The case that exercise cuts depression is stronger than ever

By Sam Downing|

More than 1 million Australians are hit by depression every year. Treatments vary, but health professionals routinely suggest exercise to alleviate the condition.

That's founded on a number of studies suggesting physical activity not only treats depression, but makes it less likely to develop in the first place.

But the link between physical activity and mental health is less cut-and-dried and more chicken-and-egg. Are those who move more really less likely to be depressed — or does depression make people less likely to move more?

A study published today in the journal JAMA Psychiatry asserts it's the former, strengthening the idea that movement is as important for mental health as for physical health.

"Our study supports the idea that physical activity has a causal role in reducing risk for depression," the study's lead author Dr Karmel Choi, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health, told Coach.

"Our findings add a new layer of evidence that is not only consistent with previous research but also makes a unique case that physical activity can make a difference for depression."

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Choi and her team analysed genetic data drawn from more than 300,000 people (including Australians) in the UK Biobank study, using a statistical method called Mendelian randomisation.

"In a Mendelian randomisation study, we can think of gene variants like a natural experiment where some people are 'assigned', on average, to higher versus lower levels of physical activity in their lifetime," Choi said.

"Because we inherit our gene variants somewhat randomly before we are born, we can use the variants associated with physical activity as proxies to estimate the true, non-genetic relationship between physical activity and depression."

As explained in a JAMA editorial on the study, that means that if exercise directly reduces depression, you'd expect to find that people with genes which make them prone to exercise more should be less likely to get depressed — which is what Choi's analysis demonstrated.

"We see this study as fitting into a larger puzzle that is looking highly promising for the role of physical activity in mental health," she said.

"Although Mendelian randomisation is not without its own limitations, it can be used to answer familiar questions using a very different approach from what has been done before."

The study also revealed the importance of the way physical activity is measured: objectively measured exercise (recorded by an wrist-worn accelerometer, akin to an Apple Watch or Fitbit) showed a significant effect on lessening depression, but self-reported exercise (relying on people's memories, which are not terribly reliable) did not.

"This is a two-way relationship: you exercise to prevent depression, and you exercise to treat depression," said Professor Ian Hickie, from the University of Sydney's Brain and Mind Centre and a member of the Major Disorders Working Group of the Psychiatric Genomics Consortium, which contributed data to the project.

"There are a lot of underlying mechanisms to why this might happen, including the regulation of your body clock, your hormonal cycle, metabolic system and mood," he said in a statement to the Australian Science Media Centre.

"But your body and mind are not two separate things: the brain responds to what the body is doing."

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It's difficult to translate Choi's analysis into a physical activity recommendation, but she suggested that replacing sedentary behaviour with 15 minutes of vigorous-intensity activity (such as running) or an hour of moderate-intensity activity (brisk walking) may significantly reduce the odds of depression.

Those levels of exercise could change health guidelines: the World Health Organisation (and Australian government health authorities) recommends at least 150 minutes of moderate-intensity physical activity a week, or about 22 minutes a day.

"People would need to do substantially more than the ... recommendations to reduce their risk of depression," noted Dr Rosanne Freak-Poli from the School of Public Health and Preventative Medicine at Monash University.

But it's important to just move, regardless of how long you do it: "On average, doing more physical activity appears to protect against developing depression," Choi said. "Any activity appears to be better than none."

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