Is there an association between physical activity and the risk of breast cancer in post-menopausal women?

Prospective cohort study with mean follow-up of 4.7 years.

During follow-up, 1780 new cases of breast cancer were diagnosed. Post-menopausal women exercising strenuously ⩾ 3 times per week at age 35 years had a significantly reduced risk of breast cancer, compared with sedentary women (RR 0.86, 95% CI 0.78 to 0.95). Strenuous exercise at age 18 and 50 years was not significantly associated with a reduction in breast cancer risk (p⩾0.08). Women currently walking briskly for 1.25–2.25h per week (or equivalent) had a significantly reduced risk of breast cancer compared with sedentary women (RR 0.82, 95% CI to 0.68 to 0.97). Further analysis showed that increasing total current physical activity was related to a lower risk of breast cancer in women whose BMI⩽24.13 (p-value for trend=0.03) but not where BMI⩾28.44 (p-value for trend = 0.30). There was some evidence for breast cancer risk reduction with low levels of physical activity by women in the middle weight range.

The risk of breast cancer was reduced in healthy-weight and moderately overweight women who undertake increased amounts of low intensity physical activity after menopause. Regular vigorous activity at age 35 was linked to breast cancer risk reduction.