Longitudinal study of body composition changes associated with weight change and physical activity

Weight changes result in fat-free mass (FFM) and body fat (BF) changes. This study determined FFM and BF changes after weight gain or loss and whether these changes differ by gender, physical activity, and age.

Healthy volunteers, recruited between 1991 and 2003, were followed for 1 y (n = 400) or 3 y (n = 305). Active subjects performed >3 h of physical activity of > or =4.0 metabolic equivalents/wk, sedentary subjects performed <3 h/wk. Body weight and body composition by bioelectrical impedance analysis were determined at year 0, 1, or 3.

At years 1 and 3, FFM and BF decreased with weight loss and increased with weight gain. BF was more sensitive (P < 0.03) to weight change than FFM. Compared to weight-stable individuals at year 1, weight gains of 1.0-1.9, 2.0-2.9, and > or =3.0 kg changed FFM by -0.04 (P = 0.90), +0.48 (P = 0.15), and +1.39 kg and BF by +1.35, +1.87, and +3.09 kg, respectively (all P < 0.001). Comparable FFM and BF decreases were observed for weight losses (FFM -0.28 kg, P = 0.38; -0.75 kg, P = 0.04; -1.51 kg, P < 0.001; BF -1.01 kg, P < 0.01; -1.55 kg, P = 0.01; -3.13 kg, P < 0.001). These relations were similar across gender and age strata. At year 1, active individuals were less likely to gain BF with weight gain and more likely to lose BF with weight loss than were sedentary individuals, except for weight losses >3 kg. At year 3, the association between body weight and FFM and BF change was similar between active and sedentary individuals.

Greater weight changes (>3 kg) are necessary for weight change to have a significant effect on FFM than to have an effect on BF.