Flickr CreativeCommons
Source: Flickr CreativeCommons

Exercise and fresh air have always been folk prescriptions for a glum mood. And most of us have, indeed, felt cheerier after a nice walk or a fun workout. But clinical depression is a different story—could regular exercise prevent a full on depressive episode from occurring? Researchers aimed to find out in a study published this week online in the American Journal of Psychiatry.

Here’s the theory of how exercise might help treat or prevent depression or anxiety: We’ve pretty much settled that a lot of clinical depression and anxiety are caused by chronic or severe acute stress, leading to a state of inflammation in the body and brain. When it comes to physiology, this means the sympathetic “fight or flight” nervous system is activated too strongly for too long, and the parasympathetic “rest and digest” nervous system doesn’t come online robustly enough to undo all the stress from the “fight or flight” overactivity. Exercise, via parasympathetic vasovagal activation during the recovery period, can, in real time, strengthen that “rest and digest” action. However, you need quite a bit of exercise to really engage the parasympathetic recovery: To get the famous “endorphin” effect you need quite a bit of high-intensity exercise, like a long and fast run, and for a pharmacologic effect it would need to be fairly frequent. In theory, exercise should work better for anxiety, which is more directly related to stress, than for clinical depression (though the two often come together). 

So what did the researchers in the HUNT protocol do? They designed a very large prospective study following thousands of participants in Norway for 9-13 years. The subjects were found by inviting the population of an entire county, aged 20 or older, to fill out a basic screening form for depression and anxiety. The happiest 70% were asked to participate in order to get a healthy baseline (and an additional 8,400 or so were excluded for serious physical illness), and of those, nearly 34,000 were included in the study. Other data, such as smoking status, social supports, BMI, resting heart rate, and demographic data were collected at various visits over the years. All in all about 22,500 completed the study. 

Large prospective observational studies like this one can’t truly prove causal relationships. (Does exercise truly prevent or treat depression? You need a randomized trial for that sort of thing.) But they are interesting, because they can teach us about trends one way or another, or sometimes disprove a causal relationship.

Flickr Creative Commons
Source: Flickr Creative Commons

Once all the numbers were crunched, the researchers ended up with a surprising finding. Those who exercised were, indeed, less likely to develop depression over those 9-13 years than those who didn’t. The weird thing is, it didn’t matter how much exercise as long as you did some deliberate physical activity for at least an hour each week. Yes, that’s right, only one hour total for the entire week. And it also didn’t matter how intense that exercise was…could be a walk in the park or three sessions of intense spinning class or a daily 5K run. This finding belies the “parasympathetic vasovagal reactivity” theory of how exercise might prevent depression by stopping the sympathetic overactivity that characterizes the disease. Also, exercise was not related to lower risk for the development of anxiety disorders—only depression.

All in all, in this very large group, the researchers figured exercise may have prevented 12% of the cases of depression from happening. Given the enormous burden of depressive disorders all over the world, this is a lot of cases of depression.

What do we really learn from this study? The researchers postulate that it’s the social benefits of exercise—getting out and about, feeling good about oneself, self care, that sort of thing—that may make the real difference, though the number-crunchers felt that a lot of the mechanism and wherefore and whys remain unexplained. We also know that people who develop a clinical depression have less get up and go, less motivation, less of the frontal lobe's saying “get out of bed and get to the gym,” so once a depression kicks in, exercise is likely to drop as a matter of course. The same isn’t really true for anxiety. 

Also, straight up parasympathetic training and sprint intervals aren’t the likely mechanism for preventing depression or anxiety, but anyone is welcome to do these more intense exercises for other reasons.

The real takeaway is that the first thing that tends to drop off our schedule if we are sick or tired or overwhelmed is that trip to the gym or early morning walk around the block to get the day started. However, this data suggests that keeping up even a brief but regular schedule of light activity may lower the chances of developing depression in the future. If you have a free half hour, stop by the park, take a walk by the river. Or pull up YouTube and run through your favorite cardio kick-boxing routine for 15 minutes before work or school. Twelve percent of depression cases prevented? That’s a significant amount, and you may not have to be a gym rat to make it happen.

Images: Yoga mats; Child

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Copyright Emily Deans MD

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