The study of more than 73 000 postmenopausal women found that walking at a moderate pace for an hour a day was associated with a 14% reduced breast cancer risk, compared to leading a sedentary lifestyle. An hour or more of daily strenuous physical activity was associated with a 25% reduced risk, the study found.
This is welcome news for women who aren't very athletic.
"The nice message here is, you don't have to go out and run a marathon to lower your breast cancer risk," said study researcher Alpa Patel, senior epidemiologist at the American Cancer Society, which funded the study.
"Go for a nice, leisurely walk an hour a day to lower your risk," Patel advised.
Breast cancer is the leading cancer among women. In the United States, about one in eight women will develop the disease in her lifetime.
The women who reported moderate exercise walked about three miles an hour, or about a 20-minute mile. The more vigorous exercisers participated in such activities as fast walking – about 4.5 miles in an hour, the equivalent of a light jog, Patel said – moderate cycling or lap swimming.
For the study, published online in Cancer Epidemiology, Biomarkers & Prevention, Patel and her team identified more than 73 000 women past menopause who were enrolled in an American Cancer Society study on cancer incidence.
When they enrolled in 1992, the average age was nearly 63. The women completed a questionnaire about medical, environmental and demographic factors at the start and repeated the reports every two years between 1997 and 2009. The study participants also reported on their physical activity and time spent sitting, including watching television and reading, and reported any diagnosis of breast cancer.
During the follow-up, which was roughly 14 years, 4 760 women developed breast cancer.
The researchers compared the exercise habits of women who developed breast cancer and those who did not. About 9% never participated in physical activity, while about half reported walking as their sole activity.
Those who walked seven hours or more a week, even without engaging in other recreational physical activity, reaped protective benefits compared to those who walked three hours or less a week.
The message is encouraging, Patel said.
A reachable goal
However, the study only found an association between moderate exercise and reduced breast cancer risk, not a direct cause-and-effect relationship.
While other studies have found that exercise lowered risk of breast cancer more for women with a lower body mass index (BMI) a calculation of body fat based on height and weight this study found the effect held regardless of BMI, weight gain in adulthood or use of postmenopausal hormone therapy.
Other studies have found a link between time spent sitting and breast cancer risk, but Patel's group did not find this link.
This is "a good news study for women," said another cancer expert, Dr Laura Kruper, who was not involved with the research.
The findings add to the accumulating evidence about exercise lowering breast cancer risk, and present a goal that is reachable for most women, said Kruper, co-director of the breast cancer programme at the City of Hope Cancer Centre in Duarte, California.
"This is something nearly every woman can do," Kruper said of exercising moderately for an hour daily.
"This is not running a marathon," she said.
Her advice to sedentary women who want to reduce their breast cancer risk: "If you get off the couch and walk around, it would help."
Why does exercise appear to lower breast cancer risk? The mechanism is mostly hormonal, Patel said. Breast cancer risk is affected by lifetime exposure to oestrogen, with more exposure increasing risk. Older, physically active women have lower levels of oestrogen than their sedentary peers.
Besides exercising, women who want to reduce breast cancer risk should maintain a healthy body weight, Patel said, and if they drink, they should limit alcoholic beverages to no more than one daily.
To learn more about risk factors for breast cancer, visit the American Cancer Society.