Women who perform mild to moderate exercise each week have a lower risk of breast cancer, according to a study published Monday in the journal Cancer. The study suggests that physical activity might be especially beneficial for adult and older women, even if they are overweight or have not been active before.
The latest research examined the relationship of exercise and weight to breast cancer risk in more than 3,000 women who participated in the National Cancer Institute’s Long Island Breast Cancer Study Project.
Researchers included women 20 to 98 years old, about half of whom had been diagnosed with breast cancer. Participants reported their own weight and exercise habits during each decade of life.
Women who reported 10 to 19 hours of weekly exercise during their child-bearing or post-menopausal years experienced about a 30 percent lower risk of breast cancer, compared with women who were inactive. The study also found that exercise duration was more important than intensity of physical activity.
“You don’t have to be in the gym running on the treadmill, and it doesn’t all have to be 10 consecutive hours,” said lead author Lauren McCullough, an epidemiologist at the University of North Carolina, Chapel Hill. Even walking the dog, taking the stairs, or doing housework could be beneficial, she said.
Among obese women with a body mass index (BMI) above 30, those who exercised the most had a lower risk of breast cancer compared with those who remained inactive.
“Heavy women can still exercise and even if they’re not losing weight, the exercise is still helping them,” said Dr. Michelle Holmes, an epidemiologist at Harvard School of Public Health.
For women who had reached menopause, significant weight gain appeared to neutralize some of the benefits of exercise. McCullough explained that weight gain among older women often involves accumulation of fat around the abdomen -- tissue that can release hormones and other chemicals that could increase cancer risks.
But the good news for these women is that exercise in later decades was particularly powerful in lowering breast cancer risk. In contrast, physical activity during adolescence and early adulthood had no apparent effect.
“It’s not saying that that early exercise is harmful. After all, that’s when you form your habits,” said Holmes. “It probably has to do with lowering hormone levels that may not even be in play until the reproductive years or later.”
Earlier research by Holmes and others has shown that women who have already been diagnosed with breast cancer may have better chances of survival and lower rates of recurrence with more exercise.
The biological processes that link exercise and breast cancer are still unclear. One theory is that exercise lowers estrogen levels in the blood. Women with high estrogen levels tend to have higher rates of breast cancer and breast cancer recurrence. McCullough said more research is needed to test this hypothesis by comparing the effects of exercise on different types of breast cancers -- some of which respond to hormones and others that don’t.
The current study focused on a mostly Caucasian and affluent population. McCullough believes additional data are needed to verify the study’s results in different racial, ethnic, and socioeconomic groups. The study also does not show that exercise causes a reduction in cancer risk; it showed only an association.
Still, the findings should encourage good habits for all women, she said. “From a public health perspective, I would recommend you should always be active from adolescence,” said McCullough. “But the results of this study say it’s never too late to start.”