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 O-18-labeled water could not ful ly account for lower resting metabolic rates (RMR) in old men and wome n compared to RMR in young men. Methods. Since age-related changes in the distribution of water between extracellular and intracellular spac es could lead to overestimation of FFM in the old, we reanalyzed our d ata using estimates for total body and intracellular water (TBW and IC W, respectively) derived from published equations and included data fr om 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). Whi le 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 explai ned by changes in body water or its distribution. Investigators should be cautious when selecting models and equations to estimate body wate r compartments.