There is a distinct difference between "physical activity can reduce the effects of depression" and "depression causes a decrease in the levels of physical activity." The Massachusetts General Hospital (MGH) researchers sought to determine what causal relationship applied to exercise and mood. They published their findings in the journal JAMA Psychiatry.
To achieve this end, they employed Mendelian randomization, an approach which used gene variants to analyze the effects of a non-genetic factor. They looked at the genetic variation of people and then checked if there was any connection between a particular gene variant and certain levels of physical activity.
Genetic variants are usually inherited at random. This randomness reduced the potential bias in participants, thereby increasing the accuracy of the experimental outcome.
Furthermore, the Mendelian randomization approach could identify causative traits. It could show which trait led to another, making it a good way of figuring out if exercise affected depression or vice versa. (Related: Prevent depression with one hour per week: Landmark study proves importance of consistent exercise.)
For data on gene variants, the MGH researchers tapped genome-wide association studies conducted in the U.K. and around the world. The U.K.-based studies focused on physical activity, while the global ones covered depression.
The data of the physical activity-centered studies came from two different populations: The self-reported levels by a total of 377,000 participants, and the readings from accelerometers worn by more than 91,000 people. Meanwhile, the depression-focused studies compiled data from 143,000 participants who were either healthy or diagnosed with major depressive disorder.
The researchers evaluated the information from the different studies and compared them. They found that the accelerometer-measured data lent credence to the theory that physical activity protected against depression.
However, analysis of the self-reported activity data did not produce similar results. The researchers attributed this dissimilarity to several factors.
First, the participants might have been mistaken about their memories regarding their levels of physical activity. Second, they could have been trying to make themselves look good during the study.
Last and possibly most likely, objective sensors like accelerometers could have been measuring movements that were not part of physical exercise sessions but still counted as physical activity. Climbing stairs, lawn mowing, and walking to destinations are examples of surprisingly healthy physical exertions.
The MGH researchers observed that higher levels of physical activity reduced the chances of depression. Furthermore, the intensity of the activity did not really matter, although less vigorous exercises would need to last longer in order to obtain health benefits.
Based on their calculations, a 15-minute-long period of strenuous physical exercise would be enough to trigger the protective effects against depression. If the patient is somehow limited to less vigorous exercises, he or she must maintain those activity levels for at least one hour.
Of course, there is the challenge of persuading people to increase their physical activity. Merely saying that moving more often is good for them will not be persuasive enough.
"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," explained MGH lead researcher Dr. Karmel Choi.