Does physical activity reduce depression, or does depression reduce physical activity?
It’s a quintessential chicken and egg scenario — and a question that’s plagued scientists for some time.
Now, thanks to the power of modern genomics, a new study published in JAMA Psychiatry provides the “strongest evidence” yet that exercise has a protective effect against depression.
Using the genetic data of 300,000 adults, researchers at Massachusetts General Hospital found people with higher levels of physical activity had lower odds of major depressive disorder, according to lead researcher Karmel Choi.
“We found evidence that higher levels of physical activity may causally reduce risk for depression,” Dr Choi said.
In fact, the research shows that replacing sedentary behaviour with 15 minutes of vigorous activity each day can reduce depression risk by roughly 26 per cent.
“On average, doing more physical activity appears to protect against developing depression … and any activity appears to be better than none.”
While the study showed physical activity could prevent depression, it found no evidence that being diagnosed with depression affected a person’s ability to exercise.
But people diagnosed with depression are still at an increased risk of reduced physical activity, according to Joseph Firth, a senior research fellow at Western Sydney University who was not involved in the study.
“It’s still the case that people with depression are less active than the general population, but [the study] is saying it’s not necessarily the depression itself that’s driving that relationship,” Dr Firth said.
“It could be social factors, rather than the actual genetics of depression.
“So, it’s still worth thinking about physical activity interventions for people with depression.”
Using exercise as prevention
Dr Firth said the study provided “the strongest evidence” to date for using exercise as a potential strategy to reduce the risk of depression across the general population.
“Depression is generally regarded as an epidemic, particularly across Western societies — many countries are struggling with high rates of it,” he said.
“These findings could ultimately inform new public health schemes, which use physical activity and exercise to not only reduce the risk of physical health problems, but also to combat the mental health epidemic.”
Dr Choi said it was one thing to know that physical activity could be beneficial for preventing depression, and another to actually get people to be physically active.
“More work needs to be done to figure out how best to tailor recommendations to different kinds of people with different risk profiles,” she said.
“We currently are looking at whether and how much physical activity can benefit different at-risk groups, such as people who are genetically vulnerable to depression or those going through stressful situations.
“[We] hope to develop a better understanding of physical activity to promote resilience to depression.”
Using a genetic framework
For years, research has demonstrated an association between increased physical activity and a reduced risk of depression.
However, until now it has been difficult to establish a clear “cause and effect” relationship, and to conclusively rule out other confounding variables.
So, researchers turned to their attention to genetics — since our genes are randomly assigned to us before birth, and (largely) independent of environmental and social factors.
The team looked at people who carry genetic variants associated with increased physical activity, and whether these variants impacted their risk of depression.
Their reasoning was that, if exercise does reduce the incidence of depression, then people carrying gene variants that predispose them to exercise should proportionally be less likely to get depressed.
“If A causes B in the real world, any factor that influences A should also influence B in a similar way,” Dr Choi said.
And that was exactly what researchers found: higher levels of physical activity (objectively measured, and indicated by associated gene variants) were linked to lower levels of depression.
The findings were not replicated when people self-reported physical activity, suggesting people may not be good at accurately reporting their true levels of exercise.
“Knowing whether an associated factor actually causes an outcome is important, because we want to invest in preventive strategies that really work,” Dr Choi said.
It’s not a two-way street
To work out whether the relationship between exercise and depression goes both ways, researchers also looked at whether genetic variants that predispose people to depression were associated with changes in rates of physical activity.
Researchers found this was not the case — there was no evidence to support the idea that depression itself reduces rates of exercise.
“We did not see such a pattern in the genetic data, regardless of how physical activity was measured,” Dr Choi said.
Alex Parker, a professor of physical activity and mental health at Victoria University, said the study’s finding that replacing sedentary behaviour with movement could reduce depression risk was in line with previous research.
“There’s definitely some similarity between what the authors proposed in this study as conferring a proactive effect … with what we know from the bulk of the studies that have looked at exercise for treating depression,” Professor Parker said.
“There seems to be a fair way to go in terms of being certain about the dose of exercise required for anti-depressant effects.
“But there is a suggestion that to get mental health benefits from engaging in physical activity — the dose of activity might be less than it is to get a physical health benefit.”