Abstract: Thirty-nine trials (2326 participants) fulfilled our inclusion criteria, of which 37 provided data for meta-analyses. There were multiple sources of bias in many of the trials; randomisation was adequately concealed in 14 studies, 15 used intention-to-treat analyses and 12 used blinded outcome assessors. For the 35 trials (1356 participants) comparing exercise with no treatment or a control intervention, the pooled SMD for the primary outcome of depression at the end of treatment was -0.62 (95% confidence interval (CI) -0.81 to -0.42), indicating a moderate clinical effect. There was moderate heterogeneity (I² = 63%). When we included only the six trials (464 participants) with adequate allocation concealment, intention-to-treat analysis and blinded outcome assessment, the pooled SMD for this outcome was not statistically significant (-0.18, 95% CI -0.47 to 0.11). Pooled data from the eight trials (377 participants) providing long-term follow-up data on mood found a small effect in favour of exercise (SMD -0.33, 95% CI -0.63 to -0.03). Twenty-nine trials reported acceptability of treatment, three trials reported quality of life, none reported cost, and six reported adverse events. For acceptability of treatment (assessed by number of drop-outs during the intervention), the risk ratio was 1.00 (95% CI 0.97 to 1.04). Seven trials compared exercise with psychological therapy (189 participants), and found no significant difference (SMD -0.03, 95% CI -0.32 to 0.26). Four trials (n = 300) compared exercise with pharmacological treatment and found no significant difference (SMD -0.11, -0.34, 0.12). One trial (n = 18) reported that exercise was more effective than bright light therapy (MD -6.40, 95% CI -10.20 to -2.60). For each trial that was included, two authors independently assessed for sources of bias in accordance with the Cochrane Collaboration 'Risk of bias' tool. In exercise trials, there are inherent difficulties in blinding both those receiving the intervention and those delivering the intervention. Many trials used participant self-report rating scales as a method for post-intervention analysis, which also has the potential to bias findings. Exercise is moderately more effective than a control intervention for reducing symptoms of depression, but analysis of methodologically robust trials only shows a smaller effect in favour of exercise. When compared to psychological or pharmacological therapies, exercise appears to be no more effective, though this conclusion is based on a few small trials.

Plain-language summary: Exercise is moderately more effective than no therapy for reducing symptoms of depression. Exercise is no more effective than antidepressants for reducing symptoms of depression, although this conclusion is based on a small number of studies.  Exercise is no more effective than psychological therapies for reducing symptoms of depression, although this conclusion is based on small number of studies. The reviewers also note that when only high-quality studies were included, the difference between exercise and no therapy is less conclusive. Attendance rates for exercise treatments ranged from 50% to 100%. The evidence about whether exercise for depression improves quality of life is inconclusive. The reviewers recommend that future research should look in more detail at what types of exercise could most benefit people with depression, and the number and duration of sessions which are of most benefit. Further larger trials are needed to find out whether exercise is as effective as antidepressants or psychological treatments.