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It is clear that exercise in women affects hormonal balance, as exemplified by menstrual changes in response to changes in body mass composition. Since cumulative exposure to estrogen and progesterone may influence the likelihood of development of breast cancer, and exercise may alter levels of these hormones, examination of the breast cancer-exercise relationship appears worthy of inquiry. Additionally, caloric restriction and reduction of body mass in some animal studies inhibits carcinogenesis and reduces mammary gland proliferative activity; similar avenues have not been thoroughly investigated in humans.
A cohort of 25,624 Norwegian women were followed for a median of almost 14 years. Physical activity was assessed by self report on a 1-4 scale, with 1 being sedentary and 4 being regular (several times weekly) vigorous physical activity.
Women who exercised regularly were found to have a relative risk of 0.63 for breast cancer when compared with sedentary women. Risk was lowest in women of body mass index less than 22.8, who exercised at least four hours weekly (relative risk, 0.28). The effect was more pronounced in premenopausal than postmenopausal women and was present for physical activity both as part of work or leisure activity. In obese women, higher triglycerides may displace estradiol from sex hormone-binding globulin (which is typically lower in obese women), resulting in increased levels of free estradiol, hence greater breast exposure to estrogen.
Increasing physical activity and avoiding obesity may have a protective effect against breast cancer.
Thune I. N Engl J Med 1997;336: 1269-1275.