Exercise linked to reduced breast cancer risk

Study suggests even small amount of regular activity may help

Both black women and white women who regularly exercise have a decreased risk of breast cancer compared to women who do not exercise, according to researchers at the Keck School of Medicine of the University of Southern California.

Numerous studies have linked physical activity to lower breast cancer risk in white women, but the Keck School study-published in the Nov. 16 issue of the Journal of the National Cancer Institute-extends the finding to black women.

“A growing body of evidence has linked recreational activity to lowered breast cancer risk, but we really haven’t known whether that lowered risk applies to all subgroups of the population,” says study lead author Leslie Bernstein, Ph.D., AFLAC Chair in Cancer Research and professor of preventive medicine at the Keck School. “This study indicates that exercise may be just as much a modifiable risk factor among black women as it is among white women.”

Bernstein and her colleagues conducted the Women’s Contraceptive and Reproductive Experiences, or CARE, Study, a National Institutes of Health-supported project. Researchers interviewed 4,538 black women and white women between ages 35 and 64 who had been diagnosed with invasive breast cancer in the mid-1990s, and matched them to 4,649 women without breast cancer. The multi-center, case-control study had sites in Los Angeles, Atlanta, Seattle, Philadelphia and Detroit.

The researchers asked the women about a variety of lifestyle factors that might influence breast cancer risk, such as smoking habits, alcohol consumption, age at first menstruation, number of pregnancies, use of oral contraceptives, and more. They also asked about the types of physical activities women participated in for at least an hour at week for at least four months a year-going back as far as age 10, to develop a lifetime measure of each woman’s exercise activity. Popular activities included walking, aerobics and cycling.

Of the 4,538 women with breast cancer, 1,132 (about 25 percent) reported no exercise activity since age 10. Of the 4,649 women who had not had breast cancer, 1,083 (about 23 percent) reported no exercise activity since age 10.

When researchers compared women who exercised to those who did not, they found that women exercising at least 1.3 hours a week on average since age 10 had about a 20 percent lower risk of breast cancer than women who were inactive. This relationship held true for both black women and white women. Researchers saw the relationship even after controlling for other factors such as body mass index and reproductive history. However, physical activity did not decrease breast cancer risk among women who had a family history of breast cancer, suggesting that hereditary factors might interfere with or overpower the beneficial effects of exercise.

Epidemiologists have sought to understand whether exercise is more beneficial at certain ages than others; however, researchers in the Women’s CARE Study did not find any particular time of life when physical activity had a greater influence on breast cancer risk.

Bernstein says that researchers have proposed several mechanisms under which exercise might lower breast cancer risk.

For one, studies among serious athletes and recreational athletes show that exercise activity can lower levels of female hormones circulating in the blood, particularly estrogen and progesterone. This is particularly apparent during adolescence and early adult years. Even postmenopausal women who exercise may have lower estrogen levels. Researchers believe that higher circulating levels of female hormones may raise breast cancer risk by stimulating breast cells to divide and multiply.

Women who exercise regularly also appear to be more sensitive to insulin and have lower levels of insulin in their blood. Higher concentrations of insulin in the blood may result in higher levels of female hormones. In addition, women who exercise are more likely to maintain normal body weight; excess fat is thought to contribute to greater circulating levels of female hormones and insulin-related hormones.

“Unfortunately, black women in the study were more likely than white women to be inactive,” Bernstein says. “Physical activity not only appears to be associated with lower breast cancer risk, but has other widely known health benefits, reducing the risk of cardiovascular disease and diabetes. Increasing activity among black women may be an area of potential intervention for better health.”