A systematic review 1 was based on a literature search in the years 1997 to 2001. Randomized controlled trials with at least 1-year duration were included in the review. The quality of the trials was assessed. In total 14 trials were found. In the exercise group, the weekly duration of exercise was 3 to 5 hours. In this group, the change in weight after 1 to 2 years was on average -1.8 kg (range -5.8 to 0.7). The respective change in the control group was +0.5 kg (-0.4 to 1.1), so the net change was -2.7 kg (-5.1 to 0.5).
Comment: There are surprisingly few methodologically sound exercise intervention studies performed among overweight persons. Also a part of the studies included in this review were methodologically poor, e.g. with a small number of participants in each intervention group. One problem with the study design may be that the participants change their diet towards a healthier one despite the instructions, at least in short-term interventions.
A Cochrane review 2 (abstract , review [Abstract]) included 43 trials on the effect of physical exercise on overweight, with a total of 3476 subjects. Study duration had to be at least 3 months and drop-out rate not more than 15%. Exercise intervention was mainly compared with controls who had no energy restriction in their diet. When compared with no treatment, exercise resulted in small weight losses across studies. Exercise combined with diet resulted in a greater weight reduction than diet alone (WMD -1.1 kg; 95% CI -1.5 to -0.6). Increasing exercise intensity increased the magnitude of weight loss (WMD -1.5 kg; 95% CI -2.3 to -0.7). Exercise as a sole weight loss intervention resulted in significant reductions in diastolic blood pressure (WMD -2 mmHg; 95% CI -4 to -1), triglycerides (WMD -0.2 mmol/l; 95% CI -0.3 to -0.1) and fasting glucose (WMD -0.2 mmol/l; 95% CI -0.3 to -0.1). Higher intensity exercise resulted in greater reduction in fasting serum glucose than lower intensity exercise (WMD -0.3 mmol/l; 95% CI -0.5 to -0.2).
Comment: The main problem with the inclusion criteria is that not all the trials necessarily aimed at weight reduction. It was not reported how many of the participants showed a clinically significant reduction of overweight (e.g. 5 or 10% from baseline). It would be more sensible to measure changes in the relative amount of fat tissue or at least changes in body mass index than changes in sole weight, the latter being the only measure reported in this review.
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