There is growing evidence that exercise can help to treat symptoms of depression and lift mood, although the reasons have been unclear. Now, a new study finds that the body's endocannabinoid system may play a role.
The researchers propose that a better appreciation of the relationship between exercise, mood, and the endocannabinoid system could result in better treatments for depression.
The study investigated changes in mood and endocannabinoid levels in people with major depression who exercised at different intensities on a stationary bike.
The investigators found a significant increase in endocannabinoid levels and improvement in mood after a session of prescribed exercise of moderate intensity.
The endocannabinoid system is a cell signaling system comprising the body's own cannabinoids, or endocannabinoids, and receptors. It extends throughout the body and influences the function of the immune, cardiovascular, and nervous systems.
When a particular endocannabinoid binds to a matching cell receptor, it sends a signal into the cell that alters its behavior and functioning.
Need to find new treatments for depression
In a recent paper about the research, which features in the Medicine & Science in Sports & Exercise journal, the authors conclude that the mood-enhancing effects of exercise likely involve several biological mechanisms, of which the endocannabinoid system is just one.
"Finding alternatives to medication is important for the treatment of depression," says first study author Jacob D. Meyer, Ph.D., an assistant professor of kinesiology at Iowa State University, in Ames.
"If we can figure out how exercise works with the endocannabinoid system," he adds, "we could then design optimal exercise interventions."
Feeling sad or lonely can be a part of the everyday ups and downs of life, especially when times are hard due to loss or stress. Depression, however, is a condition in which these and other symptoms persist.
According to information from the Centers for Disease Control and Prevention (CDC), depression affected 8.1% of adults aged 20 and older in the United States during 2013–2016.
Major depression is a treatable, common condition in which changes in mood and other mental and physical symptoms last for at least 2 weeks.
Prescribed versus preferred exercise
For the new study, the team investigated 17 adult females aged between 20 and 60 years with a current diagnosis of major depression.
Each participant completed two 30-minute exercise sessions on a stationary bike. In one session, the participant cycled at a prescribed moderate intensity, and in the other session, they could cycle freely at any intensity that they preferred.
All the participants completed both types of session, with a gap of at least 1 week between them.
The researchers took blood samples before and immediately after the sessions. In addition, the participants filled in questionnaires before and at 10 minutes and 30 minutes after the sessions.
The results showed that mood improved after both sessions, but that a rise in endocannabinoid levels only occurred after the prescribed moderate-intensity exercise session. There was no such rise following the preferred-intensity session.
Meyer says that he and his colleagues found the results of the preferred-intensity session surprising.
One explanation could be the small number of participants and the variation in intensity levels in the preferred-intensity session. Some participants completed the preferred session at a constant, light intensity, while others varied the intensity.
Another explanation for the difference in results between the preferred and prescribed exercise sessions could be that exercising at a level that someone else prescribes has a psychological as well as a biological effect.
Lasting effect of moderate-intensity exercise
Another significant result of the study was the long lasting effect of moderate-intensity exercise.
The increase in endocannabinoid levels appeared to correlate to mood improvements up to 30 minutes after the session.
This would suggest, says Meyer, that the endocannabinoid system was contributing to the mood-lifting effects of the prescribed exercise.
"These results show us that the effect of prescribed moderate-intensity exercise on mood is generally pretty quick and lasts for a while at a relatively sustained rate," he concludes.
"However, given that people felt better after the preferred exercise session, even though it didn't change endocannabinoid levels, [it suggests that] multiple factors are likely involved in how exercise makes people feel better."
Jacob D. Meyer, Ph.D.