Total body water data for white adults 18 to 64 years of age: The Fels Longitudinal Study

Background. Total body water (TBW) volume is reported to decrease with age, but much of the published data are 20 to almost 50 years old and are cross -sectional. proper interpretation of clinical levels of TBW and trends with age necessitates the availability of current longitudinal data from health y individuals. Methods. Mixed longitudinal data for TBW of 274 white men and 292 white wom en (18 to 64 pears of age) in the Fels Longitudinal Study were collected on a regular schedule over a recent eight-year period. The concentration of d euterium was measured by deuterium nuclear magnetic resonance spectroscopy. Body composition estimates were made with dual-energy x-ray absorptiometry , and random effect models were used to determine the patterns of change ov er time with and without covariates. Results. The mean TBW data for the Fels men are either similar to or approx imately 2 to as much as 6 liters greater than that reported by most other i nvestigators 20 to 50 years ago. For Fels women. the mean TBW ranges from a pproximately 2 to as much as 5 liters less than that reported previously. T hese comparisons with much earlier studies reflect cohort effects and the s ecular changes in overall body size that have occurred during the past 60 t o 70 years. These findings are reinforced by the fact that some early data sets included individuals born almost 140 years ago. After adjusting for th e covariate effects of total body fat (TBF) and fat-free mass (FFM) with ag e, there were no significant age or age-squared effects on TBW in the men. In the women, after adjusting for the covariate associations of TBF and FFM with age, there was a small, but significant, negative linear association of TBW with age. In the men and women, the mean ratio of TBW to weight decl ined with age as a function of an increase in body fatness and more so for the men than the women. Conclusion. The findings from these mixed longitudinal data indicate that T BW volume, on average, maintains a reasonable degree of stability in men an d women through a large portion of adulthood. These TBW data are recommende d as current reference data for healthy adults.