Gathering evidence for the effects of physical activity on the mind is much harder than you might expect.
There’s something intuitive about the idea that exercising makes you feel better about yourself. Several decades of research have been carried out on the topic, and some family doctors prescribe fitness classes to patients diagnosed with depression, often alongside counselling or medication. So could recent reports in the popular media proclaiming that a new study showed exercise did not improve depression levels mean that a consensus of medical opinion has been wrong?
All is not quite as straightforward as the media reports made out. Gathering evidence for the effects of exercise on depression is harder than you might expect.
The gold standard for research is the randomised controlled trial, where half the people receive the treatment and the other half don’t. No one knows which arm of the study they are in. With a new drug, this is relatively easy, you receive either the real pill or a placebo that looks like it. With exercise it’s much more difficult. People are going to notice whether they are exercising or not. It is also difficult to force people to exercise, so some studies allow people to choose which group they’d prefer to be in, skewing the results because the people who enjoy exercise are likely to benefit more.
Some researchers in the field found plenty to criticise about the way in which the latest study was carried out. The study divided people into two groups: one was given plenty of advice about exercising on top of what they referred to as “usual care”, the other had “usual care” alone. The problem is that for some people “usual care” included an exercise component, which could have confused the results. Secondly, the actual amount of exercise people took wasn’t measured. Instead they simply filled in a diary, which raises the question of whether people might have exaggerated the amount of exercise they were doing just to please the researchers. Finally, people who had previously failed to respond to antidepressants were excluded from the study. For all we know these might have been just the people who would actually benefit from exercise.
So where does this leave us? The recent study is just one of many conducted in this area, of course, so for a more comprehensive assessment I turned to the Cochrane collaboration, a not-for-profit organisation that creates systematic reviews of health studies. Combining the data from thirty different trials from countries as diverse as Thailand, Denmark and Australia they found that on the whole exercise has been shown to have some benefits for those with depression. But when only the very best-designed studies were included, this effect was very small.
So it seems that exercise might help a bit, which then begs the question of why. And there have been some extraordinary attempts to answer this question. One theory is that exercise releases endorphins and the neurotransmitter dopamine, which make you feel good. There is also a theory originating in the saunas and steam baths of 1970s Scandinavia known as the thermogenic hypothesis, which states that the rise in body temperature from exercising releases endorphins. Those participants who sat in the sauna did feel better than those who simply sat on a bench. One curious side-note about this study: the participants were paid in beer.
But could exercise improve your mood even if you don’t get hot and sweaty? Researchers have tried everything from making brave volunteers use exercise bikes while wearing scarves filled with ice to cycling in cold water with only their heads above the surface. (For a review of these studies see Morgan, W. (1997) Physical Activity and Mental Health. Philadelphia: Taylor Francis.) The mood of the underwater cyclists didn’t improve, but it did for those who were packed with ice, although one could imagine they were just relieved to remove their freezing scarves and the rectal thermometers that had been measuring their temperatures.
Perhaps it is not a chemical effect, perhaps getting out of the house, being in a group, or the pleasure of mastering new skills or seeing improvements in fitness levels makes a difference. All but one of the studies in the Cochrane review involved supervised exercise such as a class at a gym, while the new study simply recommended exercise of any kind – the authors explained that they “asked a pragmatic question concerning a feasible intervention that could be used in primary care.” The guidelines for the British organisation NICE which gathers evidence for the cost-effectiveness of different treatments are very specific in their recommendations, advising structured, supervised exercise programmes in groups, three times a week.
Finally, Professor William Morgan at the University of Wisconsin-Madison in the US has proposed a simple idea. It’s called the distraction hypothesis, and you’ve guessed it, the theory is that exercise relieves depression because it distracts us from worrying. He also found that the effect wears off after 24 hours, so we need to keep topping it up [Ref: Morgan, W. Physical Activity and Mental Health]. But his findings may be reassuring for anyone who’s not so keen on exercise. Sitting in a quiet room in a comfortable, old, leather armchair for a period of time seemed to make people feel just as good.
Whatever the reason, an important point to end with is that exercise may help, but it is not a panacea for depression. The nature of the condition means that after six months many people will feel better regardless of their treatment. Exercise isn’t going to cure the world of depression, but it is still worth a try. Ultimately, if it makes you feel better, then do it.
You can hear more Medical Myths on Health Check on the BBC World Service.
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