A rigorous new study, led by a team of scientists from Massachusetts General Hospital, has confidently suggested a strong causal connection between higher levels of physical activity and reduced risk of depression. The novel and comprehensive genetic study is the first to empirically suggest that exercise can actively prevent, or reduce, symptoms of depression.

We know exercise is beneficial for the human body. A huge assortment of studies have effectively homed in on a variety of different biological mechanisms that are triggered by physical activity, from inducing hormones that protect against dementia to improving the composition of your gut microbiome.

The positive correlation between physical activity and well-being has been strongly demonstrated across a variety of research, however, the directional relationship has always been in question. Does exercise causally reduce symptoms of depression? Or does depression lead to reduced physical activity. Sure, depression may lead to a person staying inside, moving less, and eating more but does that simply explain the link between physical activity and depression?

The new study sought to tease out the causal relationship between physical activity and depression using a method called Mendelian randomization. This technique was developed as a way to weed out some of the more spurious conclusions often generated through epidemiological studies. Essentially, the method hinges on our knowledge that certain genetic variants can influence the onset of certain diseases.

So, in this case, exciting recent advances in our genomic knowledge allowed the targeting of several specific genetic variants that are associated with habitual physical activity, as well as several variants that are associated with major depressive disorder. Using data from the UK Biobank, the researchers could effectively identify subjects with those genetic variants and also track physical activity, both self-reported and calculated using objective wrist-worn accelerometers, as well as follow which subjects ultimately suffer from symptoms of depression.

The results from the study compellingly concludes the relationship between physical activity and depression is not bi-directional, meaning physical activity does seem to reduce depression, but depression does not seem to reduce physical activity. This implies a causative connection between an increase in exercise and a decrease in depression.

"On average, doing more physical activity appears to protect against developing depression," says lead author on the new research, Karmel Choi. "Any activity appears to be better than none; our rough calculations suggest that replacing sitting with 15 minutes of a heart-pumping activity like running, or with an hour of moderately vigorous activity, is enough to produce the average increase in accelerometer data that was linked to a lower depression risk."

This study ultimately suggests that physical exercise can function effectively as a preventative tool against depression. The next steps for the researchers are to investigate best-practice advice that can be widely recommended for people with different mental health risk profiles.

"We currently are looking at whether and how much physical activity can benefit different at-risk groups, such as people who are genetically vulnerable to depression or those going through stressful situations and hope to develop a better understanding of physical activity to promote resilience to depression," says Choi.

The new study was published in the journal JAMA Psychiatry.