Fat-free mass, total body water, and intracellular water in the aged rat.

Total body water (TBW) and intracellular water (ICW = TBW minus extracellular water), when related to fat-free body mass (FFB), were recently observed to remain stable in healthy aged men and women. To expand these findings, a group of male Sprague-Dawley rats was followed from weaning to death. The design minimized several perturbations common to aging studies. At about 100-day intervals from 297 to 888 days, representative animals were killed for chemical studies. TBW/FFB was stable from 362 to 715 days, then significantly higher at 800 and 888 days. Extracellular water content of the fat-free body (ECW/FFB) increased significantly during middle life (362--579 days), then remained stable through 888 days. Changes of ICW/FFB were not significant. Aside from a uniquely higher value at 638 days, protein content did not differ significantly at any age. The stability of these major constituents suggested approximate maintenance of cell mass in healthy rats at late ages, a conclusion supported by additional observations of this colony and of an aged human population. We are unable to confirm the hypothesis that loss of FFB is part of the normal aging process.