What would you say about a remedy for depression that lifted your mood, restored your energy, realigned your brain chemistry, improved your sense of self—and cost nothing?

Then put on your walking shoes or find a workout class. Researchers are finding that physical activity works at least as well against mild to moderate depression as any other treatment. It may be an alternative to current forms of treatment or a complement to them, even in severe cases.

Mental health experts are carefully building a scientific dossier on exercise as therapy for depression. Already, mounting evidence indicates that regular activity combats depression on many fronts:

• Animal studies show that exercise alters neurotransmitters such as norepinephrine in the brain's frontal cortex and the hippocampus, both involved in circuits of emotion; the changes are similar to those produced by antidepressants.
• It appears to stimulate the vagus nerve, improving function of the brain's emotion-sensing network as well as of the heart. The vagus nerve is increasingly recognized as an important pathway of depression.
• By acting on neurohormones that govern the stress response, exercise seems to improve the ability of the body to tolerate stress and to meet changing demands. A dysregulated stress response, perhaps stemming from early psychological trauma, is implicated as a cause of depression.
• Exercise has definite cognitive effects; it changes people's perception of themselves, providing a sense of personal mastery and positive self-regard. It also reduces negative thinking.
• Exercise keeps on working, making it especially promising for preventing depression recurrences. People who continue to exercise have continuing reductions in symptoms.

A push to document the effect of exercise in depression began after a report by the U.S. Surgeon General linked physical inactivity to numerous modern diseases. James Blumenthal, Ph.D., professor of medical psychology at Duke University, was one of the first to step up to the plate, testing a four-month program of aerobic exercise in people diagnosed with moderate to severe depression.

The patients, sedentary men and women over 50, worked out for 30 minutes three times a week in groups. Their response was compared with that of similarly depressed patients who received either standard therapy with Zoloft or drug and exercise combined.

Patients in all three groups experienced equally significant reductions in depression symptoms, and remission rates were comparable—60% to 65%. Surprisingly, combining two effective treatments, drug and exercise, added no benefit.

In fact, the only difference among the groups was in speed of response. The medication-alone group began improving within a few weeks; the exercisers took a few weeks longer.

But the effects of exercise lasted longer. Six months after treatment ended, fewer patients in the exercise group had relapsed into depression. Among the patients no longer depressed, only 8% of the exercisers relapsed, compared with over 30% in the medication group. "That was a big surprise," Dr. Blumenthal confides.

Contrary to his own expectations, he found that exercise was beneficial against severe depression. "Those with moderate to severe depression responded as well as those with mild depression," he says.

Adherence rates to exercise programs turn out to be at least as good among the depressed as among the general population, and they are comparable among those getting medication and those exercising. Only 20% of patients dropped out of the Duke exercise program sometime before the full 16 weeks.

There is a large pool of people looking for an alternative to current therapies, says Andrea Dunn Ph.D., exercise psychologist at the Cooper Institute for Aerobics in Dallas. In recruiting patients for a study, she found "many people who knew they had symptoms of depression but didn't want to take drugs." Besides, medication doesn't work in 30% to 35% of cases.

How exercise works against depression may be tied to its effect in improving aerobic capacity of heart and lungs. Depression and heart disease are linked through many channels. Even mild depression is a serious risk factor for the development of cardiovascular disease. Depression increases the mortality of pre-existing heart disease. People with depression have low heart-rate variability, indicating diminished ability of the heart to respond to stress. And exercise lowers mortality of heart patients.

Exercise undertaken to combat depression improves aerobic capacity and cardiovascular fitness, and Dr. Blumenthal makes a strong case for aerobic activities such as walking and jogging. "It was a surprise to us but we found that the largest improvements in aerobic capacity correlated with the greatest reductions in depression."

Still, there's more to the effect of exercise than fitness level. Several studies show that resistance exercise, or weight training, is as effective against depression as aerobic activity. At the very least, both likely restore a sense of personal effectiveness. Both may also act on neurotransmitters and on the vagus nerve.

So to help yourself or someone you love, together take a 30-minute walk daily.