Scientists and the public alike have long suspected that exercise can reduce depression, but the problem with proving it is the same as with many long-held general associations: it’s just an association. Researchers have shown repeatedly that people who move more tend to have fewer depressive symptoms—but what if people who are less depressed have more motivations to get up and move? Or what if the people who have enough time to stay active tend to have lifestyles less prone to triggering depression?
To figure this out, researchers working for the Major Depressive Disorder Working Group of the Psychiatric Genomics Consortium used a clever new method called Mendelian randomization, which allows us to understand causal relationships between modifiable behaviors, such as exercise, and health outcomes. The basic idea goes something like this:
It’s hard to know whether exercise influences depression or whether depression influences exercise (or both!) because there are lots of other factors associated with each that could influence the other. People who exercise more might have higher incomes, for instance, and be better able to access therapy and medication. Those with worse depression might be less motivated to move around, or they might have physical pain associated with their depression that makes exercising less appealing. It could even be reverse causation—getting less exercise could make you more prone to depression. These extra associations muddy the picture. But your genes are a lot less prone to these problems. They’re assigned to you before birth in a relatively random way, which mitigates some environmental associations (like having a higher socioeconomic status) as well as eliminating the reverse causation issue.
That means we can use natural genetic variation as an experimental tool. We know that some gene variants make you more likely to exercise, so if physical activity is causing a decrease in depression it follows that people with those variants should be less prone to depression since they’ll tend to exercise more.
Of course, that’s assuming that those genes don’t influence anything else. As an editorial accompanying this study points out, this is an important caveat for all Mendelian randomization studies. “For example,” psychiatrist Adam Chekroud writes, “if the exercise gene variants also relate to low energy, and low energy relates to depression, this relationship would represent another path through which exercise gene variants might affect depression risk.” But he goes on to note that the study authors were aware of this potential misstep and did everything they could to minimize it. Even after removing genes associated with traits like body mass index or education level from the equation, their finding still held.
According to their study, for every one standard deviation increase in physical activity, depressive symptoms reduced by 26 percent. “One standard deviation” is an unhelpful measure of exercise, and though it’s difficult to draw direct parallels the researchers note that it’s approximately equal to one hour of moderate activity like walking, or replacing 15 minutes of sedentary behavior with vigorous exercise like running.
Last modified: February 9, 2019