Changes in total body water with age.

Extensive cross-sectional studies demonstrate a diminution of total body water in elderly and very old subjects. These findings are supported by less extensive longitudinal studies. Cross-sectional studies indicate that the decrease in total body water is mainly due to decreased intracellular water, but this is not supported by the findings of longitudinal studies. Despite the observed changes in total body water, both animal and human studies indicate little or no change in the relationship between total body water and fat-free mass with aging.