Exercise can lower women’s risk of breast cancer, but how much exercise is enough and at what age do women have to be physically active to benefit?
Those are the questions that Lauren McCullough, a doctoral candidate in epidemiology at the University of North Carolina, Chapel Hill, and her colleagues sought to answer in a new study published in the journal Cancer. They found that women who exercised about two hours a day five days a week were about 30% less likely to develop breast cancer than less active women. The intensity of the exercise didn’t seem to matter; all it took was moderate physical activity, which could include gardening, walking or doing household chores, for the women to benefit.
McCullough’s study included 1,504 women with breast cancer and 1,555 similar women without the disease, aged 20 to 98 years old, enrolled in the Long Island Breast Cancer Study Project. As part of the project, the women answered questions about their physical activity over their lifetimes — any recreational exercise they did for at least an hour per week for three months or more. The researchers then calculated a lifetime composite score for physical activity that they used to compare across the participants.
Overall, women who did any exercise had a 6% lower risk of breast cancer than those who did not, but certain subgroups of women enjoyed even larger benefits. The effect was strongest among women who had children and exercised about 10 to 19 hours each week — either during their reproductive years or after menopause; for them, exercise was associated with a 30% lower risk of breast cancer during the study period, compared with women who exercised less, or not at all.
(MORE: Dropping a Few Pounds Could Lower Breast Cancer Risk)
“I was excited by the results because as women tend to age, they get set in their habits, and think that if they haven’t been active their whole life, why start now,” says McCullough. “But it’s important to show that there is research-based evidence that says that you can start exercising after menopause and still enjoy really good benefits.”
That even applies to the women who were heavier to start with: among overweight or obese women, with BMIs over 30, those who exercised had a lower risk of breast cancer than those who were not physically active. In fact, says, McCullough, their risk was similar to that of normal weight women who did not exercise. “We are excited by that, because it tells women that even if they are overweight or obese, they can still engage in physical activity and while they won’t lower their risk of breast cancer [below average], they will not be increasing their risk of the disease,” says McCullough.
There is a caveat, however — women who gained weight even as they exercised were likely to negate the reduction in cancer risk linked with physical activity. That’s because most of the weight women gain after menopause is in the form of visceral fat around the abdomen, which is more metabolically active, promoting unhealthy insulin and sugar levels, leading to accelerated fat deposition and increasing breast cancer risk.
(MORE: Study: Weight Gain May Boost Survivors’ Risk of Breast Cancer Recurrence, Death)
The researchers note that weight reflects a balance between diet and physical activity, so benefits to breast cancer risk — and overall health — are likely associated not just with exercise alone, but also with the maintenance of a healthy diet.
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.