"What the studies are showing is that exercise, at least when performed in a group setting, seems to be at least as effective as standard antidepressant medications in reducing symptoms in patients with major depression," said researcher James Blumenthal, a professor of medical psychology at Duke University in Durham, N.C.

According to Blumenthal, other studies are beginning to suggest that solitary exercise, such as workouts at the gym or a daily jog, can be just as effective as group activities in beating the blues, and that "duration of exercise didn't seem to matter -- what seemed to matter most was whether people were exercising or not."

Blumenthal was lead author on a much-publicized study released five years ago that found that just 10 months of regular, moderate exercise outperformed a leading antidepressant (Zoloft) in easing symptoms in young adults diagnosed with moderate to severe depression.

And another study released earlier this year, by researchers at the University of Texas Southwestern Medical Center at Dallas, found that 30-minute aerobic workouts done three to five times a week cut depressive symptoms by 50 percent in young adults.

Theories abound as to how revving up the body helps uncloud the mind.

Robert E. Thayer is a professor of psychology at California State University, Long Beach, and the author of Calm Energy: How People Regulate Mood with Food and Exercise. He said that while workouts probably affect key brain chemicals like serotonin and dopamine, physical activity may also trigger positive changes in other areas, too.

"Depression is a condition characterized by low energy and moderate tension, something I call 'tense tiredness,'" he said. But exercise has a clear "mood effect" that seems to ease that anxious but lethargic state, he said.

According to Thayer, moderate exercise -- a brisk 10-minute walk, for example -- results in a boosting of energy, although it may not be quite enough to relieve stress.

"More intense exercise -- the amount you'd engage in with a 45-minute aerobic workout -- does give a primary mood effect of reducing tension. It might also leave you with a little less energy because you'd be tired, of course," he said. "However, there's also some indication from the research that there's a 'rebound' effect an hour or so later, in terms of [increased] energy."

Blumenthal pointed to the more lasting psychological boost regular workouts can bring. "People who exercise might also have better self-esteem; it may help them feel better about themselves, having that great sense of accomplishment," he said.

Still, the experts acknowledged that truly depressed individuals often find it tough to jump into an exercise routine.

"Why do people not do the thing that's perhaps the most important thing for them to do?" said Thayer. "It's because a drop in energy is such a central component of depression -- you just don't have the energy to do the exercise."

He said the key to breaking that cycle is to start small.

"Thinking about going to the gym and doing all the stuff that's involved with that can be overwhelming for a depressed person," Thayer pointed out. "But if you think 'Hey, maybe I'll just walk down the street 30 yards or so, at a leisurely pace,' that's a start. And it turns out that your body becomes activated then -- you have more of an incentive to walk farther, to do more."

Loved ones can play a key role, too, urging a depressed friend or family member to join in with them as they work out. "Social support, peer pressure, family support -- all of that can be helpful, certainly in getting people to maintain exercise," Blumenthal said.

No one is saying that exercise is always a substitute for drug therapy, especially for the severely depressed. "But we also know that these drugs aren't effective for everyone -- about a third of people aren't going to get better with medication," Blumenthal said.

For those patients, exercise may prove a viable, worry-free alternative -- with one great fringe benefit.

"In addition to its mental health benefits, there are some clear cardiovascular benefits to exercise which we don't see with antidepressant drugs, of course," Blumenthal noted. So, he said, what keeps the mind fit strengthens the body, too. "You're killing two birds with one stone.

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