(CNN) - The benefits of exercise are widely known: it helps you live longer and lowers risk of heart disease, stroke and diabetes. A new study published Wednesday in The Lancet Psychiatry journal suggests that when done in moderation, it leads to better mental health as well.
The researchers analyzed responses from 1.2 million adults in the United States taken from a US Centers for Disease Control and Prevention survey given in 2011, 2013 and 2015. The average number of days of poor mental health per person in the past month was around 3.4, according to the study.
Those who reported exercising had about 1.5, or 43%, fewer days of "bad self-reported mental health" in the past month compared to those who did not exercise. Poor mental health was reported in the study as stress, depression and trouble with emotions.
"I think in comparison to all other treatments, when a patient successfully exercises at the right dose, there is a sense of self-efficacy and confidence that the patient develops that is absolutely also a remarkably positive thing," said Dr. Madhukar Trivedi, Director of the Center for Depression Research and Clinical Care at the Peter O'Donnell Jr. Brain Institute at UT Southwestern Medical Center in Dallas. He was not involved with the study but recently published a paper in JAMA Psychiatry linking midlife fitness with lower risk of depression.
"I think it's a huge deal," said Adam Chekroud, an author of the study and Assistant Professor of Psychiatry at Yale University. "Even just walking just three times a week seems to give people better mental health than not exercising at all. I think from a public health perspective, it's pretty important because it shows that we can have the potential for having a pretty big impact on mental health for a lot of people."
The sweet spot for exercising was found to be 45-minute sessions three to five times a week. There wasn't a big difference in benefit for exercising beyond 90 minutes in a session -- until one hit the three-hour mark. After that, there appeared to be worse mental health associated with those people compared to others who did not exercise at all.
"I think it makes sense," Chekroud said. "If you're not exercising enough, perhaps that's not giving you the biological aspect of exercise, maybe you're not putting your body through the intensity and through the changes that it needs to stimulate those biological changes in the brain. And on the high end, anecdotally we hear a lot about people who get addicted to exercise or maybe you're kind of running yourself into the ground."
Though all forms of exercise resulted in better mental health compared to doing nothing, the strongest association was found in people who played popular team sports (a 22% lower mental health burden), cycling (22%) and other aerobic and gym activities (20%). Even completing household chores led to about a 10% drop in days of poor mental health in a month, the researchers found.
"Exercise in group settings could have a slightly higher benefit than exercise alone," Trivedi said. "There's not enough evidence to be very strong but that could be what happened."
As for cycling, Chekroud, who is also chief scientist at Spring Health, a mental health company based in New York that provides mental health services to large employers and an avid cyclist himself, said, "There's... the biological benefit of exercise. And you'll have increases in your respiratory rate and your heart rate and that kind of thing. But also, it's an opportunity where you're not working for a relatively long period of time and you get to think things through, perhaps reevaluate situations that happen in your life."
The authors adjusted for various physical and sociodemographic factors like age, race, gender, marital status, socioeconomic status, education, self-reported physical health and previous diagnosis of depression.
Still the improvement seen from exercise was more than what could be seen from any other modifiable social or demographic factor such as education, body mass index or household income.
Seventy-five types of "exercise" were included in the report, leading to some experts preferring another label instead.
"In the current study, we see the inclusion of activities such as childcare, housework, lawn-mowing, carpentry, fishing, and yoga as forms of exercise," wrote Dr. Gary Cooney, a psychiatrist at Gartnavel Royal Hospital in the United Kingdom in an accompanying commentary published with the article. "The study... in its all-encompassing approach, might more accurately be considered a study in physical activity rather than exercise."
Though the study is purportedly the largest of its size, and "unprecedented in scale", it does have a few limitations, Cooney said. Mental health disorders are not a monolith and there are discrete factors involved in research and clinical purposes of various conditions like dementia, substance misuse or personality disorder, he said.
"There is an uncomfortable interchangeability between mental health and depression, as if these concepts were functionally equivalent, or as if other mental disorders were somewhat peripheral," wrote Cooney, who also cited the authors' choice to research previous studies regarding exercise and primarily depression. Consequently, the study may offer the most guidance in depression research.
"I think that particular concern is more of an academic concern rather than a practical concern," Chekroud countered. "I think part of the reason that we were less concerned about that than the particular commentary was that we know that depression and anxiety are the most common mental health conditions. And when we talk about mental health, most people have mood or anxiety disorders. So it's true that some people will have things like schizophrenia or bipolar disorder but those conditions make up a very small percentage of the population."
"In the future we can start to maybe hone in on different illness categories and maybe we would see a different pattern, but I think overall though it's kind of an edge case in this situation," he added.
And because the answers to the survey are self-reported, individuals who have conditions like schizophrenia, schizoaffective disorder or bipolar affective disorder, may have a diminished ability to do so accurately, Cooney said.
The next step in research is asking more detailed questions and following up with people long-term, Trivedi said. The authors of the study also propose collecting data from wearable trackers, like Fitbit, to more precisely determine how frequency, duration and intensity of exercise and mental health burden are connected.
"People and patients should actually get well informed and become informed consumers and ask their doctors about whether this is a valid treatment for them or not," Trivedi said. "And if the doctor says yes then you try to figure out a plan to make sure like any other treatment -- if you get pills, then you figure out a way to take them regularly. If your doctor and you decide exercise is your treatment, then you develop strategies to make sure you basically swallow the pill."
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