Energy metabolism and obesity.

Over the long-term, most adult humans are able to maintain body energy stores through the process of energy balance, which regulates how much energy is consumed to match how much energy is expended. Energy expenditure is required for resting metabolic rate to maintain basic physiologic functions (e.g., heart beat, muscle function, respiration) and metabolize, digest, and store food that is consumed as well as for physical activity. Resting metabolic rate is the largest component of daily energy expenditure, and physical activity-related energy expenditure is the most variable. Cross-sectional studies in children and adults have shown that energy expenditure, including physical activity-related energy expenditure, are similar in lean versus obese subjects, especially after controlling for differences in body composition. A major limitation for most studies that have examined the role of energy expenditure in the cause of obesity is their cross-sectional design. Some longitudinal studies support the idea that reduced energy expenditure is a risk factor for the development of obesity, but most do not. There are several possibilities that could account for such discrepant findings. First, the ambiguous findings in the literature might be explained by the possibility that differences in energy expenditure and physical activity and their impact on the development of obesity are different at the various stages of maturation. Second, there could be individual differences in the impact of altered energy expenditure on the regulation of energy balance. The impact of energy expenditure on the cause of obesity could vary among different subgroups of the population (e.g., boys versus girls and different ethnic groups) and could have a differential effect within individuals at different stages of development. A specific example is the lower energy expenditure in Pima Indians, which predisposes to increased risk of obesity. It remains to be seen whether the lower metabolic rates that have been observed in African-Americans will relate to subsequent weight gain. It is conceivable that susceptible individuals fail to compensate for periodic fluctuations in energy expenditure. Third, given that obesity can arise as a result of a small energy balance over time, it is unlikely that existing techniques are capable of measuring such small differences. Finally, it can be argued that a focus on energy metabolism as a possible explanation of obesity is unlikely to yield interesting information because of the wide range in energy expenditure in the population even after adjusting for body composition. The major dependent variable that needs to be examined in relation to the cause of obesity is not energy expenditure but change in energy balance over time and the ability to regulate body energy stores. Given that the sudden change in obesity prevalence has occurred during a time of rapid environmental and cultural changes, additional focus on the behavioral and environmental effects on regulation of energy balance is warranted.