Exercise’s positive effect on depression is well-known, but not how it produces its anti-depressant effects. After reviewing a broad range of studies into how depression and physical activity affect motivation, researchers have now come up with a hypothesis.
People suffering from depression can experience apathy, a lack of enthusiasm, and anhedonia, the inability to derive joy or pleasure from activities that they’d usually find pleasurable. Both contribute to a decrease in motivation that can prevent people from engaging in things like work and social activities.
It’s known that physical activity, especially aerobic exercise, can reduce or even prevent depressive symptoms, but how it does so remains unclear. Researchers from University College London (UCL) reviewed a broad range of human and animal studies that explored the mechanisms underpinning depression and exercise and have proposed a novel hypothesis for exercise’s antidepressant effects.
“The antidepressant effect of aerobic exercise has been convincingly demonstrated through randomized controlled trials, but its mechanism is not well understood,” said Dr Emily Hird from the Institute of Cognitive Neuroscience at UCL and the study’s lead and corresponding author. “This is, in part, because it likely involves a variety of biological and psychological processes."
Previous studies have suggested that anhedonia is associated with a disruption of the brain’s reward pathways. These pathways are activated whenever we experience something rewarding, like nice-tasting food, sex, or addictive drugs. When exposed to a rewarding stimulus, the brain responds by releasing more dopamine, which provides a sense of pleasure and the motivation to do something again because it feels good. The disruption to these pathways in depressed people means that they’re less willing – less motivated – to expend effort to receive a reward.
Research has also shown that depression, particularly anhedonia, is associated with inflammation and that inflammation, which is responsible for a wide range of medical conditions, disrupts dopamine activity. Studies have found that inhibiting the inflammatory chemicals the body produces reduces depressive symptoms and anhedonia. In summary, the evidence suggested that inflammation reduced dopamine transmission, which decreased motivation in response to the mental and/or physical effort required to achieve a reward, leading to anhedonia and fatigue. On the other hand, the researchers found good evidence that physical activity reduced inflammation and improved dopamine functioning, reward processing, effort exertion, and motivation.
“[A]longside its positive effect on inflammation, dopamine and reward processing, exercise also reduces oxidative stress and improves self-esteem and self-efficacy,” Hird said. “However, we are proposing that exercise – particularly aerobic activities that make you sweaty and out of breath – decreases inflammation and boosts dopamine transmission, which in turn increases the desire to exert effort, and therefore boosts motivation generally.”
The researchers hope their hypothesis will inform new treatment strategies for depression, like personalized exercise programs.
“Understanding the mechanisms that underly the antidepressant effects of physical activity in depression could also inform our understanding of the mechanisms causing depression and the development of novel intervention strategies, in particular personalized intervention and social prescribing,” said Hird.
Further study using large randomized controlled trials is needed to assess the effect of exercise on depression and measure its effect on variables including inflammation, dopamine transmission, and motivation. To that end, the researchers are running a trial involving 250 participants aged 18 to 60.
“Addressing barriers to exercise – particularly in people with depression – is crucial, as regular physical activity may be able to alleviate symptoms, enhance mood and empower individuals on their path to recovery,” said Hird. “As part of this, finding strategies to encourage exercise is key.”
The study was published in the journal Translational Psychiatry.
Source: UCL