variables that influence energy intake and expenditure—and the evidence of these changes is readily available and consistent.

Changes in the food and physical activity environments have caused sustained positive energy balance, leading to weight gain. For example, energy-dense foods and beverages have become more inexpensive, heavily advertised, and readily available than in the past, and their portion sizes continue to increase. Technological innovation in the home, workplace, and schools has reduced the need for physical effort, leading to sedentary behavior. Many schools have reduced or eliminated recess and physical education programs (IOM, 2005). The field of obesity prevention has made advances in determining the causes of the epidemic, but the complex interrelationships among those many causes have been difficult to address adequately within a given intervention. For this reason, a systems approach to obesity prevention is advocated in this report (see Chapter 4).

Taking a population perspective on obesity does not negate the importance of genetic or biologically mediated influences on the problem. But the increasing prevalence of obesity in diverse populations reveals the limitations of biological controls on energy balance in the face of high caloric availability and diminished caloric expenditure through routine physical activity. Nor does the recognition of environmental influences on obesity negate the importance of individual weight control efforts; however, the level of effort required makes individuals’ attempts to control their weight notoriously ineffective over the long term. Changes in aspects of society that encourage excess caloric consumption and sedentary behavior are therefore necessary to support individual efforts to achieve and maintain energy balance. In fact, measures that have been recommended for populationwide obesity prevention would enhance the potential for obesity treatment to be effective over the long term. Moreover, unless both weight gain trends and average population weight levels decline, progress achieved through treatment for those who are already obese will be offset by the tide of new people needing such treatment.

Calls for multifaceted efforts to prevent obesity have been made repeatedly by governments, scientific and professional societies, advocacy groups, and funding agencies at the local, state, national, and international levels. The case for obesity prevention is especially strong in relation to children and youth. The case was made for U.S. children in a 2005 Institute of Medicine (IOM) report Preventing Childhood Obesity: Health in the Balance—a report that was generated in response to a congressional request for a national action plan addressing the problem. Protecting children from harm and creating social and environmental conditions that support healthy growth and development are fundamental responsibilities of all societies. The physical and psychosocial consequences of obesity begin in childhood and may have a lifelong impact, especially—although not only—if obesity persists into adulthood. Indeed, most obesity and obesity-related diseases develop during adulthood—an important justification for obesity prevention directed at the adult population as well. Another

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