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Melissa Godar

November 20, 2014

CPH 531: Contemporary Issues and Research

Term Paper

The influence of plate size on energy intake in adults – a systematic review

Abstract

Study Objective: Influences outside of individuals can affect eating behavior and energy intake. This

review was conducted to better understand what effect a specific environmental cue, plate size, may have

on energy intake.

Methods: A search of the PubMed database was conducted with criteria including: experimental study

where participants were assigned different plate sizes, the target population was adults 17-64, and the

outcome measure was energy intake/food consumed.

Results: This review contains a small number of studies, therefore findings are not definitive. Four of the

five studies reviewed showed no significant effect of plate size on energy intake.

Conclusion: More research is needed before recommending the use of smaller plate sizes. Further

research combining plate size with other environmental factors such as distractions or presenting smaller

plate sizes as a tool for restrained eaters should be considered.

Key Words: obesogenic environment, energy intake, food consumption, environmental cues, eating

behavior

Background

Obesity has become a global public health issue associated with a growing list of negative health

outcomes. Although the negative effects of and the contributing factors to obesity have been widely

researched, interventions that have been enacted have not made significant progress on the issue.1

According to a global analysis of the prevalence of overweight and obese people from 1980 to 2013 by

Ng et al. (2014), “not only is obesity increasing, but no national success stories have been reported in the

past 33 years.” In 2013, there were 2.1 billion people that fell into the categories of overweight or obese

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based on their BMI. In the United States, 70.9% of men and 61.9% of women were overweight or obese

in 2013. The U.S. has been a leader in the increase of obesity and “accounted for 13% of obese people

worldwide in 2013, with China and India jointly accounting for 15%.” The prevalence of diabetes,

osteoarthritis, cancers, and major vascular diseases are expected to increase with increased overweight

and obesity. The final recommendation of the analysis was to “help countries to more effectively

intervene against major determinants such as excessive caloric intake, physical inactivity, and active

promotion of food consumption by industry, all of which exacerbate an already problematic obesogenic

environment.”

An obesogenic environment contains influences that promote excess food consumption and

inadequate physical activity making it easier for individuals to gain weight and harder to lose weight.2

With the lack of success found in interventions focused on educating people to eat better and exercise

more, some researchers are looking for solutions outside of the individual for ways to make a larger and

longer-lasting impact.2 Environmental factors or cues can influence energy intake (amount of calories

eaten) in ways that are undetectable by individuals.2-4 Environmental factors (elements of food and eating

environments) including: lighting; music; distractions; accessibility, variety, abundance, and portion size

of food; package size; marketing; advertising; and other people have been investigated and found to

influence the food consumption of unknowing individuals.3,4 Wansink (2004) provides a list of

alterations that individuals could use to influence their own eating behavior to make reducing

consumption easier, but higher level approaches need to be considered in conjunction. Understanding

how environmental cues can influence energy intake could be an essential component of the population-

wide obesity intervention that has been called for by the authors of the 2014 analysis on the global

prevalence of overweight and obesity.1

Purpose of Review

This review was conducted to better understand how a specific environmental cue may influence

energy intake. To be consistent, energy intake will be used throughout this review with the thought that

increased food intake or consumption would result in increased energy intake. There may be great

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potential in the exploration of interventions that make healthy choices easier for individuals.

Manipulating elements of food and eating environments may be part of a large-scale solution to the

obesity epidemic. Before resources are invested in any intervention, research must show the intervention

to be effective at reducing morbidity and mortality.5 There has been some evidence that larger container

or package sizes could increase energy intake, but little research has been done to examine the effect of

plate size on energy intake.3 As many meals in the U.S. are eaten off of plates, there have been

recommendations to use smaller size plates in order to reduce portion size and food consumption, but it is

necessary to evaluate the existing evidence before such recommendations are spread.6,7

Methods

Articles included in this review were collected via searches of the PubMed database. The search

terms that resulted in 23 possible articles were: “(plate size) AND (meal intake OR calori intake OR

energy intake OR energy consumption OR calori consumption) AND ("last 10 years"[PDat] AND

Humans[Mesh])”. Articles found in this search were excluded if they did not meet the following criteria:

experimental study where participants were assigned different plate sizes, the target population was adults

17-64, and the outcome measure was energy intake/food consumed. After reviewing article titles and

abstracts, five articles were selected for inclusion in this review. The Sharp and Sobal (2012) study using

plate mapping was included because it met the first two criteria and presented an innovative study design.

One of the outcome measures was plate coverage and food size, which have been shown to influence

energy intake/food consumed.8

Description of Study Designs/Methods

Five studies were selected to be included in this review. Three studies used a cross-over

experimental design in a clinical setting in order to have participants serve as their own controls. Two

studies did not use a control group. For all studies, the independent variable relevant to this review was

plate size and the dependent variable relevant to this review was energy intake. Control variables will be

discussed within each study.

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Cross-over experimental studies

A study conducted by Rolls et al. (2007) sought to further understand how environmental

components contribute to increased energy intake. Until this study there has only been research that

shows container size could affect amount of food served, but not necessarily energy consumed. Three

single-blind cross-over experiments were done in a clinical setting. All of the experiments were

completed over a two or three week period with participants having a week washout period. Participants

were recruited, interviewed, and pre-tested to meet inclusion standards. Control variables for participant

requirements included: adult, non-dieter, not in athletic training, no food allergies or restrictions, regularly

ate three meals a day, non-smoker, not pregnant or breast-feeding, not taking medication that affects

appetite, not depressed, and BMI between 18 kg/m2 and 40 kg/m2. Pre-trial instructions included usual

food intake and physical activity the day before the study. A controlled breakfast was provided the

morning of the study and no other food was to be consumed before lunch. Participants in all studies were

asked to eat meals at the clinic, which consisted of individual dining cubicles. Different plate sizes (6.75

inches, 8.5 inches, and 10.25 inches) were randomly assigned to participants. Participants ate off of each

size plate used in their specific study. The brand and design of the plates were the same across sizes.

Each week each participant was given a different size plate while all other conditions including lunch

time, instructions, questionnaires, food type and amount, side dish, and beverage were the same

throughout the particular study. The first experiment provided each of the 45 participants with their own

dish of macaroni and cheese and allowed self-serving (used all three size plates). The second experiment

provided each of the 30 participants with a large, pre-served portion of macaroni and cheese on the plate

(used 8.5 inch and 10.25 inch plates). During this study, participants were given a larger spoon to eat

with when they received the larger plate. The third experiment allowed the 44 participants to self-serve

from a personal buffet of five foods (used all three size plates). In all three experiments, plate size did not

show a significant effect on energy intake (study one p = 0.29; study two p = 0.41; study three p = 0.61).

A single-blind pilot study conducted by Shah et al. (2011) tested whether plate size affected

energy intake in 10 normal and 10 overweight/obese women. Control variables for participant

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requirements included: female, adult, non-dieter, does not do vigorous physical activity, without an

eating disorder, non-smoker, not taking medication that affects appetite, and BMI less than 40 kg/m2.

Participants were asked to each lunch in the laboratory for two days with a mean 10 day washout period

between each meal. Before each study day, participants were instructed to eat normally and maintain

usual physical activity. Breakfast was not provided on study days, so participants were asked to eat the

same breakfast each time with no other food to be consumed before lunch. Participants were randomly

assigned which plate size (8.5 inches or 10.79 inches) they would receive first. All other conditions were

the same-identical pre- and post-questionnaires and instructions, individual dishes with the same amount

of spaghetti, self-service of food while eating alone at a private dining table, and requirement to drink all

of the water provided. Results showed no effect of plate size on energy intake for either weight status

group (p = 0.60 for plate size effect, and p = 0.55 for plate size by weight status effect).

Yip et al. (2013) tested whether plate size influenced energy intake in 20 overweight women

when hunger was increased in an unblinded study. Participants were recruited and pre-tested to meet

inclusion standards. Control variables for participant requirements included: female, adult, BMI between

25 kg/m2 and 40 kg/m2, non-dieter, non-smoker, not taking medication that affects appetite, no

cardiovascular disease, no hypertension, no diabetes mellitus, not depressed, not breast-feeding, and

willingness to eat study foods. Pre-trial instructions were to fast from 8pm the night before the study.

Lunch was eaten in a laboratory setting for two days with at least a three day washout period between

meals. A controlled low-energy breakfast was provided to participants on the day of the study to facilitate

a raised state of hunger at lunchtime. Participants ate lunch in individual dining rooms with a buffet of

food from which to serve themselves. The order of plate size (7.68 inches or 10.43 inches) given was

randomized while all other conditions remained the same including breakfast and lunch time,

questionnaires, food type and amount, food crockery, cutlery, no distractions allowed during lunch, and

requirement to remain at the lab between breakfast and lunch. No effect of plate size was found on

energy intake (p > 0.05).

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Quasi-experimental studies without the use of a control group

Koh and Pliner (2007) tested the effect of a combination of environmental cues on energy intake

in 57 females in a quasi-experimental single-blind study design. For this review, only the effect of plate

size on energy intake will be discussed. Recruited participants were asked to bring a female friend with

them to the study to eat either lunch or dinner on one occasion. Control variables for participant

requirements included: female and undergraduate student or friend of undergraduate student. Pre-trial

instructions were to refrain from eating for three hours prior to the study meal. Participants were assigned

to eat in pairs with either the friend they brought or with a stranger. Plate size was assigned to each pair

(either 7.17 inches or 9.25 inches) although whether or not assignment was random was not stated in the

article. Participants served themselves from either individual or shared (between the assigned pair) dishes

of pasta. Instructions, questionnaires, food type, food amount, meal preparation, and water provided were

controlled across all groups. No main effect of plate size on energy intake was found (no p-value

reported).

In an innovative quasi-experimental single-blind study design, Sharp and Sobal (2012) sought to

examine whether plate size had an effect on energy intake. The researchers developed a method called

plate mapping where participants were asked to accurately draw and label the type and amount of food

they would like to eat at a meal on a paper plate. Two different university courses in the same department

that met at the same time on different days were selected to be the experimental groups. One group was

given a small paper plate (9 inches) and the other was given a large paper plate (11 inches) on which to

draw. Control variables included: identical instructions, questionnaires, and materials provided; the study

took place during class time; and no examples, pictures, or food cues were presented. The dependent

variables relevant to this review were plate coverage and food area, which may be able to estimate energy

intake differences. Results from the 270 participants showed that plate size would have affected energy

intake if plate mapping is a valid method to predict real food consumption. Participants with the large

plates averaged 50% plate coverage with their drawings versus 62% for small plates, but the average total

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food area for the large plates was 26% bigger than the smaller plates (p < 0.001). In other words, the

large plates were less full, but contained more food than the small plates.

Summary of Major Findings

As mentioned in the introductions to many of the reviewed studies, previous findings about how

the size of containers other than plates (bowls, cups, packages, etc.) affects food portions/consumption

have been discrepant.6,7,9,10 This review contains a small number of studies, therefore findings are not

definitive. Four of the five studies reviewed showed no significant effect of plate size on energy

intake.6,7,9,10 One study, which did show an effect of plate size on potential energy intake, used the most

different study design, so it is not directly comparable to the other four studies.8 Plate size may have an

effect on energy intake when combined with other environmental cues such as distraction.10 The studies

that asked individuals to eat alone in a no- or low-distraction environment showed that plate size does not

affect energy intake.6,7,9 The study that tested a combination of environmental cues, which may be more

related to natural settings in which people eat, did show an effect of plate size on energy intake under

certain combinations of cues.10 These studies were conducted with people who were not dieting and

without an increased awareness of food intake; researchers hypothesized that plate size may have an

effect on those who are dieting or more conscious of food intake, but research needs to be conducted.6,9

Strengths and Limitations of Studies

Eating behaviors can be automatic and undetectably influenced by environmental factors.3 A

variety of environmental cues can increase food consumption and therefore energy intake.3,4 The

possibility of confounding by a number of other variables is high, which stresses the importance of using

rigorous methods in order to validly determine the effect of specific cues or combinations of cues on

energy intake. As with any study, there are strengths and limitations that should be considered when

interpreting the findings. Generalized strengths and limitations of the five reviewed studies are discussed

in this section.

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Strengths

While the studies varied, a common strength was relatively high internal validity. Recruitment

criteria were used in to increase internal validity by excluding individuals with factors that may have

influenced food intake such as dieting, athletic training, smoking, and taking medications that affect

appetite.6,7,9 All of the reviewed studies used consistent processes, instructions, food type, food amount,

and materials within and across experimental groups outside of the independent variable.6-10 Four of the

studies were clinical trials, which allowed researchers to control for a number of possible confounding

environmental factors and to have confidence in the effect of the independent variable on the

outcome.6,7,9,10 For the three cross-over experiments, participants were used as their own controls making

all demographic factors the same and removing any bias due to those factors.6,7,9 The cross-over studies

included washout periods, which reduced the chance of participants becoming aware of the plate size

difference and consequently changing their behavior as well as controlled for satiation of the same food,

which would have confounded the results.6,7,9 In the studies where actual food was provided, the amount

of food provided was in excess of what the participants were expected to eat, which allowed participants

to consume as much as they wanted.6,7,9,10 Limiting the available amount of food could have impacted

potential energy intake and served as a confounder. The plate mapping study also did not limit the food

choice or amount that participants could draw, which could provide more accurate representations of

individual energy intake.8 Three studies randomized which plate size participants received first, which

increases internal validity by allowing researchers to compare the plate size order between groups and

examine a possible confounding effect.6,7,9 A controlled breakfast was provided on the study days in two

studies, which helped rule out the possibility that differences in energy intake at lunch were affected by

breakfast size differences rather than or in addition to plate size differences.6,9

While external validity was not a strength of any of the five studies, there were some efforts to

increase the generalizability of the results. All but one study blinded participants to the study objective in

order to increase external validity by protecting against the Social Desirability effect where participants

may try to please the experimenter at the expense of authentic responses.6-8,10 Two experiments combined

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previously studied cues that have shown an effect on energy intake such as effort/proximity of food,

variety of food, social distractions, and portion size to test causal effects of plate size across other major

environmental cues, which is more comparable to real-life eating situations and increases external

validity.6,10 Measurements in all studies were standardized and objective, which increases the reliability

of the results. The large sample sizes in three studies increases confidence in the results by increasing the

power of the statistical tests used.6,8,10

Limitations

In general, the reviewed studies had several threats to the generalizability of results, which

lowered their external validity. The Hawthorne effect could be applicable to all studies. In the four

studies with actual food, participants ate a meal in a clinical setting and were given questionnaires about

their hunger and about food before, during, and after eating.6,7,9,10 These processes made it clear that

participants were taking part in a study related to food. The awareness of being in a study could have

caused participants to change their eating behavior, which limits the generalizability of the experiments’

results to people outside of a study context. Participants ate off the different plate sizes on only one

occasion.6-10 Effects of plate size may be different in a longer term trial or results may be confirmed as

the participants eating behavior may become more natural over time reducing the Hawthorne effect.

Dining in a lab setting is unnatural for most people and could have made it difficult for participants to

maintain their normal eating behavior.6,7,9,10 More research would be required to determine to what extent

eating behaviors recorded in an unnatural setting are generalizable to a natural setting since natural

settings are more complex with a number of factors influencing behavior. Common in all studies was the

context of food typically eaten as a meal versus snack food.6-10 As several of the researchers suggested,

eating norms for meals may be more fixed than those for snacks, which could have influenced the results

and limits the applicability to other types of food.8,10 Several studies measured or asked about

participants’ height and weight before the study possibly making participants self-conscious about how

much food they consumed; this could have changed their energy intake making the results less valid due

to the additional influence on participant behavior.6,7,9

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Measured demographics of the samples such as age, gender, and weight status were not

representative of the general adult population for the westernized countries in which the experiments were

conducted.6-10 Gender has been shown to have an effect on energy intake, so the results from the studies

restricted to female participants may not be applicable to men.6,7,9,10 Study participants were not dieting

or with an increased awareness of food intake; researchers hypothesized that plate size may have an effect

on those who are dieting or more conscious of food intake, so research needs to be conducted in order to

apply the results to that population.6,9

Ethical and Social Justice Issues

As should be the goal of all public health research and recommendations, this review seeks to

promote the health of all people by ensuring the health information that is spread to the public is backed

by research. Sometimes public health practices are based on assumptions rather than data, which is why a

framework has been proposed to raise the ethical awareness of public health practice.5 It is important to

consider the potential for public health practices to either promote or hinder social justice in order to

weigh the burdens before making decisions. Issues raised by this review are discussed in this section.

While the increase in plate sizes have shown temporal association with the growing obesity

epidemic, there has yet been substantial evidence to show that decreasing plate size will reduce energy

intake as discussed in this review.6 High level public health institutions such as the National Institutes of

Health have promoted using smaller plates to reduce portions and therefore energy intake.6 This is

problematic and presents a major ethical concern because the public’s trust should be respected by only

providing proven recommendations.5,11 Besides possibly losing individuals’ trust, recommending smaller

plates sizes without evidence for the stated results could be burdensome. An economic burden for

companies to produce more plates and for individuals to purchase smaller plates would not be warranted.

Should the public get rid of larger plates in exchange for new, smaller ones, it would be wasteful and

produce an unnecessary environmental burden. It is not uncommon for companies to suggest that certain

products are better than others in order to make money with no evidence to support their claims. A

possible, unintended consequence of trusted, high-level public health organizations producing

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recommendations without evidence is that companies can use the information to back up their claims and

make money off of the public’s ignorance. Consumers may expect to lose weight or easily change their

eating behavior by using smaller plates, which may not happen and cause undue stress.

Should further research demonstrate the effectiveness of using smaller plates, there are still social

justice and ethical issues to be raised. Obesity disproportionately affects low-income people who may not

be able to purchase new plates or who would suffer a higher economic burden to do so.12 Public health

should consider underserved populations and promote fairness. The reviewed studies either did not

provide information on the diversity of participants or explicitly stated that future studies should take care

to include more diverse populations as other characteristics of populations may cause the results to

differ.6-10 If restaurants or other food serving organizations were to use smaller plate sizes with the

intention to manipulate the eating behavior of consumers, an ethical issue of limiting the liberty and self-

determination of individuals must be considered. A consumer’s right to choice is still present though as

individuals can choose to purchase additional food. However there still may be a risk of social burden as

Koh and Pliner (2009) investigated that there may be social stigma or embarrassment associated with

taking second helpings. Consequences of public health practices can be far reaching and need to be

carefully weighed before being implemented.

Recommendations for Public Health Practice

The results of the existing studies examining the effect of plate size on energy intake do not

warrant a recommendation to use a smaller plate. More evidence is needed and suggestions for future

research can be found in the next section. Two of the researchers hypothesized that eating alone may

have made participants more aware of their food intake allowing them to pay attention and stop eating

when they were no longer hungry.7,10 A recommendation for dining facilities as well as individuals could

be to limit the number of distractions in the eating environment in order for the focus of mealtime to be on

the food. Wansink (2004) reviewed the literature from several disciplines to compile a wide number of

environmental factors that have been shown to influence food intake. Public health practice should take

an interdisciplinary approach to help reduce energy intake as there are many factors that influence a

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person’s eating behavior.2-4 Redesigning eating environments to promote healthy consumption norms

could help individuals reduce energy intake with less effort.3

Some environmental cues have been proven effective at reducing energy intake, one being

portion size. Rolls et al. (2007) recommends using strategies to reduce portion size in efforts to reduce

energy intake. Sharp and Sobal (2012) suggest health education about portion control and potentially

using a smaller plate to help guide appropriate portion size. Yip et al. (2013) proposes yet another step

further to develop portion control plates with clear visual cues for the appropriate amounts of different

types of food. With the enormity of the obesity crisis, public health should focus on including these

recommendations into a large scale intervention that has the potential to shift norms, which would provide

more sustainable and far-reaching health improvements.

Suggestions for Future Research

The reviewed studies presented several limitations, which could be considered areas for further

study on this topic. While controlled, clinical research is a high-quality approach to determining causal

relationships, it is an unnatural setting that may not be able to replicate the complexity of the

environments in which people eat. Future research should examine a range of environmental cues in

more likely eating environments such as a restaurants, workplace lunchrooms, or cafeterias.7 All of the

studies presented food in the context of a meal and, for the most part, showed no significant effect of plate

size on energy intake, which was unexpected due to past studies that have demonstrated that container

size did influence energy intake with snack foods.6-10 Further investigations should vary the types of

foods eaten and the context of eating being a meal or a snack.6,7,10 Sharp and Sobal (2012) found that

women with smaller plates drew less vegetables on their plates. If plate size is determined to be effective

at reducing food consumption, research on the composition of food on smaller plates should be included

to see if energy intake is reduced at the expense of nutritious food.8 Koh and Pliner (2009) loosely

collected information on taking second helpings. Although their data is not highly credible, they found

that second helpings did not determine differences in food intake and suggested that those who wanted to

avoid taking a second helping put more food on their plate to start. Future research on whether taking

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second helpings influences food intake and what social norms affect an individual taking a second helping

is a suggested area of investigation.8,10 Other research about which social norms are associated with food

intake would contribute valuable information to a higher level, more wide-spread approach to reducing

energy intake.10

Additional limitations to the reviewed studies include the short-term nature of the studies and the

lack of diversity in the populations studied. Longer term studies should investigate if a smaller plate size

reduces energy intake when used for a substantial period of time.10 Restrained eaters (people trying to

restrict intake) were not included in the studies presented, but Sharp and Sobal (2012) hypothesize that

plate size may be a helpful cue for that population to reduce energy intake. In future investigations,

diverse populations should purposefully be included from a social justice standpoint as well as to ensure

the applicability of any results as there may be unique characteristics of different groups that would

necessitate tailored recommendations.6,7

In conclusion, this review did not find significant results to support the use of smaller plate sizes

to reduce energy intake. There is research demonstrating the influence that certain environmental cues

have on eating behavior. A promising focus may be to control portion size in order to reduce energy

intake and help ignite a decline in obesity rates. More research needs to be done to support a large-scale

intervention.

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References

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obesity in children and adults during 1980-2013: A systematic analysis for the global burden of disease

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2. Lake A, Townshend T. Obesogenic environments: Exploring the built and food environments. J R Soc

Promot Health. 2006;126(6):262-267.

3. Wansink B. Environmental factors that increase the food intake and consumption volume of

unknowing consumers. Annu Rev Nutr. 2004;24:455-479.

4. Cohen DA. Obesity and the built environment: Changes in environmental cues cause energy

imbalances. Int J Obes (Lond). 2008;32 Suppl 7:S137-42.

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meals. Appetite. 2007;49(3):652-660.

7. Shah M, Schroeder R, Winn W, Adams-Huet B. A pilot study to investigate the effect of plate size on

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615.

8. Sharp D, Sobal J. Using plate mapping to examine sensitivity to plate size in food portions and meal

composition among college students. Appetite. 2012;59(3):639-645.

9. Yip W, Wiessing KR, Budgett S, Poppitt SD. Using a smaller dining plate does not suppress food

intake from a buffet lunch meal in overweight, unrestrained women. Appetite. 2013;69:102-107.

10. Koh J, Pliner P. The effects of degree of acquaintance, plate size, and sharing on food intake.

Appetite. 2009;52(3):595-602.

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12. Blumenthal S. Poverty and obesity: Breaking the link. The Huffington Post Web site.

http://www.huffingtonpost.com/susan-blumenthal/poverty-obesity_b_1417417.html. Published 4/11/12.

Updated 2012. Accessed 11/20, 2014.