How useful is exercise for people with severe depression, anxiety, or chronic mental illness? According to the December issue of the Harvard Mental Health Letter, hundreds of studies show that it can help- but there are qualifications.

The Harvard Mental Health Letter reports that possible explanations for the mood-enhancing effect of exercise include

• 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.

Also in this issue:

• Benzodiazepines and the alternatives
• Anxiety sensitivity: The fear of fear
• Sources of post-traumatic stress disorder
• A doctor answers: What’s the significance of the new suicide warning on the ADHD drug Strattera?