Factors determining energy expenditure during very-low-calorie diets

There is an important variability in short- and long-term weight loss success among obese subjects, regardless of the strategy used. It is still unclear whether this variability is a therapy-specific or a patient-specific problem. Changes in energy expenditure are probably a key factor in the phenomena of weight loss or weight regain. Factors influencing resting metabolic rate (RMR) and diet-induced thermogenesis (DIT) in obese patients are considered: age, fat mass, fat-free mass (FFM), sex and thyroid hormones are all related to the components of energy expenditure. Fat distribution estimates [expressed by waist-to-hip ratio (WHR)] seem to be related to DIT rather than to RMR. Short- and long-term effects of very-low-calorie diets (VLCDs) on RMR and DIT were investigated. Short-term weight loss by VLCD induced a significant (P less than 0.001) decrease of FFM with a concomitant decline of RMR values. A nonsignificant decreasing trend exists for thermogenesis. No further decrease of FFM was found after long-term weight loss without differences in RMR/FFM or thermogenesis. Changes in FFM are known to be the major component of changes in RMR, but other factors may be involved. Also some other factors may account for the possible changes in diet-induced thermogenesis during VLCD. Candidates for this include total fat mass, FFM, sex hormones, insulin concentrations, and fat distribution, of which insulin concentrations seem to determine RMR changes to an important extent. Fat distribution patterns per se or changes in WHR seem to account consistently (P less than or equal to 0.05) for changes in DIT during VLCD.