The obesity epidemic shows no signs of abating. There is an urgent need to push back against the environmental forces that are producing gradual weight gain in the population. Using data from national surveys, we estimate that affecting energy balance by 100 kilocalories per day (by a combination of reductions in energy intake and increases in physical activity) could prevent weight gain in most of the population. This can be achieved by small changes in behavior, such as 15 minutes per day of walking or eating a few less bites at each meal. Having a specific behavioral target for the prevention of weight gain may be key to arresting the obesity epidemic.

This paper describes the interplay among energy intake, energy expenditure and body energy stores and illustrates how an understanding of energy balance can help develop strategies to reduce obesity. First, reducing obesity will require modifying both energy intake and energy expenditure and not simply focusing on either alone. Food restriction alone will not be effective in reducing obesity if human physiology is biased toward achieving energy balance at a high energy flux (i.e. at a high level of energy intake and expenditure). In previous environments a high energy flux was achieved with a high level of physical activity but in today's sedentary environment it is increasingly achieved through weight gain. Matching energy intake to a high level of energy expenditure will likely be more a more feasible strategy for most people to maintain a healthy weight than restricting food intake to meet a low level of energy expenditure. Second, from an energy balance point of view we are likely to be more successful in preventing excessive weight gain than in treating obesity. This is because the energy balance system shows much stronger opposition to weight loss than to weight gain. While large behavior changes are needed to produce and maintain reductions in body weight, small behavior changes may be sufficient to prevent excessive weight gain. In conclusion, the concept of energy balance combined with an understanding of how the body achieves balance may be a useful framework in helping develop strategies to reduce obesity rates.

The low-carbohydrate diet produced a greater weight loss (absolute difference, approximately 4 percent) than did the conventional diet for the first six months, but the differences were not significant at one year. The low-carbohydrate diet was associated with a greater improvement in some risk factors for coronary heart disease. Adherence was poor and attrition was high in both groups. Longer and larger studies are required to determine the long-term safety and efficacy of low-carbohydrate, high-protein, high-fat diets.

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A consensus meeting was held in Bangkok, 21-23 May 2002, where experts and young scientists in the field of physical activity, energy expenditure and body-weight regulation discussed the different aspects of physical activity in relation to the emerging problem of obesity worldwide. The following consensus statement was accepted unanimously. 'The current physical activity guideline for adults of 30 minutes of moderate intensity activity daily, preferably all days of the week, is of importance for limiting health risks for a number of chronic diseases including coronary heart disease and diabetes. However for preventing weight gain or regain this guideline is likely to be insufficient for many individuals in the current environment. There is compelling evidence that prevention of weight regain in formerly obese individuals requires 60-90 minutes of moderate intensity activity or lesser amounts of vigorous intensity activity. Although definitive data are lacking, it seems likely that moderate intensity activity of approximately 45 to 60 minutes per day, or 1.7 PAL (Physical Activity Level) is required to prevent the transition to overweight or obesity. For children, even more activity time is recommended. A good approach for many individuals to obtain the recommended level of physical activity is to reduce sedentary behaviour by incorporating more incidental and leisure-time activity into the daily routine. Political action is imperative to effect physical and social environmental changes to enable and encourage physical activity. Settings in which these environmental changes can be implemented include the urban and transportation infrastructure, schools, and workplaces.'