The positive effects of exercise on depression are well known, but how it produces its anti-depressant effects is unknown. After reviewing a wide range of studies on how depression and physical activity affect motivation, researchers have now come up with a hypothesis.
People suffering from depression may experience apathy, lack of enthusiasm, and anhedonia, or an inability to derive pleasure or enjoyment from activities they normally enjoy. Both contribute to decreased motivation that can prevent people from participating in things like work and social activities.
It is known that physical activity, especially aerobic exercise, can reduce or even prevent depressive symptoms, but How Whether this is the case or not remains unclear. Researchers from University College London (UCL) have reviewed a wide range of human and animal studies examining the mechanisms underlying depression and exercise, and have proposed a new hypothesis for the antidepressant effects of exercise.
“The antidepressant effect of aerobic exercise has been convincingly demonstrated in randomised controlled trials, but its mechanism is not well understood,” says Dr Emily Hird, from the Institute of Cognitive Neuroscience at UCL and the study's leader and corresponding author. “This is partly because it probably involves a range of biological and psychological processes.”
Previous studies have suggested that anhedonia is associated with a disruption in the brain’s reward pathways. These pathways are activated when we experience something rewarding, such as delicious food, sex, or addictive drugs. When we are exposed to a rewarding stimulus, the brain responds by releasing more dopamine, which provides a sense of pleasure and motivation to do something again because it feels good. The disruption of these pathways in depressed people means they are less willing to put in the effort—less motivated—to get a reward.
Research has also shown that depression, particularly anhedonia, is associated with inflammation, and inflammation, which is responsible for a wide range of medical conditions, disrupts dopamine activity. Studies have found that blocking the inflammatory chemicals the body produces reduces depressive symptoms and anhedonia. In summary, evidence suggests that inflammation reduces dopamine transmission, which reduces motivation in response to the mental and/or physical effort required to obtain a reward, leading to anhedonia and fatigue. On the other hand, researchers have found good evidence that physical activity reduces inflammation and improves dopamine function, reward processing, effort, and motivation.
“[A]“In addition to its positive effects on inflammation, dopamine, and reward processing, exercise also reduces oxidative stress and increases self-esteem and self-efficacy,” Hird said. “However, we propose that exercise—especially aerobic activities that make you sweat and get out of breath—reduces inflammation and increases dopamine transmission, which increases willingness to exert effort and therefore increases motivation in general.”
The researchers hope their hypothesis will shed light on new treatment strategies, such as personalized exercise programs for depression.
“Understanding the mechanisms underlying the antidepressant effects of physical activity in depression may also contribute to our understanding of the mechanisms that cause depression and to the development of new intervention strategies, particularly personalised intervention and social prescribing,” said Hird.
More studies using large randomized controlled trials are needed to assess the impact of exercise on depression and to measure its impact on variables such as inflammation, dopamine transmission, and motivation. To this end, researchers are conducting a trial involving 250 participants aged 18 to 60.
“Addressing barriers to exercise is crucial – particularly in people with depression – as regular physical activity can relieve symptoms, improve mood and empower individuals on the road to recovery,” Hird said. “As part of this, finding strategies to encourage exercise is crucial.”
The study was published in the journal Translation Psychiatry.
Source: UCL