Relationship of body water compartments to age, race, and fat-free mass.

Water compartments were studied in 72 black and 128 white women, aged 20 to 70 years. Total body water (TBW) was measured by tritiated water dilution, and extracellular water (ECW) was measured by using delayed gamma neutron activation for the determination of total body chloride. Intracellular water (ICW) was assessed as the difference between TBW and ECW. Fat-free mass (FFM) was estimated by the measurement of total body potassium (TBK) and total body fat (TBF) by measurement of total body carbon (TBC). Total body protein was calculated from total body nitrogen (TBN). TBW was found to decline with age and to also be significantly influenced by body size. The extracellular water space was 41.5% of TBW. The decline of TBW with age is due primarily to a decline in ICW. The hydration of the FFM increased with age for the overall population because of a decline in TBK and TBN and an increase in ECW. Body composition models that have assumed constancy of hydration should be adjusted not only for gender but for age and body size.