2. The ratio of hydrogen atoms to oxygen atoms infatty acids always greatly exceeds the 2:1 ratio in carbohy-

drates. Simply stated, lipid molecules have more hydrogen

atoms available for cleavage and subsequent oxidation for en-

ergy than carbohydrates and proteins.

INTEGRATIVE QUESTION

Respond to a student who asks: How can the oxy-

gen required to burn food indicate the number of

calories in the meal Im going to eat tonight?

One can conclude from the above discussion that lipid-

rich foods have a higher energy content than relatively fat-free foods. One cup of whole milk, for example, contains 160

kcal, whereas the same quantity of skim milk contains only 90

kcal. If a person who normally consumes one quart of whole

milk each day switches to skim milk, the total calories in-

gested each year would decrease by the equivalent of the

calories in 25 pounds of body fat. In 3 years, all other things

remaining constant, the loss of body fat would approximate

75 pounds! Such a theoretical comparison merits serious con-

sideration because of the almost identical nutrient composi-

tion between whole milk and skim milk except for lipid con-

tent. Drinking skim rather than whole milk also significantly

reduces saturated fatty acid intake (0.4 versus 5.1 g; 863%)

and cholesterol (0.3 versus 33 mg; 910%).

Net Energy Value of Foods

Differences exist in the energy value of foods when the heat

of combustion (gross energy value) determined by direct

calorimetry is compared with the net energy actually avail-

able to the body. This pertains particularly to protein because

the body cannot oxidize the nitrogen component of this nutri-

ent. In the body, nitrogen atoms combine with hydrogen to

form urea (NH2CONH2), which the kidneys excrete in the

urine. Elimination of hydrogen in this manner represents a

loss of approximately 19% of the protein molecules potential

energy. This hydrogen loss reduces proteins heat of combus-

tion to approximately 4.6 kcal per gram instead of 5.65 kcal

per gram released during oxidation in the bomb calorimeter.

In contrast, the physiologic fuel values of carbohydrates and

lipids (which contain no nitrogen) are identical to their heats

of combustion in the bomb calorimeter.

COEFFICIENT OF DIGESTIBILITY. The efficiency of the digestive

process influences the ultimate energy yield from the food

macronutrients. Numerically defined as the coefficient of di-

gestibility, digestive efficiency indicates the percentage of in-

gested food actually digested and absorbed to meet the bodys

metabolic needs. The food remaining unabsorbed in the in-

testinal tract is voided in the feces. Dietary fiber reduces the

coefficient of digestibility; a high-fiber meal has less total en-

ergy absorbed than does a fiber-free meal of equivalent

caloric content. This variance occurs because fiber moves

food through the intestine more rapidly, reducing time for ab-

sorption. Fiber also may cause mechanical erosion of the in-testinal mucosa, which is then resynthesized through energy-

requiring processes.

Table 4.1 shows different digestibility coefficients, heats

of combustion, and net energy values for nutrients in the var-

ious food groups. The relative percentage of the macronutri-

ents digested and absorbed averages 97% for carbohydrate,

95% for lipid, and 92% for protein.Little difference exists in

digestive efficiency between obese and lean individuals.

However, considerable variability exists in efficiency per-

centages for any food within a particular category. Proteins in

particular have digestive efficiencies ranging from a low of

about 78% for protein in legumes to a high of 97% for protein

from animal sources. Some advocates promote the use ofvegetables in weight-loss diets because of plant proteins rel-

atively low coefficient of digestibility. Those who choose

vegetarian-type diets should consume adequate, diverse pro-

tein food sources to obtain all the essential amino acids.

From the data in Table 4.1, one can round the average net

energy values to whole numbers referred to as Atwater gen-

eral factors.

These values, named for Wilbur Olin Atwater (18441907),

the 19th-century chemist who pioneered human nutrition and

energy balance studies at Wesleyan College, indicate the net me-

tabolizable energy available to the body from ingested foods. If

precise energy values for experimental or therapeutic diets are

not required, the Atwater general factors provide a good esti-mate of the energy content of the daily diet (see In a Practical

Sense). For alcohol, 7 kcal (29.4 kJ) represents each g (mL) of

110 SECTION 2 Energy for Physical Activity

ATWATER GENERAL FACTORS

4 kcal per gram for dietary carbohydrate

9 kcal per gram for dietary lipid

4 kcal per gram for dietary protein


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pure (200-proof) alcohol ingested. In terms of potential energy

available to the body, alcohols efficiency of use equals that of

other carbohydrates.

Use of Tabled Values

Computing the kilocalorie content of foods requires consider-able time and labor. Various governmental agencies in the

United States and elsewhere have evaluated and compiled nu-

tritive values for thousands of foods. The most comprehen-

sive data bank resources include the United States Nutrient

Data Bank (USNDB), maintained by the U. S. Department of

Agricultures Consumer Nutrition Center, and a computerized

data bank maintained by the Bureau of Nutritional Sciences

of Health and Welfare Canada. Appendix B presents the en-

ergy and nutritive values for common foods, including spe-

cialty and fast-food items.

A brief review of Appendix B indicates that large dif-

ferences exist between the energy values of various foods.

Consuming an equal number of calories from different foods

often requires a tremendous intake of a particular food or

relatively little of another. For example, to consume 100

kcal from each of six common foodscarrots, celery, green

peppers, grapefruit, medium-sized eggs, and mayonnaiseone must eat 5 carrots, 20 stalks of celery, 6.5 green peppers,

1 large grapefruit, 1 1/4 eggs, but only 1 tablespoon of may-

onnaise. Consequently, a typical sedentary adult female who

expends 2100 kcal each day must consume about 420 celery

stalks, 105 carrots, 136 green peppers, 26 eggs, yet only

1 1/2 cup of mayonnaise or 8 ounces of salad oil to meet

daily energy needs. These examples illustrate dramatically

that foods high in lipid content contain considerably more

calories than food low in lipid and correspondingly high in

water content.

TABLE 4.1 FACTORS FOR DIGESTIBILITY, HEATS OF COMBUSTION,AND NET PHYSIOLOGIC ENERGY VALUESa OF PROTEIN, LIPID,AND CARBOHYDRATE

FOOD GROUP DIGESTIBILITY (%) HEAT OF COMBUSTION (KCAL G1) NET ENERGY (KCAL G1)

Protein

Animal food 97 5.65 4.27

Meats, fish 97 5.65 4.27

Eggs 97 5.75 4.37Dairy products 97 5.65 4.27

Vegetable food 85 5.65 3.74

Cereals 85 5.80 3.87

Legumes 78 5.70 3.47

Vegetables 83 5.00 3.11

Fruits 85 5.20 3.36

Average Protein 92 5.65 4.05

Lipid

Meat and eggs 95 9.50 9.03

Dairy products 95 9.25 8.79



Average Lipid 95 9.40 8.93

Carbohydrate


Cereals 98 4.20 4.11

Legumes 97 4.20 4.07

Vegetables 95 4.20 3.99

Fruits 90 4.00 3.60

Sugars 98 3.95 3.87


Average 97 4.15 4.03

Carbohydrate

aNet physiologic energy values are computed as the coefficient of digestibility heat of combustion adjusted for

energy loss in urine.

From Merrill AL, Watt BK. Energy values of foods: basis and derivation. Agricultural Handbook no. 74,

Washington, DC: USDA, 1973.


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INTEGRATIVE QUESTION

What factors could account for a discrepancy be-

tween computations of the energy value of daily

food intake using the Atwater general factors and

those from direct measurement via the bomb

calorimeter?

Also note that a calorie reflects the food energy regard-

less of the food source. Thus, from an energy standpoint, 100calories from mayonnaise equals the same 100 calories in 20

celery stalks. The more a person eats of any food, the more

calories that person consumes. However, a small amount of

fatty food represents a considerable number of calories; thus,

the termfattening often describes these foods. An individuals

caloric intake equals the sum ofall energy consumed from ei-

ther small or large quantities of foods. Celery would become

a fattening food if consumed in excess! Chapter 3 considered

variations in daily energy intake among diverse groups of

male and female athletes.

Summary

1. A calorie or kilocalorie (kcal) represents a measure

of heat used to express the energy value of food.

2. Burning food in the bomb calorimeter permits di-

rect quantification of the foods energy content.

3. The heat of combustion quantifies the amount of

heat liberated in the complete oxidation of a food.

Average gross energy values equal 4.2 kcal per

gram for carbohydrate, 9.4 kcal per gram for lipid,and 5.65 kcal per gram for protein.

4. The coefficient of digestibility represents the pro-

portion of food consumed that is actually digested

and absorbed.

5. Coefficients of digestibility average approximately

97% for carbohydrates, 95% for lipids, and 92%

for proteins. Thus, the net energy values equal 4

kcal per gram of carbohydrate, 9 kcal per gram of

lipid, and 4 kcal per gram of protein. These values,

known as Atwater general factors, provide an accu-


IN A PRACTICAL SENSE

DETERMINING A FOODS MACRONUTRIENT COMPOSITION AND ENERGY CONTRIBUTION

Food labels must indicate a foods macronutrient content (g)

and total calories (kcal). Knowing the energy value per gram

for carbohydrate, lipid, and protein in a food allows one toreadily compute the percentage kcal derived from each

macronutrient. The net energy value, referred to as Atwater

general factors, equals 4 kcal for carbohydrate, 9 kcal for

lipid, and 4 kcal for protein.

CalculationsThe table shows the macronutrient composition for one large

serving of McDonalds French fries (weight, 122.3 g [4.3 oz]).

[Note: McDonalds publishes the weight of each of the

macronutrients for one serving along with the total kcal value.]

1. Calculate kcal value of each macronutrient (column 4).

Multiply the weight of each nutrient (column 2) by

the appropriate Atwater factor (column 3).2. Calculate percentage weight of each nutrient

(column 5).

Divide weight of each macronutrient (column 2) by

the foods total weight.

3. Calculate percentage kcal for each macronutrient

(column 6).

Divide kcal value of each macronutrient (column 4)

by foods total kcal value.

Learn to Read Food LabelsComputing the percentage weight and kcal of each

macronutrient in a food fosters wise decisions in choosing

foods. Manufacturers must state the absolute and percent-

age weights for each macronutrient, but computing their ab-

solute and percentage energy contributions completes the

more important picture. In the example for French fries, lipidrepresents only 17% of the foods total weight. However,

the percentage of total calories from lipid jumps to 48.3%,

or about 195 kcal of this foods 402 kcal energy content.

This information becomes crucial for those interested in

maintaining a low-fat diet.

Similar computations can estimate the caloric value of

any food serving. Of course, increasing or decreasing por-

tion sizes or adding lipid-rich sauces or creams, or using

fruits or calorie-free substitutes, affects the caloric content

accordingly.

MACRONUTRIENT ENERGY CONTENT AND PERCENTAGE COMPOSI-TION OF MCDONALDS FRENCH FRIES, LARGE (TOTAL WEIGHT,

122.3 G [4.3 OZ])

(1) (2) (3) (4) (5) (6)

ATWATER (g) % OF

NUTRIENT WEIGHT FACTOR KCAL WEIGHT % OF KCAL

Protein 6 4 kcal g1 24 4.9 6.0

Carbohydrate 45.9 4 kcal g1 183.6 37.5 45.7

Lipid 21.6 9 kcal g1 194.4 17.7 48.3

Ash 3.2 0 2.6 0

Water 45.6 0 37.3 0

Total 122.3 402 100 100


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Segal KR, Gutin B. Thermic effects of food and exercisein lean and obese women. Metabolism 1983;32:581.

Considerable research has linked obesity and impaired

thermogenesisa diminished capacity to increase metabo-lism in response to different stimuli. These studies note a

lower rise in metabolism for obese individuals than for lean

individuals after ingestion of a meal, exposure to cold, infu-

sion of noradrenaline, or the combination of eating and ex-

ercising. A diminished thermogenic response probably plays

an accessory role in total energy conservation, contributing

to the onset and persistence of human obesity.

The research of Segal and Gutin evaluated thermogenic

difference between obese and lean women in response to

food intake, two levels of exercise, and the possible poten-

tiation of the thermic effect of food with physical activity.

Subjects included 10 obese (% fat, 37; body mass, 77.9 kg)

and 10 lean (% fat, 18.8; body mass, 53.2 kg) women, mea-

sured under six different conditions: (a) resting metabolism(V

O2) for 4 hours; (b) V

O2 for 4 hours following consumption

of a 910-kcal meal (14% protein, 46% carbohydrate, 40%

lipid); (c) V

O2 during exercise at a constant submaximal in-

tensity of 300 kg-m min1 (cycling for 5 min every 0.5 h for

4 h); (d) V

O2 during exercise at an intensity equal to the sub-

jects lactate threshold (cycling for 5 min every 0.5 h for 4 h);

(e) and (f) same as protocols c and d, except the subjects

consumed the test meal before exercising.

The figure indicates that consumption of the 910-kcal

meal increased exercise V

O2 more for the lean than for the

obese women. Stated somewhat differently, a greater dif-

ference emerged between the fed and fasting conditions for

the lean group at both exercise intensities. The postprandialexercise V

O2 for the lean group also remained elevated

above the corresponding fasting value at the end of the 4

hours, while for the obese group, the postprandial value at 4

hours equaled their fasting exercise metabolism. Thus, us-

ing a 4-hour measurement underestimated the total amount

that eating augmented energy expenditure during exercise

for the lean women. These subjects exhibited a larger ther-

mic effect of food during exercise than during rest. Obese

subjects, on the other hand, showed similar thermic effects

of food for exercise and rest conditions, with no added ther-

mogenic bonus from exercise after eating.

The researchers concluded that exercise significantly

potentiated the thermic effect of food for lean but not for

obese women. The large differences in response to thecombination of food and subsequent exercise emerged de-

spite similar thermogenic responses of the lean and obese

women to food alone and exercise alone. Therefore, the cu-

mulative effect of a lower metabolic rate of the obese (com-

pared with lean subjects) during exercise that follows eating

favors energy conservation rather than energy dissipation.

Focus on Research

25

23

21

A

6

0

Time (min) Time (min)

30 240 30 240

Lean Obese

VO2(mL

kgLBM1

min1)

B

C

A

B

C

Effects of exercise and a 910-kcal

meal on metabolic rates of lean and

obese men and women. A, Exercise

at lactate threshold; B, exercise at300 kg-m min1; and C, rest. The red

circles represent postprandial (after

the meal) data; yellow circles repre-

sent postabsorptive (after fasting)

data. The shaded areas indicate the

thermic effect of food under each

condition.

Obesity-Related Thermogenic Response


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rate estimate of the net energy value of typical

foods a person consumes.

6. The Atwater calorific values allow one to compute

the caloric content of any meal from the carbohy-

drate, lipid, and protein compositions of the food.

7. Calories represent heat energy regardless of the

food source. From an energy standpoint, 500 kcal

of peppermint ice cream topped with whipped

cream and Brazil nuts is no more fattening than

500 kcal of watermelon, 500 kcal of cheese and

pepperoni pizza, or 500 kcal of an egg bagel with

salmon, onions, and sour cream.

Suggested Reading

Atwater WO, Woods CD. The chemical composition of American food ma-terials. USDA Bulletin no. 28. Washington, DC: USDA, 1896.

Boyle M. Personal Nutrition. 4th ed. Belmont, CA: Wadsworth Publishing,2001.

Brody T. Nutritional biochemistry. New York: Academic Press, 1998.Brown J. Nutrition now. Belmont, CA: Wadsworth Publishing, 1999.Brooks GA, et al. Exercise physiology: human bioenergetics and its appli-

cations. 3rd ed. Mountain View, CA: Mayfield, 2000.Gibson RS. Principles of nutritional assessment. New York: Oxford Univer-sity Press, 1990.

Groff JL, Gropper SS. Advanced nutrition and human metabolism. Bel-mont, CA: Wadsworth Publishing, 1999.

Guyton AC. Textbook of medical physiology. 10th ed. Philadelphia: WBSaunders, 2000.

Health and Welfare Canada. Nutrient value of some common foods. Ot-tawa, Canada: Health Services and Promotion Branch, Health and Wel-fare, 1988.

Katch FI. U.S. government raises serious questions about reliability of U.S.Department of Agricultures food composition tables. Int J Sports Nutr1995;5:62.

Mahan IK, Escott-Stump S. Krauses food, nutrition, & diet therapy.Philadelphia: WB Saunders, 2000.

McCance RA, Widdowson EM. The composition of foods. 5th ed. London:Royal Society of Chemistry. Ministry of Agriculture, Fisheries andFood, 1991.

Miles CW, et al. Effect of dietary fiber on the metabolizable energy of hu-man diets. J Nutr 1988;118:1075.

Pennington JAT, Church HN. Bowes and Churchs food values of portionscommonly used. 17th ed. Baltimore: Lippincott Williams & Wilkins,1989.

Rand WM, et al., eds. Food composition data: a users perspective. Tokyo:United Nations University, 1987.

Rumpler WV, et al. Energy value of moderate alcohol consumption by hu-mans. Am J Clin Nutr 1996;64:108.

Shils ME, et al. Modern nutrition in health and disease. 9th ed. Baltimore:Lippincott Williams & Wilkins, 1999.

US Department of Agriculture. Composition of foodsraw, processed, and

prepared. No. 8. Washington, DC: US Department of Agriculture,19631987.


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