At his research clinic in Dallas, psychologist Jasper Smits is working on an unorthodox treatment for anxiety and mood disorders, including depression. It is not yet widely accepted, but his treatment is free and has no side effects. Compare that with antidepressant drugs, which cost Americans $10 billion each year and have many common side effects: sleep disturbances, nausea, tremors, changes in body weight.

This intriguing new treatment? It’s nothing more than exercise.

Smits says his exercise treatment appeals to patients for two main reasons. First, exercise doesn’t carry the same stigma among patients (and some providers) that depression medication and psychotherapy do. Second, the mood-enhancing benefits of exercise can kick in fast — a lot faster than, say, its impact on weight loss or cardiovascular health. “By and large, for most people, when they exercise 30 minutes — particularly when it’s a little bit more demanding and they get their heart rate up — they feel better,” Smits says. “You get an immediate mood lift.”

Yet for all the potential clinical benefits, the big questions about exercise treatment remain unanswered: How much? How long? In which patients? In their recent book for therapists, Exercise for Mood and Anxiety Disorders (Oxford University Press, 2009), Smits and co-author Michael Otto at Boston University suggest precise exercise doses that they hope will aid psychologists and primary-care doctors in prescribing exercise as treatment — which can be administered in combination with other treatments, of course.

Smits and Otto recommend the familiar 30 minutes of moderate-intensity aerobic exercise, like walking, five times per week, or 30 minutes of high-intensity aerobic exercise three times a week. These doses, which are regularly recommended for physical fitness, are the only ones that have been well tested for depression.

Read more: http://www.time.com/time/health/article/0,8599,1998021,00.html#ixzz1Q8ONbIcr