The report, published Tuesday in the American Journal of Psychiatry, is based on an analysis of nearly 34,000 Norwegian adults whose levels of exercise and symptoms of anxiety and depression were monitored over 11 years.
Of the cases of depression reported by the group, 12 percent could have been prevented had the participants engaged in one hour or more of physical activity each week, according to the authors.
"We've known for some time that exercise has a role to play in treating symptoms of depression, but this is the first time we have been able to quantify the preventative potential of physical activity in terms of reducing future levels of depression," said lead author Samuel Harvey, an associate professor at the Black Dog Institute and the University of New South Wales in Australia.
"We are still trying to determine exactly why exercise can have this protective effect, but we believe it is from the combined impact of the various physical and social benefits of physical activity.”
The team used data from the Health Study of Nord-Trondelag County – one of the largest population-based health surveys ever organized – which was conducted from January 1984 to June 1997.
A healthy group of participants was asked to initially report how frequently they exercised and at what intensity: without becoming breathless or sweating, becoming breathless and sweating or exercising to exhaustion. As a follow-up, the participants later completed a self-report questionnaire – the Hospital Anxiety and Depression Scale – to indicate any developing anxiety or depression.
The researchers also accounted for variables that could impact the connection between exercise and common mental illness, including socioeconomic and demographic factors, body mass index, new physical illnesses, substance use and perceived social support.
Their findings show that participants who reported doing no exercise at the onset of the study had a 44 percent higher likelihood of developing depression compared to those who exercised for an hour or two each week.
"With sedentary lifestyles becoming the norm worldwide, and rates of depression growing, these results are particularly pertinent as they highlight that even small lifestyle changes can reap significant mental health benefits," Harvey said.
The benefits did not seem to carry over to preventing anxiety, however, as the team failed to identify an association between level and intensity of exercise and the chances of developing the disorder.
A 2013 survey by the U.S. Centers for Disease Control and Prevention estimates that nearly 80 percent of adult Americans do not get the recommended amounts of exercise each week – 2.5 hours of moderate-intensity aerobic or 1.25 hours of vigorous-intensity activity, or a combination of both.
"These results highlight the great potential to integrate exercise into individual mental health plans and broader public health campaigns,” Harvey said.
“If we can find ways to increase the population's level of physical activity even by a small amount, then this is likely to bring substantial physical and mental health benefits."
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