Exercise and wellbeing have historically gone hand in hand, but scientists could never define the exact relationship between exercise and depression. What causes what? Does lack of exercise lead to depression? Or does depression lead to lack of exercise?
Researchers from the Psychiatric & Neurodevelopmental Genetics Unit at Mass General recently published a study in JAMA Psychiatry that set out to gain a deeper understanding of the relationship between the two. The research team, led by Jordan Smoller, MD, ScD, and Karmel Choi, PhD, found that exercise serves as a protective factor against depression, and that it may serve as an effective means of preventing it.
The problem
According to the Centers for Disease Control, depression affects approximately one in six people at some point in their life. Since common symptoms include feeling tired or withdrawing from activities, many scientists have been eager to test whether increasing activity levels could help.
However, the issue becomes complicated when it comes to pinpointing the exact cause of depression and comparing it to an individual’s rate of exercise and its effects. Some believe being an overall less active person increases risk for depression, while others believe depression causes people to be less active.
Scientists often run into issues when trying to measure activity because if study participants are asked to report their own levels of activity, there is a chance of bias and confounding.
Bias can occur if someone does not accurately report their activity levels, which could be for a variety of reasons such as poor memory, mood or simply wanting to have the “correct” answer. The inaccurate answers become problematic because they can lead to confounding, which is when one variable influences both variables being tested in a way that cannot be controlled. Consider the following:
How could a researcher come to an accurate conclusion if they aren’t sure their information is correct?
“Knowing whether an associated factor actually causes an outcome is important, because we want to invest in preventive strategies that really work.”
Karmel Choi, PhD
The outcome
Since answers to a survey can be subjective, the research team decided to include objective variables that are harder to manipulate: activity monitors and genetics.
Researchers turned to a technique called Mendelian randomization, which treats genetic differences between people as a kind of natural experiment to answer causal questions. Using this technique, they drew on data from prior genome-wide studies in over 600,000 people to test whether exercise affects depression, or vice versa.
As expected, the team could not find a strong connection between self-reported activity levels and depression, but things changed once they looked at the objectively-measured data from the activity monitors.
The team took several gene variants that were linked to a person’s exercise levels and looked at how these variants were linked to a person’s likelihood to develop depression. By mathematically combining this information, they drew the conclusion that more exercise could lead to lower risk of developing depression.
On the other hand, doing the same analysis in the opposite direction using variants linked to depression, they didn’t find that depression necessarily made people less active.
But how much exercise is enough to help prevent depression?
When the team dug a little deeper, they found that approximately 15 minutes of intense physical activity (e.g. running) or about an hour of light to moderate activity (e.g. walking or household chores) could be sufficient in boosting mood and protecting against depression.
Moving forward
Depression is one of the most common mental illnesses in the United States, according to the National Institute of Mental Health, and finding the right treatment and having access to said treatment can be tricky.
While more work needs to be done, the team is hopeful their study is a step in the right direction. By discovering a one-way relationship between exercise and depression, they have identified a new preventative measure that is accessible to a wide variety of individuals and has shown to produce positive results.
“Physical activity is good for a lot of things. It may have benefits not only for all kinds of aspects of your health, but also, it looks like, your risk of developing depression.”
Jordan Smoller, MD, ScD
Jordan Smoller, MD, ScD, is a Physician Investigator in the Department of Psychiatry at Massachusetts General Hospital, Professor of Psychiatry at Harvard Medical School and a Tepper Family MGH Research Scholar 2014-2019.
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Research at Massachusetts General Hospital is interwoven through more than 30 different departments, centers and institutes. Our research includes fundamental, lab-based science; clinical trials to test new drugs, devices and diagnostic tools; and community and population-based research to improve health outcomes across populations and eliminate disparities in care.
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