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Exercise May Reduce Breast Cancer Risk

Hormones and energy balance may affect breast cancer risk, and studies have shown a protective effect of physical exercise. Investigators in Norway studied the relationship between physical exercise and breast cancer in a prospective trial of 25,624 women from 1974 to 1978 and 1977 to 1983. Questionnaire data on the women's level of physical activity at work and during leisure time were linked to the Norwegian cancer registry and adjusted for age, body-mass index, height, parity, and county of residence.

During 13.7 years of follow-up, 351 cases of invasive breast cancer were identified. The overall relative risk for breast cancer was 0.63 (95% CI, 0.42 to 0.95) for women who exercised regularly compared with sedentary women. Risk reductions were greater in younger women (<45) and in lean women who exercised at least four hours per week (relative risk, 0.28; 95% CI, 0.11 to 0.70).

Comment: While the exact mechanism is not clear, this study supports earlier cohort and case-control evidence for a protective effect of exercise on breast cancer risk. Possible explanations include the effects of exercise on estrogens, progesterone, fat metabolism, and energy balance. This is just one more reason why clinicians should counsel patients to be physically active.

— W Levinson

Published in Journal Watch Women's Health June 1, 1997

Citation(s):

Thune I et al. Physical activity and the risk of breast cancer. N Engl J Med 1997 May 1 336 1269-1275.

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• Medline abstract (Free)

McTiernan A. Exercise and breast cancer -- time to get moving? N Engl J Med 1997 May 1 336 1311-1312.

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• Medline abstract (Free)