RELATIONSHIP BETWEEN RESTING METABOLIC-RATE AND THE COMPOSITION OF THE FAT-FREE MASS

Although a low resting metabolic rate (RMR) has been shown to be a ris k factor for future weight gain, little is known about the mechanisms determining its level. We tested the hypothesis that the composition o f the fat-free mass (FFM) is a main determinant of RMR. If this hypoth esis is true, a regression model including internal organ masses as in dependent variables should explain a larger fraction of the variance i n RMR than is explained using only FFM as a predictor. We measured fat mass by hydrodensitometry, liver and kidney volumes by computed tomog raphy (CT), heart mass by echocardiography, muscle mass by dual-energy x-ray absorptiometry (DEXA), and RMR by calorimetry in 40 subjects. F FM and fat mass explained 83% of the variability in RMR (standard erro r of the estimate [SEE], 420 kJ/d) in a multiple regression analysis. Combinations of organ and muscle masses performed as well as but not b etter than stepwise multiple regression models. The fact that the comp osition of the lean mass could not improve the prediction of RMR in co mparison to the traditional FFM-fat mass model suggests that the weigh t of internal organs is not a main determinant of RMR. This may indica te that the variability in RMR is associated with variation in energy expenditure per kilogram of tissue of the individual organs. Copyright (C) 1997 by W.B. Saunders Company.