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Several recent studies have found that mild to moderate exercise can reduce your chances of getting or dying of breast cancer--although nobody knew for sure why. Now, a new study may have found at least part of the answer.

For years we have known that women who exercise regularly have a lower risk of breast cancer. Then last May, Harvard reserachers reported stunning results indicating that women with breast cancer could double their chances of survival by spending three to five hours a week walking briskly. The researchers, who published their findings in the Journal of the American Medical Association, weren't exactly sure why exercise provided such a strong protective effect. But they speculated that the positive effects were due to the fact that exercise decreases the amount of circulating estrogen in the body.

"The most logical explanation is that physical activity lowers hormone levels, and the lower hormone levels reduce the chances of a recurrence," said Michelle Holmes, the lead investigator.

This month, Canadian researchers have published results of a new study which found that increased melatonin levels may be responsible for the cancer-reducing benefits of exercise. Melatonin is a hormone produced in the pineal gland. It is responsible for regulating the body's circadian rhythms, or sleep/wake cycles. It also appears to play a role in the body's immune function.

Several previous studies have suggested that higher melatonin levels are protective against breast cancer and cancer in general. The new Canadian study found that the amount of exercise women got had a strong impact on how much melotonin they produced. Suprisingly, the amount of sunlight they were exposed to had a much weaker influence. (The pineal gland is though to be stimulated by light.)

“It is possible that the protective effect of exercise with respect to breast cancer may operate in part through an effect on melatonin,” the authors concluded.

The bottom line: Over the course of a lifetime, women have a 1 in 8 chance of being diagnosed with breast cancer, which is the second leading cause of cancer deaths in women. A brisk walk every day is a simple and inexpensive step that offers powerful protection. (At the same time, you''ll be reducing your risk of heart disease, diabetes, and osteoporosis!)

For more information: Walking for Health

Sources: Holmes MD, Chen WY, et al. Physical activity and survival after breast cancer diagnosis. JAMA. 2005 May 25;293(20):2479-86. Link to article
Knight JA, Thompson S, Raboud JM, Hoffman BR. Light and exercise and melatonin production in women. Am J Epidemiol. 2005 Dec 1;162(11):1114-22. Link to article