Many studies conclude that an active lifestyle will reduce a woman's risk of breast cancer. Some research shows that it's the total amount of exercise a woman gets in her lifetime that matters. But even inactive women who don't start exercising until they're older seem to lower their risk.

Researchers aren't exactly sure how physical activity reduces risk. Some think that exercise reduces the amount of fat in a woman's body. Fat cells produce the hormone estrogen, which may stimulate the growth of breast cancer cells.

Exercise also seems to lower insulin. This hormone may cause the growth of breast cancer cells. Another theory is that exercise helps boost a woman's immune system, helping her ward off disease.

If you're inactive, start with smaller amounts of exercise and gradually build up to at least 30 minutes on most days of the week. You'll be more likely to stick to exercise if you choose activities you enjoy. Examples of moderate activities range from brisk walking and dancing to raking leaves and mopping floors.

Women who are already active may want to aim for longer and harder workouts. Some studies show that the more exercise women get—and the more vigorous—the lower their risk of breast cancer. For instance, women age 40 and younger who averaged at least four hours a week of jogging or other aerobic exercise cut their risk by 50% compared to less active women.