What is the effect of physical activity on a women's risk for breast cancer?

Updated: Dec 26, 2019
  • Author: Graham A Colditz, MD, DrPH; Chief Editor: Chandandeep Nagi, MD  more...
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Among postmenopausal women, physical activity may lower breast cancer risk by reducing fat stores, which convert androstenedione to estrone. Physical activity may also increase levels of sex hormone-binding globulin (SHBG), which would reduce bioavailable estrogens. [61] Increased physical activity also reduces insulin resistance and hyperinsulinemia, which has been hypothesized to be related to breast cancer. The relation of physical activity to breast cancer risk has been addressed in over 100 case-control and cohort studies, which were summarized by an IARC working group, which concluded that the evidence was sufficient to establish that physical activity is protective against breast cancer. [62]

The most compelling evidence for a relationship between long-term physical activity and breast cancer risk comes from studies among premenopausal women. Bernstein noted that higher levels of physical activity related to later onset of regular menses and showed that sustained higher activity levels through premenopausal years produced a substantial reduction in risk of breast cancer. [63] Risk was reduced by approximately 40% among those who were consistently most active.

Such findings have been replicated in the prospective Nurses Health Study II cohort, in which lifetime activity was assessed in 1997 and women were followed for 6 years. Sustained activity from ages 12-22 years showed a 25% reduction in premenopausal breast cancer. [64] These levels of activity may not be sufficient to modify hormone levels in premenopausal women but are sufficient to impact insulin metabolism and risk of type 2 diabetes in women. [65]

Other studies show a significant increase in estrogen among premenopausal women with low leisure time physical activity [66] and positive associations between insulin levels and postmenopausal breast cancer. [67] The insulin receptor is a significant major predictor of reduced disease-free survival in women with node-negative breast cancer. [68] This finding supports the potential importance of the insulin pathway in breast cancer biology.

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