Reduction in resting energy expenditure in relation to lean tissue loss in obese subjects during prolonged dieting.

Resting energy expenditure (REE) was measured by indirect calorimetry during dieting (2.74-3.30 MJ/day) for up to 200 days in overweight patients undergoing continuous metabolic balance studies. The initial rapid reduction in REE in the first 2 weeks of dieting was related to the extent of the immediate nitrogen deficit that follows caloric restriction and was larger (p less than 0.01) in men dieting de novo than either women (NS) or 4 patients of comparable lean body mass who had dieted prior to admission (NS). Because of their smaller metabolic energy deficit on our diets, the reduction in REE in women was small and not significant. In men, the slower long-term reduction in REE (p less than 0.05) was related to a continuing loss of lean tissue with no changes in the REE/m2, REE/kg body weight and REE/mmol creatinine excreted with time. We have previously shown that total energy expenditure (estimated from the rate of fat loss as measured by the difference between body weight changes and weight changes in fluid and protein) does not change significantly during prolonged dieting. The fall in REE in parallel with lean body mass suggests that some increase in the energy expended in daily activities must have occurred.