Study under review: Assessment of Bidirectional Relationships Between Physical Activity and Depression Among Adults: A 2-Sample Mendelian Randomization Study
Exercise has been shown to improve mood and sleep and even boost memory and thinking, perhaps through its ability to reduce insulin resistance, reduce inflammation, and stimulate the release of neurotrophic growth factors in the brain. In support of these neurotrophic effects of exercise, numerous studies have observed that the prefrontal cortex and medial temporal cortex (areas of the brain that control thinking and memory) are often larger in volume in people who exercise compared to those who do not exercise. Luckily for people who do not currently exercise, just six months of regular aerobic exercise performed at moderate intensity can increase the volume of select parts of the brain.
Increasing evidence points to size and structural changes in brain areas involved in mood, memory, and decision-making during depressive episodes. Initial evidence suggests that this remodeling of the brain can be prevented and potentially reversed with antidepressant and mood-stabilizing medications. Physical activity may offer similar therapeutic benefits without the side effects, as exercise has also been shown to improve the brain’s capacity to produce and absorb the neurotransmitters serotonin and dopamine, both of which are believed to play a role in depressed brains. Additionally, exercise reduces activity of stress hormones and stimulates the growth of new brain cells. Exercise has also been shown to partially reverse the age-related volume loss of the hippocampus, the area of the brain responsible for processing long-term memory and emotional response.
Meta-analyses of randomized controlled trials have linked physical activity to reduced depressive symptoms in people with depression, but these don’t necessarily speak to whether physical activity can prevent depression. On the other hand, prospective studies have found associations between higher levels of physical activity and reduced risk for developing depression later on, but these studies can’t establish a causal link between exercising and reducing depression risk, since depression itself can lead to less physical activity, and so may its risk factors.
To help determine whether people who exercise are at lower risk of depression or whether people at risk for depression exercise less, the study under review used genetic data in a Mendelian randomization analysis to examine the relationship between physical activity and depression.
Depression is associated with lower levels of physical activity, but it is unclear whether individuals who do not exercise are at greater risk for depression or whether individuals who are at risk for depression are at greater risk for inactivity. The study under review used a Mendelian randomization analysis to help determine the path of causality.
Other Articles in Issue #54 (April 2019)
Mini: What’s magnesium good for?
In this Mini, we give a quick summary of a recent umbrella review's findings concerning which of magnesium's many purported effects are most well supported by the evidence.
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A D-fence against cancer?
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Mini: Folic acid intake and neural tube defects
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The (mild) health risks of energy drinks
Energy drinks can make a small, potentially negative impact in certain metabolic measures in young, relatively healthy people. But do these changes really matter?
Peppermint oil soothes symptoms of irritable bowel syndrome
Enteric coated peppermint oil may help relieve some of the symptoms of IBS according to this recent meta-analysis, at least in the short term.