TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat.

At the same time, fat-free body mass declines 15 percent, which in turn slows the body's metabolism - the rate at which it converts food to energy.

Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on BMR, has been shown in the 1990s not to correlate with BMR, when fat-free body mass was adjusted for.