Vigorous exercise is believed to trigger a surge in natural brain chemicals, including endorphins, the body’s own painkillers. It also boosts serotonin, a brain chemical that can be low in some people with anxiety and depression.

“Exercise can be an appropriate add-on treatment for a lot of anxiety disorders,” said Roger A. Fielding, director of the nutrition and exercise physiology laboratory at Tufts University. “There’s a lot of evidence that exercise improves people’s sense of well-being.”

At Arizona State University in Tempe, Daniel Landers, a professor of kinesiology (the study of movement) has reviewed more than 100 studies in humans and animals. In controlled studies, he said, rats put under stress tend to freeze if put into a strange environment. But if the rats are allowed to exercise, this fearful response is diminished.

With humans given questionnaires right after exercise and after weeks of training, exercise has both an immediate and long-term effect in reducing anxiety, Landers said. Animal studies show that both endorphin and serotonin levels go up with exercise. In people with panic attacks, exercise has been found to be just as effective at reducing the number of attacks over a 16-week period as drugs given to combat anxiety and depression. There is also evidence, Landers said, that exercise increases levels of a brain chemical called BDNF, which enhances both mood and intellectual functioning.

To get the mellowing effects of exercise, both aerobic exercise, like walking briskly, and weight lifting work, provided that you’re working at 50 to 70 percent of your maximum possible effort. In general, researchers recommend 30 minutes a day — which can be done in several, shorter bouts — of moderate aerobic exercise at a pace that feels somewhat fast but still allows you to carry on a conversation.