Risk of Depression Decreases with Physical Activity
Whether it be walking around the neighborhood for 30 minutes per day or running for miles on end every weekend, we have often heard that moderate, but regular exercise is a key factor in prolonging a healthy, disease-free life. I wrote an article a few years back that touted the link between exercise, anxiety, and depression. It turns out that physical exercise is not only great for your cardiovascular health, but it is also beneficial for your mental health and well-being.
A systematic review and meta-analysis came out recently in the Journal of the American Medical Association (JAMA) which highlights the association between physical activity and the risk of depression. The study by Pearce et. al 2022, looked at a total of fifteen studies comprising of 191,130 participants. The researchers observed an inverse dose-response association between physical activity and depression, with steeper association gradients at lower activity volumes. Adults accumulating the recommended volume of 8.8 mMET (marginal metabolic equivalent task hours) per week had 25% lower risk with diminishing potential mental health benefits and higher uncertainty observed beyond that 8.8 mMET level. The metabolic equivalent of task is the objective measure of the ratio of the rate at which a person expends energy, relative to the mass of that person, while performing some specific physical activity compared to a reference, roughly equivalent to the energy expended when sitting quietly. In other words, 8.8 mMET per week seemed to be the sweet spot for the group observed, with mMET > 8.8 hrs/week, providing diminishing additional potential mental health benefits.
Based on the results above, it seems that even physical activity at levels below the public health recommendations can provide significant mental health benefits, and a reduction of depression risk. If you are skeptical that this is true, here are the two reasons why this makes sense.
Firstly, exercise causes our brain to release endorphins. Endorphins are chemicals released in our nervous system which bind to the body’s opiate receptors, causing an analgesic effect. If you’ve heard of or experienced “runner’s high”, which is a brief, deeply relaxing state of euphoria, it occurs due to a release of endorphins in the body. This release of endorphins protects your brain from feeling depressed or anxious.
Secondly, physical activity takes your mind off things that you may be worrying about. Going for a walk or run can help break you out of a cycle of rumination which feeds depression and anxiety. Next time you are on a walk or run, start being more mindful about your surroundings. Focus in on the people you are passing by, the trees around you, the path you are stepping on – focus in on the details.
Several laboratory studies have demonstrated a beneficial effect of regular exercise on important brain chemicals. Researchers found that mice with free access to a running wheel versus those which did not, had significantly higher levels of brain-derived neurotrophic factor (BDNF) production. Not only does BDNF promote regional brain growth (so called neuroplasticity), but it is also associated with cognitive improvement and the alleviation of depression and anxiety. The main target of BDNF is in the hippocampus which exerts an inhibitory influence on stress and emotion regulation systems in the brain.
Another well known effect of regular aerobic exercise is on the resting activity of the vagal nerve (vagal tone). People with anxiety, depression and chronic stress tend to have a reduction in cardiovagal tone and a lower heart rate variability. Some studies have shown that reduced vagal tone is a risk factor for heart disease. Vagal tone is regulated by brain regions involved in emotion regulation and central autonomic control, and the effect of regular exercise on the responsiveness of these regions is reflected in an increase in vagal tone. Regular exercise, like abdominal breathing are simple ways to increase your vagal tone and influence emotion regulating brain networks.
Regular exercise also has a beneficial effect on the gut microbiome and on gut microbiome brain interactions. Scientists found that exercise training alters the composition and functional capacity of the gut microbiota, independent of diet. While extreme forms of exercise, such as ultramarathons and triathlons can have a negative effect on gut health and systemic immune activation, ultimately affecting the brain, regular moderate exercise may reduce neuroinflammation in the brain and associated symptoms of fatigue and negative mood.
Don’t get me wrong, SSRI’s are highly effective medications to treat depression and have saved thousands of people’s lives. While exercise is a promising addition to treat mild anxiety and depression, it can only be considered an adjuvant treatment for severe cases. According to Dr. Michael Craig Miller, assistant professor of psychiatry at Harvard Medical School, “For some people, [exercise] works as well as antidepressants, although exercise alone isn’t enough for someone with severe depression.”
In summary, regular aerobic exercise has been shown to positively affect the brain, mood states, stress responsiveness, and even the gut microbiome. Exercise is a great addition to your lifestyle to treat mild anxiety and depression. Over hundreds of thousands of years of evolution, humans have been optimized to run, and are rewarded for this physical activity by not only feeling good, but by receiving many unexpected health benefits as well.
E. Dylan Mayer is a graduate from the University of Colorado at Boulder with both a major in Neuroscience and minor in Business. He is fascinated by the interactions of brain, gut and microbiome, and the role of nutrition in influencing the health of our microbiome, as well as our own well-being. He will begin his Master’s Degree in Human Nutrition at Columbia University this Fall.