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.