Photo: Seymour Chwast

A new study shows exercise to be a surprisingly potent vaccine. In the first randomized clinical trial to investigate the connection between physical activity and colds, sedentary, postmenopausal women were about twice as likely to get sick as those who worked out regularly for a year. Most of the exercisers chose brisk walking as their activity and averaged 30 minutes five days a week. "Increased blood flow stimulates the immune system," says Cornelia Ulrich, PhD, senior author of the study, which was published in The American Journal of Medicine, and associate member of the Public Health Sciences division at the Fred Hutchinson Cancer Research Center in Seattle. Each bout of exercise, the researchers hypothesize, may boost virus-fighting cells called leukocytes.

When O polled 1,400 readers on Oprah.com, the results confirmed that the more active people are, the fewer colds they get. Among those who work out at least four times a week, 20 percent say they never get colds, while only about 9 percent of occasional exercisers (once a week or less) stay sniffle-free. And while 3.5 percent of the occasional exercisers get four to six colds a year, only 1.85 of the frequent ones do.

The one caveat: You can overdo it. "A big exertion is immunosuppressive," says Ulrich. "It may be that a little bit of stress to the body is good and stimulating, but too much will cause problems."