Published: May 16, 2001
Overview
A review of the existing data on the relationship between breast cancer and exercise indicates that the relationship is a complicated one depending on many factors such as age, intensity level of physical activity, pregnancy history, oral contraceptive use, type of breast cancer, alcohol use, smoking, obesity levels etc. There does not seem to be any conclusive data in the literature that unequivocally links participation in exercise or sports with a decreased risk of breast cancer. However, it does appear that under some circumstances exercise and sports participation may play a protective indirect role in the risk of breast cancer. For example, studies show that women who are within acceptable weight ranges have a reduced risk of breast cancer. Other studies have demonstrated that women who exercise are more likely to be within acceptable weight ranges.
Selected Research
1. Exercise may not protect women from early breast cancer. Researchers compared the self-reported levels of vigorous physical activity during high school and up through age 22 of over 100,000 women in the Nurses' Health Study II with the incidence of breast cancer between the start of the study in 1989 (when women were age 25 to 42) and 1995 (when the women were age 31 to 48). No significant relationship was found between breast cancer rate and level of exercise in the teens and early 20s. (Journal of the National Cancer Institute, August 5, 1998, vol. 90 (15):1155-1160, 1116-1117)
2. Data from a Norwegian study of more than 25,000 women found that women who exercised regularly at least four hours a week had a 37% lower risk of developing breast cancer than did sedentary women. Researchers speculate that exercise exerts its protective effect on breast tissue by altering levels of natural hormones. The effects were greatest for women with the least body fat. The leanest third of the women cut their breast cancer risk by 72% when they exercised at least four hours a week. (New England Journal of Medicine, 1997; vol. 336 (18): 1269-1275, 1311-1312)
3. The weight a woman gains between age 18 and her post-menopausal years may as much as double her breast cancer risk. The research is part of the long-running Nurses' Health Study. (Journal of the American Medical Association, November, 1997)
4. Although the relationship between breast cancer and exercise remains unclear, analysis of all the studies on exercise and breast cancer to date suggests that recreational physical activity may decrease a woman's chance of developing breast cancer – especially if the most recent, best-designed studies are weighted more heavily. (Menopause, 1996, vol. 3 (3):. 172-180)
5. Women who exercise regularly during their reproductive years lower their risk of developing breast cancer before menopause. Almost 1100 Caucasian women, all under 40 years of age and half of whom had been newly-diagnosed with breast cancer, participated in the study. Researchers found that women who participated in an average of 3.8 hours or more of physical activity (defined as participation in sports practices and competitions, as well as exercise activities) reduced their risk of breast cancer by 50%. One to three hours of exercise a week over a woman's reproductive lifetime (the teens to about age 40) may bring a 20-30% reduction in the risk of breast cancer, and four or more hours of exercise a week can reduce the risk almost 60%. (Journal of the National Cancer Institute, 1994, vol. 86, 1403-1408)
6. A study of 5,398 college alumni women found that former female collegiate athletes had a significantly lower risk of cancer of the breast and reproductive system than did the non-athletes. (Br. J. Cancer, 1985, vol. 52, 885-891)