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Obesity Prevalence Web Resources

Centers for Disease Control Behavioral Risk factor Surveillance Survey
http://www.cdc.gov/brfss/

American Obesity Association
http://www.obesity.org/subs/fastfacts/obesity_US.shtml

National Health And Nutrition Examination Survey Data
http://www.cdc.gov/nchs/products/pubs/pubd/hestats/overwght99.htm

World Health Organization
http://www.who.int/nut/db_bmi.htm

HOW DID OUR POPULATION BECOME OBESE?

How did the obesity epidemic arise? The available data from prospective cohort studies suggests that it has arisen gradually from a small, consistent weight gain, likely produced by a small, consistent degree of positive energy balance. For example, body weight data from the Coronary Artery Disease in Young Adults (CARDIA) study suggest that the average adult has gained .5-2 kg/year for the past 15 years (8). CARDIA is an observational study in which young adult Caucasian and African-African American men and women were followed over time to identify predictors of heart disease. The greatest weight gain over time was seen in African-American women and the lowest in Caucasian women. Only about 25% of participants avoided gaining weight over the past 15 years. The gradual weight gain seen in this study could have resulted from a consistent, sustained, positive energy balance of less than 50 kcal/day. The epidemic of obesity appears to have arisen because much of the population has been in slight positive energy balance over the past 10-15 years.

CARDIA Website
http://www.dopm.uab.edu/cardia/

We do not fully understand the reasons for the gradual weight gain in the population but researchers are increasingly focusing on an environment that promotes increased energy intake and decreased physical activity. The purpose of this chapter is to consider how the current environment may have contributed to development of the obesity epidemic and to consider strategies for dealing with the environment to reduce the prevalence of obesity. We will begin by briefly discussing the nature of body weight regulation and consider how different environments could differentially impact this regulation. In particular, we will discuss why our physiology is permissive to the gradual weight gain produced by the current environment.

THE NATURE OF BODY WEIGHT REGULATION

Clearly, the etiology of obesity is very complex, involving genes and the environment, food intake and energy expenditure, and individual behavior and societal factors. Figure 1 shows a simple cartoon illustrating the relationships among energy intake, energy expenditure and body mass.

Figure 1.

There is general agreement that the body does have some ability to alter energy intake in response to perturbations in energy expenditure and vice versa in order to defend the current state of energy balance and body weight (9). However, the strength of the regulation of energy balance appears to be weak, so that body weight declines with sustained underfeeding and increases with sustained energy balance. Further, the system may be biased toward a more vigorous defense against negative energy balance than against positive energy balance (9-11).
Interestingly, as weight gain and increases in body fatness occur, these serve to limit further weight gain as shown in Figure 2. As body weight increases, this produces an increase in total energy expenditure because increased body weight produces an increase in resting metabolic rate and in the energy cost of movements. Similarly, there is evidence that increasing body weight and body fatness also produces increased levels of circulating factors that may serve to reduce energy intake. In essence, becoming obese allows reachievement of energy balance - but at a higher body weight. Seen in this light, development of obesity could be considered, not as defective physiology, but as a natural response to our current environment.

Figure 2.

THE ROLE OF GENES

Genes play a very important role in determining whether or not obesity will develop. This topic is discussed in more detail in Chapter 7 (12). While the environment is driving weight gain, differences in genotype will determine, for example, to what extent body weight gain is seen within a given environment. In figure 1, for example, genes affect each component of energy balance and can help explain why different individuals maintain different levels of body weight within a given environment. There are likely genes that protect against weight gain, and the 39% of the U.S. population not overweight or obese may include some individuals genetically resistant to weight gain. It likely also includes a number of individuals actively maintaining a behavior pattern consistent with weight maintenance.

WHY DOES OUR PHYSIOLOGY PERMIT GRADUAL WEIGHT GAIN

While developing a physiological defense against weight loss would have been beneficial for early man, there was no logical reason for us to develop a strong physiological defense against weight gain. Most of the environments in which man has lived can be characterized by an unreliable food supply and a high need for physical activity to survive (13). In these environments, our weak physiological regulation of energy balance has been effective. There is no evidence that we have ever before experienced a sustained period of population weight gain. We must assume that the gradual weight gain currently seen in many populations around the world is a relatively new phenomenon and likely attributable to a changing environment that provides a constant food supply and has reduced the need for physical activity to survive.

HOW CAN DIFFERENT ENVIRONMENTS DIFFERENTIALLY AFFECT ENERGY BALANCE?

Why is our physiological control system sufficient to maintain a healthy body weight in one environment, yet not in another? Figure 3 illustrates how the environment may impact the nature of body weight regulation. In previous environments, high levels of physical activity were required for survival. In early man, this was physical activity required for hunting and gathering. In later generations, it was the need for substantial physical activity in most occupations and in daily living. Our challenge has traditionally been getting sufficient food to match high energy requirements. Given that our early environments consistently required high levels of physical activity, it is not surprising that we developed strong physiological mechanisms to promote food intake with not much need to develop physiological mechanisms to promote food restriction or increased physical activity. In early environments, achieving energy balance was "driven" by high levels of physical activity, creating a high rate of energy expenditure. Food intake was the "follower" and it made sense to eat when food was available. We developed a number of redundant physiological systems for food intake.

Figure 3.

Further, since the food supply was inconsistent, errors in food intake were probably equally distributed above and below energy requirements. Small deviations could be countered by our physiological regulatory system. Positive energy balance and weight gain almost certainly occurred in some people, but obesity was not an issue for the vast majority of the population. In such environments, achieving a healthy body weight was an unconscious, instinctual process.
The current environment in the U.S. (and many other countries) is very different with little or no need for high levels of physical activity for survival and a constant supply of convenient, inexpensive, palatable food (9). Low levels of physical activity lead to low levels of total energy expenditure and low energy requirements. To achieve energy balance under these conditions it is necessary to constantly restrict energy intake to match the low levels of energy expenditure. It is extremely difficult for most people to consistently restrict energy intake given the constant availability of good-tasting, low cost food and a physiology that promotes eating when food is available. In fact, because energy expenditure is so low in most people, errors in energy intake are not normally distributed and most errors lead to positive rather than negative energy balance. Our physiological control system is not sufficiently strong to oppose constant unidirectional errors in energy balance. There does not appear to be a strong drive to increase physical activity in response to excess energy intake and there appears to be only a weak adaptive increase in resting energy expenditure in response to excess energy intake. Under these conditions, gradual weight gain occurs until adaptive changes described above reestablish energy balance.
An alternative to restricting energy intake is to increase voluntary physical activity to make up for the decline in physical activity required for survival and productivity. It appears that few people are able to do this. Successful body weight management requires some combination of food restriction and increases in voluntary physical activity. In other words, most people can no longer achieve a healthy body weight without conscious effort. In today's environment, cognitive management is required to maintain a healthy body weight.

WHAT FACTORS IN THE ENVIRONMENT PROMOTE OBESITY?
In order to understand how the environment promotes obesity, we must understand which behaviors are promoting obesity and how the environment encourages those behaviors.

FACTORS THAT INCREASE ENERGY INTAKE

Much attention has been focused on how the environment increases energy intake. Many factors the environment have been suggested to affect total energy intake, and some of these are listed in Table 1. There is surprisingly little research evaluating the impact of these factors on energy intake over the long-term.

Portion Size

There is clear evidence that portion sizes have increased over the past 15 years. This includes food served at restaurants, soft drinks and many kinds of snack foods. Rolls and colleagues have shown that within a single meal, total energy intake increases with increased portion size (14). It seems that the more food on the plate, the more we eat. While it seems reasonable to conclude that large portion sizes contribute to higher overall energy intake over the long-term, this has not been studied.

High Fat/High Energy Density Diets

The dietary fat content of the U.S. diet, estimated from dietary records, has been between 35-40% of total energy over the past few years (15), while recommendations have been to eat <30% of total calories from fat (16). High fat diets reliably increase total energy intake over periods of several days (17,18). Despite strong evidence that high fat diets lead to increased energy intake, there are vigorous efforts to convince the public that restricting dietary fat is not an effective strategy to prevent obesity (19).

The reason why high fat diets lead to increased energy intake may be because of their high energy density (energy per gram of food)(20). When energy density is held constant, dietary fat content does not seem to affect energy intake, at least over the course of a few days (21). Similarly, over the short-term, total energy intake varies directly with the energy density of the diet (20,21). It has been suggested that this is because we seem to eat a constant amount of food and when this food is high in energy density, we ingest more energy than when it is low is energy density (20). However, since the primary determinant of the energy density of the diet is the non-caloric content (i.e. water, fiber, etc.), it is not completely clear that energy density explains the entire reason why high fat diets are associated with obesity (22).

Glycemic Index

Eating a diet with a high glycemic index (GI) has been suggested to increase total energy intake and to promoting obesity. Glycemic index is defined as the area under the curve of the blood glucose excursion following ingestion of 50 g of carbohydrate from a given food. This is usually expressed relative to the glycemic curve obtained for glucose or white bread (set at 100). The mechanism whereby foods with a high GI lead to obesity is based on the idea that these foods lead to a greater increase and then decrease in blood glucose, leading to a relative hypoglycemia and overall increases in food intake (23).

Opponents of the GI hypothesis raise several questions about the GI as a contributing factor in the etiology for obesity. First, some studies are not able to replicate differences in GI when foods are given as mixed meals rather than a single food (24). Second, the GI of many foods has been determined in individuals with type 2 diabetes (25). Subjects with type 2 diabetes have insulin resistance and defects in insulin secretion that can be variable over time, and this leads to great variation in response to a test meal and limited generizability to other populations (24). The GI of a given food also varies with processing, method of storage, and method of cooking (23). As a practical issue, it is difficult to determine the GI of new foods or foods eaten in away from home (26). Finally, it is not clear that there is a consistent relationship between glycemic index and energy intake.

Glycemic Index Web Resources

University of Sydney
http://www.glycemicindex.com/

GI Symbol
http://www.gisymbol.com.au/pages/index.asp

Other Environmental Factors Affecting Energy Intake

FACTORS THAT DECREASE PHYSICAL ACTIVITY

There are substantial data to suggest that low levels of physical activity are associated with weight gain (9,29). It has been suggested that the environment has facilitated declines in physical activity. Table 2 shows some of the environmental factors thought to contribute to reduced physical activity. Because we have poor documentation of the amount of physical activity required for work and in daily living, there are not definitive data that allow us to quantify changes in physical activity over the past two decades. There is, however, general agreement that it has declined. Technological advances have reduced the need for physical activity in most occupations, so that little physical activity is accumulated through work. A similar situation exists in schools. Most children used to have required physical education in school that provided substantial physical activity. Few schools now require physical education (30).

While Americans have more leisure time than ever, it does not seem that they are using it for physical activity. This may not be surprising given the competition for our leisure time from attractive sedentary activities. We now have hundreds of channels on television, videos, DVDs, and the internet. Americans clearly do not spend much of their leisure time in physical activity. Less than 30% of adults report regularly engaging in physical activity during their leisure time. However, the number of adults engaging in regular physical activity during their leisure time does seem to have declined substantially over the past two decades (31).

In general, advances in technology and transportation have made physical activity unnecessary for most of our daily functions (9). Automobiles, elevators, and escalators are abundant and serve to reduce the need for walking. Cell phones, email and remote controls all reduce the need for physical activity. There is strong speculation that declines in the energy required for daily living have contributed to reduced total energy expenditure and weight gain.

STRATEGIES FOR DEALING WITH THE OBESITY EPIDEMIC

If we accept that the environment is promoting weight gain and that our physiology is permissive to this weight gain, how can we address the obesity epidemic?

Our first aim must be to prevent further weight gain. Gradual increases in body weight have created and continue to fuel the obesity epidemic. Prevention of weight gain can stop the trend toward increased prevalence of obesity and can likely prevent development of chronic diseases such as type 2 diabetes in those already overweight or obese.
We also must not forget that over 26% of the population needs to lose some weight in order to reduce their risk of other chronic diseases. However, the challenge here is not producing weight loss; we have many strategies for doing that effectively. Rather the challenge again is preventing weight gain - in this case preventing regain of body weight. It is possible that effective strategies for weight regain would differ from effective strategies for primary prevention of weight gain, but this is unlikely. It is more likely that greater behavior changes will be required for prevention of weight regain vs primary prevention of weight gain.

In the remainder of this chapter, we will discuss strategies for dealing with the environment to prevent population weight gain/regain. If factors within the current environment are promoting weight gain, then we need to consider how to modify these factors to prevent weight gain/regain. We also need to consider strategies for better managing body weight within the current environment. It is likely that it will take some time to modify the environment and given the rapidity of the increase in obesity, we must look for short-term strategies to manage weight better within the current environment.

MODIFYING THE ENVIRONMENT TO PREVENT WEIGHT GAIN?

In this section, we will consider the evidence in support of various strategies to change the environment.

MODIFYING DIET TO PREVENT WEIGHT GAIN

We have few if any success models for changing the environment to change eating behavior. The available data have focused on understanding which aspects of our diet affect obesity and there is virtually no research on how the environment affects energy intake. The most data involve assessing the effects of low fat and low glycemic index diets on energy intake, and in both cases, results are controversial. A low fat diet appears to reduce energy intake and prevent weight gain/regain (17-19), but it appears difficult for people to adhere to such a diet for long periods of time. There are some data regarding the impact of low glycemic index diets (32) and high protein diets (33) on energy intake, but results are inconclusive.

There is a surprising lack of research about how to change the environment to reduce total energy intake. While suggestions have been made to change policies to ban soft drinks, tax undesirable foods, remove competitive foods from schools, regulate advertising for undesirable foods, and reduce sugar consumption, there are no studies to suggest that these strategies would in fact reduce total energy intake.

Web Based Debate on the Health Implications of Taxing High-fat Food
http://bmj.com/cgi/content/full/320/7230/301

(Marshall T. Exploring a fiscal food policy: the case of diet and ischaemic heart disease. BMJ 2000; 320: 301-304)

MODIFYING PHYSICAL ACTIVITY TO PREVENT WEIGHT GAIN

We can next consider research showing that increasing physical activity would prevent weight gain and that the environment can be modified to increase physical activity. While we do not have definitive randomized trials assessing the impact of increasing physical activity on prevention of weight gain/regain, we do have substantial other research to suggest that this would be an effective weight gain prevention strategy.

It seems that 60-90 minutes/day of moderate intensity physical activity is effective in preventing weight regain in reduced obese individuals (34-36). This might seem like an extraordinarily high amount of physical activity given that so few adults engage in regular physical activity.
It may not be surprising that it takes considerable amounts of physical activity to prevent weight regain, since reduced obese individuals have experienced substantial reductions in energy requirements. An important question is whether it takes a similar amount of physical activity for primary prevention of weight gain.

In the general population, there is an inverse relationship between physical activity and weight gain. Information from several prospective cohort studies shows clear negative associations between amount of physical activity and likelihood of weight gain over time (29). Similarly, studies in rodents (37) and human subjects (38) have shown that increasing physical activity can prevent some or all of the weight gain associated with eating a high fat diet.

We do not have randomized intervention trials showing that increased physical activity can prevent weight gain, but the available data suggest that this should be the case. We also do know the minimal amount of physical activity required for primary prevention of weight gain. Theoretically, if the weight gain we are trying to prevent is on the order of .5-2 kg/yr it should take far less than 60-90 min/day of moderate intensity physical activity.

As with diet, we have few if any success models for how to modify the environment to increase physical activity. Most previous research has address how to change individual physical activity behavior. Research in this area is beginning, as researchers address, for example, how the built environment (eg. sidewalks, mixed use neighborhoods) affect physical activity.

CHOOSING A STRATEGY TO CHANGE THE ENVIRONMENT

Ideally, development of strategies for changing the environment to prevent weight gain/regain should proceed in a logical order. First, we need research showing which behaviors are causing weight gain and how the environment encourages these behaviors. This would be followed by using this information to develop testable hypotheses about how to change the environment in order to change behavior. These hypotheses would be tested in small-scale interventions in defined populations. The most promising interventions would be tested in more large-scale studies, and eventually could lead to public health campaigns. Why then are many obesity experts calling for public health campaigns and policy changes to modify the environment given that we know so little about how to do this? This is because the prevalence of obesity is increasing so rapidly around the world that we must do something now, or we may lose the ability to prevent obesity. Many people believe we must act now, even though we do not have a thorough understanding of how the environment is causing obesity and even though we have few if any success models for changing the environment.

Interestingly most of the suggestions for policy approaches to obesity involve prohibitions or restrictions on "undesirable" eating behavior. Some of these solutions include taxes on "unhealthy foods", restrictions on certain foods in vending machines or in schools, and restrictions on food advertising (27,28). These strategies are being proposed on a widescale basis, even though we have no small-scale successes with them and no data to suggest they would be effective in reducing energy intake. Few policy suggestions have been based on providing incentives for "desirable" behavior or on dealing with physical activity.

BEGIN BY INCREASING PHYSICAL ACTIVITY

While most of the work in modifying the environment is focused on modifying the environment to alter food intake, it might make more sense to begin by altering the environment to affect physical activity. There are several reasons for this. First, as discussed earlier, we have traditionally achieved energy balance by matching food intake to a high level of physical activity. Given the low level of physical activity seen for most people, it seems unlikely that that we can modify the environment enough to allow these people to consistently restrict food intake to match a low energy expenditure. Second, there is no consensus about the goals for environmental changes to affect energy intake. Should we aim to modify diet composition, portion size or other variables? There is much more agreement that achieving increases in physical activity would be one effective strategy for preventing weight gain. While there is no convincing data that we can modify the environment to positively affect either energy intake or physical activity, there are some promising data to suggest that increasing lifestyle physical activity (i.e. walking) may be achievable (39).
It seems infeasible, both practically and politically, to attempt to engineer a significant amount of physical activity into modern sedentary occupations, however, increasing lifestyle activity at home, at school, in the workplace and in the community would seem within reach. This can be done through a variety of means starting with building fundamental awareness and knowledge of the value and accessibility of lifestyle activity (e.g., walking) and disseminating many existing programs that have demonstrated effectiveness in increasing physical activity among children and adults alike (40-42).

The current recommendations from the U.S. Surgeon General is for adults to accumulate 30 minutes of moderate intensity physical activity on all or most days (30). If this is accumulated through lifestyle physical activity, one problem is the difficulty for most people in knowing their success in achieving their physical activity target. The new generation of electronic step counters (43) provide an inexpensive and easy way to monitor lifestyle physical activity (ie. walking). Having step counters allows people to set and monitor individual physical activity goals.
U.S. Surgeon General's Reports

The Surgeon General's Call To Action To Prevent and Decrease Overweight and Obesity
http://www.surgeongeneral.gov/topics/obesity/

NLM's Health Services/Technology Assessment Text (HSTAT)
http://hstat.nlm.nih.gov/hq/Hquest/db/local.sgen.sgact.ctaobese/screen/TocDisplay/action/Toc

Office of the Surgeon General
http://www.surgeongeneral.gov/topics/obesity/calltoaction/toc.htm

Healthy People 2010: Physical Activity and Fitness
http://www.health.gov/healthypeople/Document/HTML/Volume2/22Physical.htm

Step Counters Websites

http://www.walk4life.com/pedometers.htm

http://www.store.yahoo.com/n-e-wlifestyles/

FOLLOW WITH FOOD INTAKE

Increasing physical activity alone can have a meaningful impact on body weight control, but changes in eating behavior and the food environment are also needed. Many good suggestions have already been made in the scientific and popular press, including broadening nutrition labeling to include providing calorie and nutrient information on restaurant foods. This is especially important as more and more of today's food dollar is spent in restaurants and typical portion sizes have grown dramatically in the past decade. Most people probably have no idea how much they are eating and as indicated earlier, there are only very weak biological mechanisms to protect against "overeating". Energy and nutrient information should also be provided in cafeterias in schools and workplaces so that wherever people can get food, the information they need is there to make an informed choice.

Beyond labeling, there are many other areas to explore on the energy intake front, but none of them are easy. We have yet to succeed at creating increased consumer demand for consumption of more fruits and vegetables and foods lower in fat and calorie content. Furthermore, beyond the goal to increase fruit and vegetable consumption the scientific and medical communities rarely agree on the specifics of what else people should eat. Such controversy provides consumers with a convenient excuse to not change anything. In the end, whatever changes we can agree to pursue, they will have to make economic sense throughout the food value chain from producer to consumer.

INSTITUTIONALIZING CHANGE

Sustaining environmental and social changes in the long-term generally requires some level of institutionalization of systems that support the desired changes. So, for example, if it is desirable that communities have sidewalks and crosswalks to promote safe walking it should institutionalize mechanisms that ensure that these amenities are included in any new construction or renovation. This could, for example, lead to incentives such as tax breaks for businesses that meet these community standards. Similar principles could be applied in many other sectors that control some aspect of the physical or social environment to insure that the institutional behavior at a minimum does not preclude, and ideally, would support healthy lifestyles.

Environmental Impact Assessment
http://ceq.eh.doe.gov/nepa/nepanet.htm

Such efforts can be sustained only through continuous funding and sustainable political will. In the short term, because of the scale of the problem we face, funding will need to come from nearly every sector of society, public and private. We need to begin now to better understand the economics of physical activity and eating behaviors in order to look for incentives that can support change throughout the value chain. In other social change movements, like recycling, inventing end uses for recycled plastics (e.g., micro-fiber clothing) provided the necessary economic "pull" to support other changes that made it easier for consumers to recycle (e.g., making curbside recycling feasible) (44). Sustaining the long-term political will (i.e., public support) for healthier lifestyles needs to begin with our children. They have played a key role in previous social change movements like recycling-they learned about why recycling was critical to preserving and sustaining our planet and they took the recycling mentality home with them and reminded their parents. We need to teach our children at an early age about why physical activity and healthy eating are so important. We need to provide them with the knowledge and the skills to manage energy balance in the modern environment. If we accept that successful management of body weight within current environment requires cognitive skills, we need to ensure that our children have these skills (45). Just we teach our children cognitive skills to balance their checkbooks, we need to teach them cognitive skills to achieve energy balance. This would include developing basic understanding about the energy value of food and physical activity and how these work together to determine energy balance. This can be done in a way that doesn't need to focus on body weight, per se, so that it need not contribute to eating disorders or other issues that could develop from an overemphasis on body weight or body image. We also need to make sure their physical activity and eating environment, in school at a minimum, supports the knowledge and skill development. In other words, we need to make sure they can actually practice healthy eating and active living behaviors while at school. This principle should apply to both the physical environment as well as the social environment and the teaching curriculum. If we do this now, we can insure that our children will grow up equipped with the minimum capabilities needed to manage body weight in the modern world. In addition, we would hope to spawn generations of young people that would be advocates for healthy lifestyles-to continue to build and sustain the political will for healthy environmental change.

SUMMARY

The global epidemic of obesity is driven by an environment that encourages overeating and discourages physical activity, creating a consistent bias toward positive energy balance. We have had no reason to develop a physiological control system to vigorously oppose a small degree of sustained, positive energy balance. The result is that most of the population is gaining weight at a rate of .5-2 kg/year.

We desperately need to modify the environment to one that is less conducive to weight gain, but the available research provides few clues for how to do this effectively. So many things in the environment would seem to logically promote obesity that it is difficult to know where to start in changing the environment. We have a great need for more research to understand how factors within the environment affect energy intake and physical activity and to conduct small-scale interventions to see how to change these factors. Because the prevalence of obesity is still rapidly increasing, many feel that we cannot wait until we have this information before we begin to change the environment. We are currently seeing many suggestions for national action and for policy changes, with little or no data to support any of these suggestions.

Given the urgent need for action, we suggest that increasing physical activity may be the best place to start, and we urge that substantial efforts be directed toward modifying the environment to increase physical activity.