Meta-analysis: effect of exercise, with or without dieting, on the body composition of overweight subjects.

OBJECTIVE: To determine if physical training conserves fat-free mass (FFM) in overweight men or women during weight loss.
DESIGN: Journals published between 1966 and 1993 were searched by MEDLINE and by handsearch to obtain all reports on human subjects in which the effect of exercise on body composition was studied in at least two concurrent treatment groups, of which at least one group did, and one group did not, undergo an exercise programme designed to promote fat loss. The relation between loss of weight, and loss of FFM, was examined by linear regression analysis among exercising and non-exercising groups of men or women.
SUBJECTS: Twenty-eight publications reported results on 226 sedentary men in 13 groups, 233 exercising men in 14 groups, 199 sedentary women in 23 groups, and 258 exercising women in 28 groups.
RESULTS: Aerobic exercise without dietary restriction among men caused a weight loss of 3 kg in 30 weeks compared with sedentary controls, and 1.4 kg in 12 weeks among women, but there was little effect on FFM. Resistance exercise had little effect on weight loss, but increased FFM by about 2 kg in men and 1 kg in women. Regression analysis shows that for a weight loss of 10 kg by diet alone the expected loss of FFM is 2.9 kg in men and 2.2 kg in women. When similar weight loss is achieved by exercise combined with dietary restriction the expected loss of FFM is reduced to 1.7 kg in men, and women. It is probable that the FFM conserved by exercise during weight loss contains more water and potassium than average FFM. The subjects studied were not severely obese.
CONCLUSIONS: Aerobic exercise causes a modest loss in weight without dieting. Exercise provides some conservation of FFM during weight loss by dieting, probably in part by maintaining glycogen and water.