Just over an hour of weekly exercise reduces the risk of depression, scientists say

Physical activity lowers one’s risk of developing depression. This is a consensus derived from countless studies. Recently, a team of scientists attempted to determine just how little exercise makes a difference.

Synthesizing the results of 15 studies, a group from the University of Cambridge found that a difference starts to kick in at about half of the World Health Organization’s recommended minimum for weekly physical activity and the benefits do not increase much once you reach the full amount (creating yet another reason to never join CrossFit if you don’t want to).

Science in Action — The researchers, publishing their findings this month in JAMA Psychiatry, compiled 15 preexisting studies in which participants’ activities and health outcomes were measured over a median of 8.5 years. In total, the meta-analysis included data from 191,130 people.

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Incidents of depression were measured by a diagnosis or depression symptoms, as recorded by the researchers who completed the original study.

The researchers measured exercise using a unit called marginal metabolic equivalent task hours. A metabolic equivalent (MET) minute is the energy expended while doing any activity for one minute, expressed as a ratio compared to the energy expended while totally at rest — doing literally nothing — for one minute.

“An activity volume equivalent to 2.5 hours of brisk walking per week was associated with 25 percent lower risk of depression”

Kinesiologists have MET scores for a variety of activities, some of them stored in this handy MET calculator. For a person of 160 pounds, one minute of:

• Bicycling is 7.5 MET minutes
• Salsa dancing is 4.5 MET minutes
• Cleaning windows is 3.2 MET minutes
• Playing the guitar (while standing) is 3 MET minutes
• Walking at a moderate pace of about three miles an hour is 3.5 MET minutes
• Running at eight miles an hour is 11.8 MET minutes
• Coal mining is 5.5 MET minutes

The authors of the new report hit one snag translating the records of weekly exercise into MET time: MET minutes and MET hours are times in which a person is actually doing the activity. The studies they utilized relied on self-reports, which obviously included some breaks and interruptions. If you played basketball (6.5 MET minutes) for an hour, you probably did not subtract the time in which you, say, stopped to huddle or broke for a Gatorade or waited for your buddy to retrieve the ball he tossed in a neighbor’s yard.

So the researchers introduced a new concept: They took about 12 percent away from each activity’s MET hours to accommodate for this, creating marginal metabolic equivalent task hours.

For healthy adults under 65, the World Health Organization recommends 600 MET minutes — or 10 MET hours — a week.

The researchers found that reduced risk of depression started for people who got half that. Relative to adults not reporting any activity, people who accumulated 4.4 marginal MET hours per week had 18 percent lowered risk of developing depression, according to the accumulation of all those studies.

People who accumulated the 8.8 marginal MET hours (about the W.H.O. recommendation) per week had 25 percent lower risk and there were no certain or noteworthy benefits beyond that level of activity.

Putting it more simply, the researchers wrote:

Accumulating an activity volume equivalent to 2.5 hours of brisk walking per week was associated with 25 percent lower risk of depression, and at half that dose, risk was 18 percent lower compared with no activity. Only minor additional benefits were observed at higher activity levels.

Why It’s a Hack — The researchers considered a few reasons why exercise of such little frequency or intensity had a benefit.

• Exercise releases endorphins and other good brain chemicals. Some past research showed that less than 60 minutes of moderate exercise triggered neuroendocrine and inflammatory responses, with no greater responses from these systems with more exercise.
• Perhaps some of the reduction in depression correlated with exercise had to do with subjective measurements, like self-image and socialization.
• People often exercise in green space, another depression fighter, and of course, more MET minutes do not directly equal more time in nature.
• Conversely, exercise gets people away from noise pollution and isolating indoor environments.

How This Affects Longevity — As the study notes, depression is the leading cause of mental health-related disease burden worldwide, affecting about 280 million people. It is associated with increased mortality rates from a variety of conditions.

People with regular exercise routines are less likely to suffer from depression. Some of that could be attributed to reverse causation; people free of depression are more likely to have the get-up-and-go needed to work out. But scientists do think exercise has a protective effect on the brain.

More than a third of Americans said they do not exercise ever in a 2021 survey. If they want to put on some running shoes or hit the weight machines for better mental health, maybe it’d be good to know some scientifically informed minimum for results.

Hack Score — Three out of ten walks halfway around the block 👟👟👟