 More than half of American adults are overweight or obese, and this proportion is increasing. The health risks associated with obesity include diabetes, cardiovascular disease, arthritis, and some cancers. It is well established that on an individual level, weight gain is caused by taking in more calories than are expended. On a population basis, the prevalence of overweight and obesity is associated with a myriad of influences, including genetic and physiological factors as well as social, behavioral, cultural, and environmental factors. Behavioral factors that contribute to weight gain include eating choices that lead to increased energy intake and physical activity choices that lead to decreased energy expenditure. For a majority of overweight individuals, restoring the balance between energy intake and expenditure is difficult. 