Can a stroll help ease depression? That question preoccupied Dr. Madhukar H. Trivedi, a professor of psychiatry at the University of Texas Southwestern Medical Center in Dallas, after several of his patients, all suffering from serious depression, mentioned that they felt happier if they went for a walk. The patients in question were taking the widely prescribed antidepressants known as S.S.R.I.’s, for selective serotonin reuptake inhibitors, but not responding fully. They remained, by clinical standards, depressed. Dr. Trivedi and his colleagues began to wonder if adding a formal “dose” of exercise would increase their chances of getting better.

Certainly the possibility was worth investigating. Clinical depression, as anyone who has experienced or watched a loved one struggle with the condition knows, can be stubbornly intractable. Even if patients have been taking an antidepressant for months, recovery rates tend to hover below 50 percent.

In order to increase the odds of improvement, doctors frequently add a second treatment — often another drug, like lithium or an antipsychotic — to the S.S.R.I. regimen at some point, Dr. Trivedi said. Most patients ultimately require at least two concurrent treatments to achieve remission of their depression, he said. Studies have shown that these secondary drug treatments help an additional 20 to 30 percent of depressed patients to improve, but the medications can be expensive and have unpleasant side effects.

Which prompted Dr. Trivedi to look to exercise. His investigation joins a growing movement among some physiologists and doctors to consider and study exercise as a formal medicine, with patients given a prescription and their progress monitored, as it would be if they were prescribed a pill.

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