When it comes to alleviating depression, says Andrea Dunn, intensity makes little difference. It's not only unnecessary to go for the burn, it's unwise; too-vigorous exercise raises the risk of stopping altogether. But exactly how much energy expenditure needed is not clear. Nor is it known whether there's a "threshold effect,"—a minimum level of exercise needed to provide any antidepressant benefit.

• "Our data show that 30 minutes of aerobic exercise three times a week is sufficient for reducing depressive symptoms," says James Blumenthal. Thirty minutes thrice weekly is the minimum recommended for cardiovascular benefit.

  Exercising more often may have value, but it might set the bar too high for depressed people. What's important is getting people to be physically active. "We know that behavioral activation is a very important component of cognitive behavioral therapy," says Blumenthal. "Physical activity may be not just a necessary component but a sufficient component."

• Dunn puts more stock in the frequency of exercise. She suggests 180 to 210 minutes of activity a week, done 30 minutes a day. Such levels are at the higher end of public health guidelines for physical activity for the general population. She has undertaken the first study to quantify the precise dose of exercise needed to impact depression.
• Whatever the pattern of working out, it should be comfortable for the long haul. Blumenthal found an unequivocal relationship between exercise and depression 10 months after formal treatment ended. The more people exercised on their own, the less they were depressed.
• Call it the 50-50 rule. "Exercise was a significant predictor of being free of depression at the end of follow-up," he says. "People who engaged in 50 minutes of exercise a week had a 50 percent decrease in the likelihood of being depressed."