A study of more than 3,000 Long Island women found that those who did 10 to 19 hours of exercise a week had about a 30 percent lower risk of getting breast cancer than those who were sedentary.
The study, published Monday in the journal Cancer, also found that gaining a substantial amount of weight after menopause may eliminate the protective benefits of exercise against cancer.
The study used data from 1,504 Long Island women ages 20-98 with breast cancer and 1,555 similar women without the disease. They were part of the Long Island Breast Cancer Study Project, government-funded studies designed to look at possible environmental causes of breast cancer on Long Island.
The studies haven't identified any environmental factors that could be responsible for the incidence of the disease. The women were interviewed in the early 1990s about their exercise habits and other lifestyle factors, such as smoking and drinking, over their lifetime.
The lowest risk of breast cancer appeared among postmenopausal women who did moderate exercise, such as walking or biking, the study found.
But more was not always better. Those who exercised the most, more than 19 hours a week, showed less risk reduction than those who exercised moderately. Lead researcher Lauren McCullough, a doctoral candidate at the University of North Carolina at Chapel Hill. McCullough hypothesized that those women could be causing some cellular damage. "I tend to believe there's a balance between being physically active and the body being able to cope," she said.
Women who gained the most weight in adulthood and did no exercise had a 28 percent rise in breast cancer risk. Those who gained a similar amount of weight but exercised canceled out the risk-reduction benefit.
Postmenopausal women who gained "substantial weight" but exercised were still at risk. High weight gain was defined as gaining more than about 17 pounds for women across the life span and more than 11 pounds for postmenopausal women.
Doctors extolled the benefits of exercise to help lower the risk of breast cancer and as part of a healthy lifestyle but noted limitations in the study.
Dr. Janice Lu, director of breast oncology at Stony Brook University School of Medicine, pointed out that the study included mostly white, educated and affluent women.
Dr. Lora Weiselberg, chief of breast cancer service at the Monter Cancer Center at North Shore-Long Island Jewish Health System, questioned the amount of exercise being termed "moderate" and whether women interviewed could accurately recall how much exercise they did. "It seems like a lot of recreational physical activity. It doesn't seem overly realistic to me. That's almost two hours a day," she said.