Yet another reason to exercise, even if just for a few hours a week: It may cut the risk of developing breast disease, according to researchers from the Keck School of Medicine of the University of Southern California and the American Cancer Society. Women who exercised had a 35 percent lower risk of developing breast carcinoma in situ than did inactive women, according to a study in an upcoming issue of the journal Cancer. Alpa Patel, a researcher with the American Cancer Society, is the report's lead author.
Breast carcinoma in situ, or BCIS, consists of clusters of abnormal cells confined either to breast ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ). Most BCIS cases are found through screening mammograms. Left untreated, some BCIS develop into invasive breast cancer.
"It's critical to know which factors are important at each stage of disease-because this knowledge may help you devise interventions or earlier means of detection. By identifying risk or protective factors for BCIS, we have the opportunity intervene a bit earlier in the cancer process," says Leslie Bernstein, professor of preventive medicine at the Keck School and the study's senior author.
Bernstein and other researchers have shown that regular physical activity reduces risk for invasive breast cancer, possibly by reducing levels of female hormones. But relationships between physical activity and BCIS risk are poorly understood.
For this latest study, epidemiologists studied more than a thousand women, either white or African-American, between ages 35 and 64 in Los Angeles County. They interviewed 567 women diagnosed with BCIS, and matched them to 616 BCIS-free women based on race and age group. All women had undergone a screening mammogram within two years of identification.
Interviewers asked participants about lifetime involvement in activities such as walking, jogging, dance and swimming. The researchers came up with each woman's weekly average hours and average energy spent exercising since she began menstruating.
When researchers adjusted for known breast cancer risk factors, they found that among all women, the risk of BCIS was about 35 percent lower in those who reported exercising compared to those who were inactive.
The epidemiologists then divided participants into two groups: those who had a mother or sister diagnosed with breast cancer, and those did not. Among participants with no family breast cancer history, the risk was reduced with increasing hours per week of exercise. Women who exercised more than four hours a week had a 47 percent lower risk of BCIS than inactive women.
Physical activity did not reduce BCIS risk among women with a family history of breast cancer, though.
Scientists are unsure exactly how physical activity may deter BCIS. One theory is that physical activity lowers levels of female hormones. Heavy exercise may change menstrual function and lower levels of hormones such as estradiol and progesterone, especially during adolescence. Even among recreational athletes, exercise can decrease average hormone levels, lengthen menstrual cycles or result in menstrual cycles in which no egg is released. It might also help postmenopausal women maintain a lower weight, which can lower estrogen levels.
"Although we presume that physical activity works through a hormonal means to reduce BCIS risk, this may not be an important mechanism for women who may have a hereditary form of the disease," Bernstein says.
Some scientists also have hypothesized that exercise's immune-boosting power or its ability to increase insulin sensitivity might be protective.
Despite carefully considering the screening practices of the women studied, the researchers cautioned that screening practices still might have affected their results. Further studies are needed to understand better whether physical activity can reduce BCIS risk.
