Makers of widely prescribed antidepressants should take note: In the future, doctors may be recommending regular exercise for patients with mild to moderate depression.

Aerobic exercise performed at a moderate intensity on most days of the week and at a level consistent with public-health recommendations is effective in reducing mild to moderate depression, according to a study published in the American Journal of Preventive Medicine (2005; 28 [1], 1–8).

Researchers at the University of Texas Southwestern Medical Center conducted a study to determine whether exercise could be an effective treatment for mild to moderate depression and, if so, to identify the dose-response relationship of exercise to the reduction in symptoms. Scientists observed 80 participants, aged 20–45 years, with mild to moderate depression. Each subject was allocated to one of five groups: four participated in aerobic exercise treatment programs with various intensity and frequency levels, and one—the control group—participated in flexibility exercises 3 days per week. Investigators followed participants for 12 weeks.

Among subjects whose total energy expenditure met the American College of Sports Medicine’s exercise recommendations for improving health (consistent with government guidelines for physical activity), reductions in depression symptoms were comparable to reductions experienced by patients treated with antidepressant medications or cognitive behavioral therapy. Subjects who performed less exercise did not achieve this effect. Public-health guidelines call for all adults to engage in at least 30 minutes of moderate- intensity exercise on most and preferably all days of the week.

In another article, published in the July–August 2004 edition of a Polish medical journal, a researcher reviewed numerous studies on the effects of physical activity on emotional states—in particular, anxiety, depression and mood—and found consistent results (Psychiatria Polska, 2004; 38 [4], 611–20]. The review included studies of both healthy people and patients with emotional disorders. Studies revealed that the greatest improvements in mood resulted from rhythmic, aerobic activity using large muscle groups— for example, jogging, swimming, cycling or walking—performed at moderate or low intensities. The minimum dose necessary to create an effect was 15–30 minutes at least three times a week for 10 weeks or longer.

Scientists are not yet certain what mechanisms lead to mood improvement. Some theorize that the effects result from hormonal changes, increases in body temperature, enhanced blood circulation to the brain—with an impact on the hypothalamic-pituitary-adrenal axis— and changes in physiological reactivity to stress.