FOR THOSE in the doldrums, few things are more tiresome than being told to exercise. But unwelcome advice is not necessarily wrong. Study after study has found that exercise boosts mood and reduces anxiety. Two large analyses published earlier this year go further, suggesting it works about as well as therapy or antidepressants.

FOR THOSE in the doldrums, few things are more tiresome than being told to exercise. (Economist)FOR THOSE in the doldrums, few things are more tiresome than being told to exercise. (Economist)

The first, published in January by researchers based across Britain and Ireland, took the form of a Cochrane review—a well-regarded meta-analysis of health-care research. It pooled the results of 69 randomised-controlled trials (RCTs) conducted to measure the effects of exercise on depression. The second paper, published in February in the British Journal of Sports Medicine, was a so-called meta-meta-analysis. It drew on more than 1,000 trials involving nearly 80,000 participants. Both concluded that exercise reduces symptoms of depression and anxiety by roughly as much as conventional treatments.

There are important caveats. Meta-analyses are only as good as the studies they include, and exercise trials are prone to being skewed. For one thing, participants cannot be blinded—they know if they are doing kettlebell swings or not—which makes their self-reported mood vulnerable to any favourable expectations they might have. For this and other reasons, the Cochrane review judged all the studies it included to be at “high risk” of bias.

What’s more, the meta-meta-analysis did not include any studies that tested exercise against other interventions. The findings from the exercise trials were, instead, compared against those from separate trials of antidepressants or therapy. But unlike exercise studies, RCTs of antidepressants are typically well blinded and have strong placebo effects, making it harder for them to achieve similarly impressive results. “I don’t think it’s a fair comparison,” says Jonathan Roiser, a professor of neuroscience at University College London.

All the same, most researchers are confident that exercise helps improve mood. Aerobic workouts, such as running, walking or cycling, seem to be particularly beneficial across the board. For depression, group-based or supervised exercise is more effective than sweating alone, and the benefits of exercise accrue over several months. For anxiety, the best results seem to come from lower-intensity activity.

Why exercise works is less clear. The popular idea that exercise generates a “high” by causing the release of endorphins, a form of opioid, has little scientific support. A study published in 2021 found that blocking runners’ opioid receptors reduced neither the euphoria they reported after a session nor the drop in anxiety. Researchers instead think that endocannabinoids—chemicals produced by the body and brain that activate the same receptors as the active molecules in cannabis—might be responsible for these short-term boosts.

Other pathways are also being triggered. Exercise seems to reduce inflammation and improve brain plasticity, as well as increasing the transmission of dopamine in the brain. Dopamine is involved in the process of weighing effort against reward and so increasing transmission may help reverse the loss of motivation associated with depression.

There are purely psychological benefits, too: exercise can provide people with a sense of achievement, agency and eventually mastery, all of which are known to lift mood. Plenty of reasons, then, to work up a sweat.