Researchers analyzing data from a long term study of women in the US found low levels of exercise and watching lots of TV were each linked to a higher risk of depression compared to high levels of exercise and little TV viewing. A report of their findings appeared recently in the American Journal of Epidemiology.
Lead author Michel Lucas from Harvard School of Public Health, and colleagues, used data from women taking part in the Nurses’ Health Study who had filled in questionnaires every two years from 1992 to 2000. They selected 49,821 who were depression-free in 1996.
The questionnaire data contained information about physical activity (such as cycling, walking, running, swimming), from which the researchers were able to compile a figure for a total average number of minutes of physical activity per day.
In 1992, the participants had also been asked about their television viewing habits, and from those responses the researchers calculated how many hours a week the participants spent watching TV.
The researchers were able to assess which women developed clinical depression over the follow up (from 1996 to 2006) from their self reports of whether they had been diagnosed with such by a doctor or whether they were taking anti-depressants.
The results showed that:
- Over the follow-up, there were 6,505 incident cases of depression.
- Higher levels of physical activity were linked to lower risk of depression: even after ruling out known potential confounders such as weight, smoking and certain illnesses.
- Compared with those who exercised less than 10 minutes per day on average, those women who exercised 90 minutes a day or more on average had a 20% lower risk of depression (multivariate relative risk was 0.80; 95% Confidence Interval CI ranged from 0.70 to 0.92; Ptrend < 0.001).
- In contrast, the risk of depression went up with more hours of TV viewing.
- There was a 13% higher risk of developing depression among women who watched 21 hours or more of TV per week compared to women who watched 1 hour or less (multivariate relative risk was 1.13; 95% CI 1.00 to 1.27; Ptrend =0.01).
- Physical exercise and television viewing contributed independently to depression risk.
The researchers did point out that one limitation of their study was the possibility that some of the women may have already been depressed at the start of the study in 1996 and been diagnosed later during the follow up, and this could have been the cause of them exercising less.
Previous studies have shown a clear link between higher levels of regular exercise and lower risk of depression. But until this study it was less clear whether regular physical activity and television viewing were linked with clinical depression risk.
Written by Catharine Paddock PhD