Older women who got more exercise and less television time were the least likely to be diagnosed with depression, according to a US study of thousands of women -- with physical activity having the biggest impact.
According to findings published in the American Journal of Epidemiology, researchers found that women who reported exercising the most in recent years were about 20% less likely to get depression than those who rarely exercised.
On the other hand, the more hours they spent watching TV each week, the more their risk of depression crept up.
"Higher levels of physical activity were associated with lower depression risk," wrote study author Michel Lucas, from the Harvard School of Public Health in Boston.
More time spent being active might boost self-esteem and women's sense of control, as well as the endorphins in their blood, although the study could not prove directly that watching too much television and avoiding exercise caused depression, she added.
The report included close to 50,000 women who filled out surveys every couple of years as part of the US Nurses' Health Study, and covered the years 1992 to 2006.
Participants recorded the amount of time they spent watching TV each week in 1992, and also answered questions about how often they walked, biked, ran and swam between 1992 and 2000.
On the same questionnaires, women reported any new diagnosis of clinical depression or medication taken to treat depression.
The analysis only included women who did not have depression in 1996. Over the next decade, there were 6,500 new cases of depression.
After the researchers accounted for aspects of health and lifestyle linked to depression, including weight, smoking and a range of diseases, exercising the most -- 90 minutes or more each day -- meant women were 20% less likely to be diagnosed with depression than those who exercised 10 minutes or less a day.
Women who watched three hours or more of television a day were 13% more likely to be diagnosed with depression than those who hardly ever tuned in, but Lucas said at least part of that link might be due to women replacing time they could be exercising with TV watching.
One alternative explanation the researchers brought up is that women might have been experiencing some symptoms of depression before they were diagnosed, leading them to exercise less. A formal diagnosis could have come later.
"Previous studies have suggested that physical activity is associated with a lower risk of depressive symptoms," said Gillian Mead, who studies geriatric medicine at Edinburgh's Royal Infirmary but was not involved in the study.
"(The finding) adds to the growing body of evidence that physical activity is important to maintain brain health," she added in an email to Reuters Health.