SAN ANTONIO — The three most practical public health-type lifestyle interventions at present for reducing breast cancer risk are to encourage breast-feeding, get young girls started exercising regularly to lay the groundwork for a lifelong habit of physical activity, and tackle the postmenopausal obesity epidemic, Leslie Bernstein, Ph.D., said at a breast cancer symposium sponsored by the Cancer Therapy and Research Center.
She has spent most of her career studying the breast cancer-preventive effects of physical exercise. Indeed, she conducted the first epidemiologic study demonstrating the link. Since then more than 30 studies from around the world have followed, most showing lifetime physical activity is independently associated in dose-dependent fashion with up to a 20%–30% reduction in breast cancer risk, compared with that of women who don't exercise.
The association has held true in studies of European women, Asian women, Asian Americans, and Hispanic Americans. Most recently, Dr. Bernstein and coworkers reported the findings of the Women's Contraceptive and Reproductive Experiences (Women's CARE) study, the first-ever epidemiologic study focusing on the effects of lifetime recreational exercise on breast cancer risk in African Americans.
Women's CARE involved 1,605 black and 2,933 white women aged 35–64 years with breast cancer, along with 1,646 black and 3,003 white controls. No racial differences were found in the impact of lifetime physical activity upon breast cancer risk. The highest level of recreational physical activity—defined in this study as an average of 3 hours or more per week over a woman's lifetime—was independently associated with roughly a 25% risk reduction, compared with that of sedentary women (J. Natl. Cancer Inst. 2005;97:1671–9).
The risk reduction is greater among women without a first-degree family history, said Dr. Bernstein, professor of preventive medicine at the University of Southern California, Los Angeles.
She noted that the field of breast cancer risk reduction through physical activity “still has a long way to go.”
“The most difficult task is to learn how to motivate sedentary women to become physically active,” Dr. Bernstein added.
Lactation has clearly been shown to protect against breast cancer. It's uncertain if the benefit is solely due to prevention of ovulation, with resultant reduced lifetime exposure to estrogen and progesterone.
Pregnancy is also protective, but only provided the first term pregnancy occurs before age 30, Dr. Bernstein said.
Age at first pregnancy isn't customarily considered a readily modifiable breast cancer risk factor. However, efforts are underway to develop chemoprevention regimens that mimic pregnancy hormonally.
Turning to obesity, Dr. Bernstein noted it has been shown to be a major risk factor for breast cancer, but only in obese postmenopausal women—and in her own studies, only in those obese postmenopausal women who have a family history of breast cancer in a first-degree relative.
Regarding the potential for dietary interventions for breast cancer risk reduction, Dr. Bernstein observed that most hypotheses in this area have not panned out.
However, an intriguing study by her USC colleague Anna Wu, Ph.D., points to green tea consumption as a potential risk reduction intervention. Dr. Wu's population-based, case-control study of breast cancer among Asian American women in Los Angeles showed a dose-response effect, with voracious green tea drinkers having roughly a 50% lower risk of developing breast cancer than women who did not drink the beverage regularly (Int. J. Cancer 2003;106:574–9). Soy intake was also associated with reduced breast cancer risk in Dr. Wu's study, but mainly in those who consumed it heavily in adolescence.
Even light to moderate alcohol intake has been linked to increased breast cancer risk. However, the risk is present only in drinkers without adequate folate, she said.
'The most difficult task is to learn how to motivate sedentary women to become physically active.' DR. BERNSTEIN