A cross-sectional survey from Scotland has suggested that at least 20 minutes' physical activity a week is associated with lower levels of psychological distress.

Data were obtained from 19,842 adult respondents, of whom 54% were women. Physical activity over the previous four weeks was assessed using a self-reported questionnaire and mental health was assessed using the General Health Questionnaire (GHQ-12).

A GHQ-12 score of ≥ 4 was used to define psychological distress. Physical activity was divided into three categories: domestic, walking and sports.

After adjusting for age, gender, socioeconomic group, marital status, BMI, chronic illness and smoking, a minimum of 20 minutes per week of exercise in any of the three categories was associated with a significantly lower risk of psychological distress.

There was some evidence of a dose-response pattern, with the greatest benefit seen in participants who exercised every day (OR 0.59). When performed on four or more days per week, sports appeared to be more beneficial (OR 0.67) than domestic activity (OR 0.84) or walking (OR 0.87), but the confidence intervals overlapped. Although limited data on exercise intensity were collected, this was not included in the analysis.

There has been a dramatic increase in the prevalence of depression over the past 50 years: compared with previous generations, our children have both a higher lifetime risk and an earlier average age of onset of depression.¹ It may be no coincidence that they are also less physically active. Between 1985 and 1993, the average yearly distance walked by British children under 15 years fell from 395 to 317km, and the average distance cycled fell from 61 to 45km.²

The findings of this study are consistent with the evidence from randomised controlled trials that exercise may be effective in the short term as a treatment for mild to moderate depression.³

However, this is a cross-sectional study and cannot therefore indicate the direction of causation. The results may simply reflect the fact that depressed patients are less motivated and exercise less.

Longitudinal studies are needed to determine the temporal nature of the association but have been limited by inadequate measurements of physical activity. A recent study of middle-aged men in Wales, which used a more sophisticated activity measure, found that participants who undertook any heavy-intensity leisure activity had a reduced risk of common mental disorder at baseline (OR 0.70). However, five years later the benefit was reduced (OR 0.79, not significant) and 10 years later there was none (OR 1.16).⁴