University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA
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.
