Danish researchers Krogh and colleagues randomly 115 assigned depressed people to one of two exercise programs. One was a strenuous aerobic workout - cycling for 30 minutes, 3 times per week, for 3 months. The other was various stretching exercises. The idea was that stretching was a kind of placebo control group on the grounds that, while it is an intervention, it's not the kind of exercise that gets you fit. It doesn't burn many calories, it doesn't improve your cardiovascular system, etc. Aerobic exercise is the kind that's most commonly been proposed as having an antidepressant effect. So what happened? Not much. Both groups got less depressed but there was zero difference between the two conditions. The cyclists did get physically fitter than the stretchers, losing more weight and improving on other measures. But they didn't feel any better. If this is true, it might mean that the antidepressant effects of aerobic exercise are psychological rather than physical - it's about the idea of 'exercising', not the process of becoming fitter. While many trials have found modest beneficial effects of exercise vs a "control condition", the control condition was often just doing nothing much - such as being put on a waiting-list. So the placebo effect or the motivational benefits of 'doing something', rather than the effects of exercise per se, could be behind it. In the current study though the stretching avoided that problem. As I said in a post about a previous paper, I said that Exercise and Depression: It's Complicated
The idea that exercise is a useful treatment for depression: it's got something for everyone. For doctors, it's attractive because it means they can recommend exercise - which is free, quick, and easy, at least for them - instead of spending the time and money on drugs or therapy. Governments like it for the same reason, and because it's another way of improving the nation's fitness. For people who don't like psychiatry, exercise offers a lovely alternative to psych drugs - why take those nasty antidepressants if exercise will do just as well? But this doesn't mean it's true.
This was a moderate sized study, and one study by itself doesn't prove much - any more than one single political poll does. From personal experience I think there's a good chance strenuous aerobic exercise can boost mood... but this is a reminder that the picture on exercise and depression is not quite as clear as the recent enthusiasm for it suggests...
Krogh J, Videbech P, Thomsen C, Gluud C, & Nordentoft M (2012). DEMO-II Trial. Aerobic Exercise versus Stretching Exercise in Patients with Major Depression-A Randomised Clinical Trial. PloS one, 7 (10) PMID: 23118981