Age Changes in Body Size, Body Composition and Basal Metabolism

¹ From the Department of Physiology, University of North Carolina Medical School, Chapel Hill, North Carolina

The interrelations of body size, body composition and basal metabolism were studied in 69 albino rats ranging in age from 18–174 days. The decline in metabolic rate with age was more rapid than would be predicted from the weight^0.75 rule which eliminates the influence of body size in interspecific measurements. Body composition analyses indicated that the increase with age in metabolically inert fat and bone minerals was approximately balanced by a corresponding decrease in metabolically inert extracellular fluid, so that ‘active tissue mass’ was virtually unchanged. Calculations based on data in the literature indicate that about one-half the decline in metabolic rate with age may be due to the corresponding decrease in the relative weight of the viscera. The remainder of the decline in metabolic rate must be due to factors other than changes in the chemical or histological composition of the body.