Effect of age on body water and resting metabolic rate

Background. We previously reported that differences in fat-free mass (FFM) estimated by isotope dilution of ¹⁸O-labeled water could not fully account for lower resting metabolic rates (RMR) in old men and women compared to RMR in young men. Methods. Since age-related changes in the distribution of water between extracellular and intracellular spaces could lead to overestimation of FFM in the old, we reanalyzed our data using estimates for total body and intracellular water (TBW and ICW, respectively) derived from published equations and included data from adolescent boys and girls studied under similar conditions. Results. In both sexes, the age-related reduction in RMR remained significant after adjustment for estimated body water compartments (p < .05). While adjusted RMR differed in boys and girls (p < .0001), it did not in old men and women (p = .15). Conclusion. We conclude that aging per se reduces RMR in lean tissue, a difference which cannot be fully explained by changes in body water or its distribution. Investigators should be cautious when selecting models and equations to estimate body water compartments.