Dependence of weight loss during very-low-calorie diets on total energy expenditure rather than on resting metabolic rate, which is associated with fat-free mass.

The assumption that total energy expended is related to resting metabolic rate (RMR) has not been validated. Intuitively, weight lost should be determined by the difference between the total energy consumed and the total energy expended. The ratio of actual daily energy usage to RMR by subjects dieting for 11 wk on a 1695-kJ (405-kcal) very-low-calorie diet (VLCD) was variable 1.9 +/- 0.3 (range 1.47-2.36). Weight loss correlated with total energy expenditure measured by 2H2 18O and not RMR, body mass index or body composition. Although RMR may be a reliable indicator of fat-free mass, there is little evidence that subjects with similar RMR will lose comparable weight or have comparable difficulty in weight maintenance.