Led by epidemiologist Leslie Bernstein, the USC team interviewed 545 California women who developed breast tumors by the age of 40 and compared them with an equal number of healthy control subjects. After correcting for known risk factors, such as early menstruation and a family history of breast cancer, the researchers found that the most active women were the least likely to be stricken. Women who said they exercised at least four hours a week suffered nearly 60 percent less breast cancer than those who described themselves as inactive. Even a history of light exercise (one to three hours a week) brought a 30 percent reduction in risk.
The USC researchers suspect that a hormonal connection is at work here. The reproductive hormones progesterone and estradiol are known to foster breast tumors, and past studies have shown that exercise slows the production of those chemicals. No one knows yet whether the cancerfighting effects of jogging, tennis and swimming extend beyond menopause. But there’s no need to stay seated until the evidence is in.
