There is little doubt that the world is in the throes of an obesity epidemic. As noted by Potter,1 on a fundamental level, the basis of obesity is straightforward. Simply put, excess body fat will accumulate if the food energy consumed exceeds energy expended. However, as he also states, this truism provides little insight into the causes or the prevention of obesity. Just as the realm of epidemiologic research has primarily been human biologic conditions, there has been a tendency for epidemiologic approaches to obesity to consider biologic causes. Potter covers some of these, pointing to factors such as early exposures and genetic polymorphisms that promote overeating, tilting energy balance in favor of excess energy intake. There has also been substantial research into the roles of various metabolic factors such as gastrointestinal peptide hormones that control appetite2 or the interplay of leptin, insulin, and other hormones that influence fat metabolism.3
Although epidemiology may continue to refine the potential roles of these factors in obesity, the search for effective measures for obesity prevention will need to consider the broader societal context in which obesity occurs. Specifically, food intake and energy expenditure do not occur in a vacuum. The socioenvironmental context, and changes in that context, need to be considered. This approach requires the application of epidemiologic methods in studies that consider social trends, transportation and land use policies, zoning regulations, and even political philosophies that underlie decisions that influence food availability and opportunities for physical activity.
OPPORTUNITIES FOR PHYSICAL ACTIVITY
Although energy expenditure is a fundamental aspect of obesity prevention and management, the societal context in which physical activity promotion occurs will play a major role in its effectiveness. Powerful societal trends prevent us from incorporating physical activity into our daily lives with the result that “exercise” is thought of as an activity that needs to be scheduled competing with lunch appointments or watching a favorite television show.
In the United States in particular, the phenomenon of suburban sprawl has resulted in greater distances traveled for commuting and other routine tasks, inefficient public transportation, and growth of infrastructure that promotes sedentary travel.4 Architectural, zoning, and policy issues can promote or prevent the incorporation of activity into one's lifestyle.5 For example, suburban sprawl produces low population density, which in turn cannot support a public transportation infrastructure. Most of adults in the United States own motor vehicles, which are by far the preferred mode of transportation—a much more sedentary approach to movement than public transit, bicycling, or walking. Suburban development rarely considers the needs of the pedestrian or bicycle commuter. Neighborhoods are often designed in disconnected road networks that may appeal to a suburban esthetic ideal but make automobiles essential for trips that might otherwise have been short walking or bicycling excursions. Sidewalks are often lacking, major streets are unsafe for bicyclists, and buildings are designed with stairs intended for use only in emergencies.
Perhaps as important is the perception of personal safety—as distinguished from actual data, which document dramatic declines in homicide6 and violent and property crime rates7 since 1990. In annual surveys conducted by the Gallup Poll since 1989, a large proportion of respondents, ranging from 41% in 2001 to 89% in 1992, felt that there was more crime in the United States than in the previous year.8 In Europe, there has also been a trend toward larger proportions of the population feeling unsafe.9 The impact of feeling less safe may result in fewer opportunities for physical activity regardless of where one lives. Perceptions of lack of safety may result in fewer children walking to school or bicycling unaccompanied by an adult.4
INFLUENCES ON FOOD AVAILABILITY
There is continuing debate concerning the role of nutrient composition on obesity in addition to the effects of total energy intake. However, there is little doubt that increases in daily energy intake without compensatory increases in energy expenditure, even in relatively small daily quantities, result in increased weight gain and development of obesity. An excess of just 10 kcal per day can result in an increase of 1 lb of weight per year. There are at least 2 societal trends in this regard—increases in restaurant portion sizes and vending machine contracts that place soft drinks in schools—that illustrate the forces that increase the availability of food energy in the United States.
Using data from national food consumption surveys, Nielsen and Popkin10 observed that portion sizes for various food categories have increased substantially. For example, the typical hamburger in 1977 weighed approximately 162 g, whereas in 1994–1996, it weighed 198g—an increase of 22%. Soft drink portions increased 52%, from 387 mL to 587 mL. These increases can easily be an important contributor to the obesity epidemic. Nielsen and colleagues11 also determined that a larger proportion of food energy is being consumed at restaurants and fast food establishments—the same locations where increases in portion size were the most dramatic.10 These trends may be fueled by perceptions of increased value when larger portions are provided for the same cost and also by pervasive advertising.
The reach of advertising is apparent in the establishment of exclusive contracts to provide soft drink vending machines in schools. These contracts may bring needed revenue to school districts in an environment where property tax cuts shortchange education budgets. However, they may be a Faustian bargain in that they result in increased availability of soft drinks.12 Prospective studies show a positive association between the consumption of caloric soft drinks and weight gain.13–15
Can nutrition policies affect obesity rates? Some commentators have pointed to the 1992 Food Guide Pyramid, which both reinforced the message that all fats in the diet should be decreased and promoted 6 to 11 daily servings of cereal and grain products. However, this pyramid failed to make obvious the differential health effects of various fat sources and between whole grain foods and those high in refined cereals and sugars, leading to food choices that result in increased weight gain.16 Others have noted that, regardless of the appropriateness of nutrition recommendations, those foods most harmful to health are the ones most aggressively marketed.17
From a biologic perspective, the development of obesity results from an imbalance of excess energy intake relative to energy expenditure. However, the context in which obesity occurs is substantially more complex. We need to apply epidemiologic perspectives to the study of the socioenvironmental context in which food availability, food choices, and opportunities for physical activity occur. The extent to which socioenvironmental factors may influence obesity rates is unknown and ripe for study. The application of more complex modeling procedures such as outlined by McKeown-Eyssen18 deserves consideration. A recent request for applications from the National Institutes of Health on “Obesity and the Built Environment” is one example in which such work is being encouraged. Ultimately, effective public health strategies addressing the obesity epidemic will require epidemiologic research that ventures into realms where it has not often tread such as land use planning, transportation policy, and the politics of food.
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