Numerous studies have found that exercise boosts mood, lowers stress levels and even improves cognitive functions like attention, memory and problem solving. These benefits may be rooted, in part, in exercise's ability to increase our brains' production of the chemicals dopamine and serotonin, according to a review of studies published in the journal Brain Plasticity in March 2017.
Dopamine is a neurotransmitter that's critical in regulating motivation, memory, reward and attention, while serotonin affects mood, emotion, sleep and appetite.
Regular exercisers are likely familiar with the feel-good effects of a good workout. And these effects aren't just limited to otherwise healthy people: Exercise can help alleviate symptoms in many patients with mental health disorders. For example, a January 2019 meta-analysis of 11 trials, published in Depression and Anxiety, concluded that regular aerobic exercise was an effective antidepressant intervention for adults diagnosed with major depression.
"There are a host of studies to show that exercise helps anxiety and depression," says Philip R. Muskin, MD, chair of the scientific program committee of the American Psychiatric Association and professor of psychiatry at Columbia University Irving Medical Center. However, he adds, it's important to recognize that exercise may not help in cases of severe depression.
How Exercise Affects Dopamine and Serotonin
It's not well understood exactly how exercise improves mental health, but some research points to its effect on dopamine and serotonin function.
Because it's difficult to directly measure neurotransmitter changes in the human brain, much of the research on exercise and dopamine, as well as on exercise and serotonin, comes from animal studies.
According to the March 2017 Brain Plasticity study review, in rodent studies, researchers have found elevated dopamine levels in several brain regions, including the hippocampus, prefrontal cortex, striatum, midbrain and pons-medulla, during exercise and for approximately two hours afterward. Meanwhile, when it comes to serotonin, in rodents, exercise has been found to increase serotonin in the frontal cortex, hippocampus, striatum and midbrain. Some studies, however, have found no increase, perhaps indicating that a certain intensity or duration of exercise is required to boost serotonin.
In human studies, a bout of exercise has been shown to increase dopamine and serotonin in the blood. And some studies have linked these changes to mental benefits. For example, in an October 2016 study published in the European Journal of Applied Physiology, healthy young adults had their blood drawn and performed an attention test before and after about 30 minutes of exercise. Post-workout, serotonin levels in the blood were elevated and participants did better on the test. Furthermore, the participants who improved most on the test also had the greatest increases in serotonin.
However, it's still unclear how these changes in neurotransmitter levels in the blood translate into changes in the brain. One of the few ways to measure neurochemical changes in the human brain is positron emission tomography (PET). In a small but landmark study published in the Journal of Nuclear Medicine in August 2000, researchers used PET scans to assess the effects of 30 minutes of treadmill running on dopamine concentrations in 12 healthy, regular exercisers. Surprisingly, they didn't find the running to increase dopamine levels, but some researchers suggest that the exercise protocol in the study wasn't intense enough.
In addition, many researchers suspect that changes in relative serotonin and dopamine levels may be responsible for the feeling of fatigue that hits after prolonged exercise, according to the Brain Plasticity analysis. The theory is that, while levels of both neurotransmitters initially increase, dopamine levels eventually start to drop off while serotonin levels are still elevated, causing exhaustion sets in.
Beyond Serotonin and Dopamine: Explaining the 'Runner's High'
It's unlikely that exercise's psychological benefits are rooted solely in how it influences serotonin and dopamine. Exercise has potent effects on a range of other neurochemicals as well.
Researchers suspect that much of the mood-boosting power of exercise is due to the its effect on endorphins and other neuromodulators involved in the endogenous opioid system. The opioid system is important in pain modulation, reward, response to stress and autonomic control. In both humans and animals, such natural opioids are increased in the blood after exercise.
In a seminal November 2008 study published in Cerebral Cortex, researchers used PET to explore what was happening in the brains of 10 trained male athletes: They found that mood was improved and opioid concentrations were significantly increased in many brain areas after 30 minutes of running.
And while endorphins have traditionally gotten most of the credit for producing the so-called "runner's high," more recent research has uncovered an important role for endocannabinoids. This family of lipids — which activate the same receptors as the THC in marijuana does — are also increased in the blood after exercise, according to the analysis in Brain Plasticity.
Adapting to Stress and Stimulating New Neurons
When it comes to long-term benefits, exercise's impact on the body's stress response system may be important too. Exercise activates the hypothalamic-pituitary-adrenal (HPA) axis and stimulates the release of cortisol. One hypothesis is that this primes people to adapt to stress, so that they are protected from the negative physiological effects of future stressors.
For example, in a January 2015 study published in Psychoneuroendocrinology, young men who had done 30 minutes of aerobic exercise had a less pronounced cortisol spike and showed fewer physiological markers of stress when they later underwent a test designed to psychologically stress them out.
Exercise also increases the blood levels of various neurotrophic factors that encourage the creation of new neurons. In two rodent studies — a paper published February 2018 in Frontiers in Neuroscience, and a September 2016 meta-analysis in PLoS One — researchers linked exercise-induced increases in brain-derived neurotrophic factor (BDNF) to the growth of neurons in the hippocampus, an area of the brain that's critical for memory and learning.
Though it is much more difficult to study this process in humans, many researchers believe this ability to promote new neurons may contribute to exercise's antidepressant effect. It may also explain why regular exercise seems to help combat cognitive decline due to aging or neurodegenerative diseases like Alzheimer's disease, according to a May 2017 study in Current Neuropharmacology.
While there are still large gaps in our understanding of the exact mechanisms by which exercise changes the brain, there's little doubt about its benefits. Which is why Dr. Muskin recommends aerobic exercise to all of his patients, and the more assertive the better, as long as the person is healthy enough. "Taking a walk is good for you — taking a run can have anti-anxiety and anti-depressant effects."
As the authors of the February 2018 review article in Frontiers in Neuroscience concluded, "Exercise, when pursued in moderation, not only serves as a robust method for improving physical health, but also serves as a preventative and protective measure against numerous neurological and mental diseases."
- Frontier in Psychology: "Neuromodulation of Aerobic Exercise — A Review"
- Frontiers in Neuroscience: "Exercise-Mediated Neurogenesis in the Hippocampus via BDNF"
- Current Neuropharmacology: "The Long Run: Neuroprotective Effects of Physical Exercise on Adult Neurogenesis from Youth to Old Age"
- Brain Plasticity: "The Effects of Acute Exercise on Mood, Cognition, Neurophysiology, and Neurochemical Pathways: A Review"
- Depression and Anxiety: "Aerobic exercise for adult patients with major depressive disorder in mental health services: A systematic review and meta-analysis"
- European Journal of Applied Physiology: "The effects of different aerobic exercise intensities on serum serotonin concentrations and their association with Stroop task performance: a randomized controlled trial"
- Journal of Nuclear Medicine: "PET studies of the effects of aerobic exercise on human striatal dopamine release"
- Cerebral Cortex: "The Runner's High: Opioidergic Mechanisms in the Human Brain"
- Psychoneuroendocrinology: "The stress-buffering effect of acute exercise: Evidence for HPA axis negative feedback"
- PLoS One: "The Effect of Exercise Training on Resting Concentrations of Peripheral Brain-Derived Neurotrophic Factor (BDNF): A Meta-Analysis"