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Adapted from the NCI Cancer Bulletin, vol. 4/no. 10, March 6, 2007 (see the current issue).

California researchers have found that strenuous long-term physical activity decreases a woman's risk of invasive and in situ breast cancer, according to study results published in the February 26, 2007, Archives of Internal Medicine (see the journal abstract).

Dr. Leslie Bernstein of the University of Southern California and colleagues evaluated 107,034 participants from the National Cancer Institute-funded California Teachers Study, a prospective study of current and retired female California public school teachers and administrators established in 1995-1996. Researchers collected information on the participants' level of physical activity - moderate or strenuous - between high school and their current age (or age 54, if the participant was 55 or older), as well as activity in the past three years.

Women who annually participated in more than five hours per week of strenuous activity had a lower risk of invasive breast cancer compared with the least active women. Long-term moderate activity and strenuous and moderate activity in the past three years were not associated with invasive breast cancer. Researchers also found that women who participated in long-term strenuous or moderate physical activity had a decreased risk of estrogen receptor- (ER-) negative invasive breast cancer, but not of ER-positive invasive breast cancer.

Participants had also reported information on relevant breast cancer risk factors, including race/ethnicity, family history of breast cancer, age at menarche, reproductive history, menopausal status, use of hormone therapy and oral contraceptives, height, weight, diet, smoking history, alcohol consumption, mammography screening history, and breast biopsy history. However, these factors did not account for the relationship between exercise and breast cancer.

The authors noted, "In summary, these results provide additional evidence supporting a protective role for long-term strenuous recreational physical activity on risk of invasive and in situ breast cancer, whereas the beneficial effects of moderate activity are less clear."

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