Abstract: In most situations involving a significant change in body weight, both fat-free body mass (FFM) and body fat participate, but the relative contribution of FFM and fat to the total weight change is influenced by the initial body fat content. Overfeeding: In experiments of at least 3-weeks' duration, the weight gain of thin people comprises 60–70% lean tissues, whereas in the obese it is 30–40%. Underfeeding: In humans, there is an inverse curvilinear relationship between initial body fat content and the proportion of weight loss consisting of lean tissue. The same trend holds for animals and birds, including loss during hibernation. Another factor is the magnitude of the energy deficit: as energy intake is reduced, lean tissue makes up an increasing fraction of the total weight loss. Exercise: If individuals lose much weight with exercise, the result is usually some loss of lean tissue as well as fat, and once again the proportion of lean loss to total weight loss is greater in thin people than in those who have larger body fat burdens. Members of twin pairs often differ in weight. In thin individuals, lean accounts for about half of the intrapair weight difference, whereas in the obese it accounts for only one quarter. Body fat content must be taken into account in evaluating body composition changes induced by nutrition and exercise.