Insights into energy balance from doubly labeled water.

Obesity is defined as the excess storage of energy in the form of fat that results from imbalances between energy intake and expenditure. The study of the components of energy balance has undergone a significant advancement with the application of the doubly labeled water (DLW) method to the measurement of human energy expenditure. This manuscript includes a selective review of the studies that have utilized the doubly labeled method as it applies to the study of human obesity. Although generally now accepted, one of the major surprises from the early applications of DLW was that obese individuals have higher energy expenditures than lean controls. Moreover, weight gain, even in the already obese, is associated with an increase in energy expenditure as weight is one of the strongest predictors of total energy expenditure. Similarly, studies of weight loss treatment show a decrease in energy expenditure due to weight loss and due to adaptive changes in energetic efficiency, but these changes do not account for the common cessation of weight loss observed after 12-26 weeks of restriction. The accumulating data from the application of the DLW method suggest a need to place greater emphasis on mechanisms that lead to a mismatch between energy intake and expenditure rather than a continuing emphasis on energy intake or energy expenditure alone.