Study links moderate activity to lower breast cancer risk

It was published early online October 4, 2013 in Cancer Epidemiology, Biomarkers, and Prevention.

The researchers looked at breast cancer status and exercise levels in 73,615 postmenopausal women taking part in the CPS-II Nutritional Cohort, a study begun by the American Cancer Society in 1992. During the 17-year study, 4,760 of the women were diagnosed with breast cancer.

Among the women who reported walking as their only activity, those who walked at least 7 hours per week had a 14% lower risk of breast cancer compared to those who walked 3 or fewer hours per week. The women walked at a "moderate" pace -- about 3 miles per hour. The most active women -- those who walked and did more vigorous exercise -- had a 25% lower risk of breast cancer compared to the least active.

"Our results clearly support an association between physical activity and postmenopausal breast cancer, with more vigorous activity having a stronger effect," said Alpa Patel, PhD, leader of the study and American Cancer Society strategic director of Cancer Prevention Study-3. "Our findings are particularly relevant, as people struggle with conflicting information about how much activity they need to stay healthy. Without any other recreational physical activities, walking on average of at least one hour per day was associated with a modestly lower risk of breast cancer. More strenuous and longer activities lowered the risk even more."

Women in the study benefited from physical activity whether they were at a healthy weight, overweight, or obese, and even if they gained weight during the study. They also benefited whether or not they were taking menopausal hormone therapy. Exercise reduced the risk of both estrogen receptor positive and estrogen receptor negative cancers. (In estrogen receptor-positive breast cancer, the female hormone estrogen promotes the growth of the cancer. Hormone therapy for breast cancer works by blocking the effects of estrogen or lowering estrogen levels. Estrogen receptor-negative breast cancer is often harder to treat because it is not likely to respond to hormone therapy.)

The American Cancer Society recommends that adults get at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity activity each week, preferably spread throughout the week. Moderate-intensity activities are those at the level of a brisk walk. Vigorous-intensity activities increase your heart rate and breathing, and make you sweat. Examples include running, aerobic dance, and soccer.

How exercising lowers breast cancer risk is not fully understood. It's thought that physical activity regulates hormones including estrogen and insulin, which can fuel breast cancer growth.