http://www.health.harvard.edu/newsweek/Exercise-and-Depression-report-excerpt.htm

(This article was first printed in the Special Health Report from Harvard Medical School "Understanding Depression". For more information or to order, please go to www.health.harvard.edu/UD.)

Can a few laps around the block actually solve your emotional problems? Probably not, but a regular exercise program might help. A review of studies stretching back to 1981 concluded that regular exercise can improve mood in people with mild to moderate depression. It also may play a supporting role in treating severe depression.

Another study, published in the Archives of Internal Medicine in 1999, divided 156 men and women with depression into three groups. One group took part in an aerobic exercise program, another took the SSRI sertraline (Zoloft), and a third did both. At the 16-week mark, depression had eased in all three groups. About 60%–70% of the people in all three groups could no longer be classed as having major depression. In fact, group scores on two rating scales of depression were essentially the same. This suggests that for those who need or wish to avoid drugs, exercise might be an acceptable substitute for antidepressants. Keep in mind, though, that the swiftest response occurred in the group taking antidepressants, and that it can be difficult to stay motivated to exercise when you’re depressed.

A follow-up to that study found that exercise’s effects lasted longer than those of antidepressants. Researchers checked in with 133 of the original patients six months after the first study ended. They found that the people who exercised regularly after completing the study, regardless of which treatment they were on originally, were less likely to relapse into depression.

A study published in 2005 found that walking fast for about 35 minutes a day five times a week or 60 minutes a day three times a week had a significant influence on mild to moderate depression symptoms. Walking fast for only 15 minutes a day five times a week or doing stretching exercises three times a week did not help as much. (These exercise lengths were calculated for someone who weighs about 150 pounds. If you weigh more, longer exercise times apply, while the opposite is true if you weigh less than 150 pounds.)

How does exercise relieve depression? For many years, experts have known that exercise enhances the action of endorphins, chemicals that circulate throughout the body. Endorphins improve natural immunity and reduce the perception of pain. They may also serve to improve mood. Another theory is that exercise stimulates the neurotransmitter norepinephrine, which may directly improve mood.
Besides lifting your mood, regular exercise offers other health benefits, such as lowering blood pressure, protecting against heart disease and cancer, and boosting self-esteem. How often or intensely you need to exercise to alleviate depression is not clear, but for general health, experts advise getting half an hour to an hour of moderate exercise, such as brisk walking, on all or most days of the week.