JAMA Journals – Women who regularly engage in strenuous physical exercise may have a lower risk of developing both invasive breast cancer and in situ (early-stage) breast cancer than women who do not, according to a report in the February 26, 2007 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

“Few established risk factors for breast cancer are easily modifiable,” the authors noted. Physical activity has been associated with breast cancer risk and may be one of the few risk factors that women can control. “Questions remain regarding the amount and intensity of physical activity and the periods when activity provides the greatest breast cancer risk reduction.”

Cher M. Dallal, M.S., University of Southern California, Los Angeles, and colleagues studied 110,599 women age 20 to 79 who were part of the California Teachers Study, established in 1995 and 1996. At the beginning of the study, the women were asked about their average participation in moderate (such as brisk walking, golf or volleyball) and strenuous (including swimming laps, aerobics and running) physical activity from high school to their current age and also in the past three years. The women also provided information about other breast cancer risk factors, including race, family history and use of hormone therapy.

Through 2002, 2,649 women were diagnosed with invasive breast cancer and 593 with in situ breast cancer. Women who reported participating in strenuous exercise for more than five hours per week over the long term had a 20 percent lower risk of invasive breast cancer and 31 percent lower risk of in situ breast cancer than women who participated in less than 30 minutes of strenuous activity per week. “Long-term moderate physical activity and strenuous and moderate activity in the past three years were not associated with invasive breast cancer,” the authors write. Similarly, moderate activity did not appear to influence the risk of in situ breast cancer (Arch Intern Med. 2007;167:408-415).

The researchers also examined the link between strenuous physical activity and the risk of breast cancer by hormone receptor type, or which hormones can bind to proteins on the surface of the tumor. Strenuous exercise appeared to be associated with a lower risk of estrogen-receptor–negative but not estrogen-receptor–positive breast cancers.

“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. For invasive breast cancer, strenuous and moderate physical activity affect risk of estrogen-receptor–negative tumors, but neither affects risk of estrogen-receptor–positive tumors.” 


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