The cross-sectional and longitudinal dependence of the resting metabolic rate on the fat-free mass.

The dependence of the resting metabolic rate (RMR) on the fat-free mass (FFM) of temporarily fasted well-fed subjects has been studied by many researchers over the years. The results of 10 such studies yield an average linear dependence with a slope of 75 +/- 15 kJ/kg per day. In the work of Keys et al (The biology of human starvation. Minneapolis, MN: University of Minnesota Press, 1950) on semistarved subjects, however, the slope of the RMR dependence on the FFM was found to be 280 +/- 50 kJ/kg per day. The argument presented in this article is that the result derived for the large group of well-fed subjects is cross-sectional information, whereas that for the semistarved subjects is longitudinal data. The linear regression of the longitudinal data yields a negative offset term that when combined with the RMR vs FFM slope divides the FFM into active and inactive components, active tissue being that which interacts directly with oxygen. The linearity of the RMR vs FFM curve suggests that the elements of the active tissue mass are energetically similar regardless of their distribution in the body's organ systems. The active-inactive model implies that the longitudinal data results from the decrease in active tissue alone, whereas cross-sectional data for different individuals correspond to an admixture of both active and inactive tissue. For different individuals having the average RMR vs FFM slope of the semistarved subjects, it is calculated that a change in the FFM consists of about 27% active and 73% inactive tissue. A histogram of the individual longitudinal RMR vs FFM slopes for the 32 semistarved subjects yields an unexpected non-Gaussian distribution with a minimal value of 158 kJ/kg per day and a maximal value of 405 kJ/kg per day.