* enhanced body image * social support from exercise groups * distraction from everyday worries * heightened self-confidence from meeting a goal * altered circulation of the neurotransmitters serotonin, norepinephrine, and the endorphins.

Exercise may also serve as a form of predictable stress that supplies a kind of "vaccination" against the uncontrolled stress that leads to depression and anxiety.

It's also possible that exercise's effect on mental health is an illusion, says the Harvard Mental Health Letter. According to some surveys and observational studies, it could be that depression and anxiety prevent people from exercising, rather than the other way around. Or some feature of personality or upbringing might cause both depression and sedentary habits.

Even controlled trials on the subject often have problems, such as insufficient follow-up, the difficulty of correcting for the effect of expectations, and the fact that people who volunteer for exercise studies are not necessarily typical.

These doubts may not matter, because exercise has many health benefits and does little harm. But low motivation is a problem. People are often told to find an activity they enjoy, but depressed people don't enjoy anything much. So it's necessary to begin slowly and remember that exercise does not have to be strenuous to be helpful. Walking, gardening, or household work will do.