Working out wards off breast cancer
Exercising more than four hours a week may cut a woman's risk of developing breast cancer by almost half, new American research suggests.
The effect is probably a result of lowered female hormones, say the researchers, whose work has just been published in the online version of the journal Cancer.
The finding adds to a growing body of evidence that physical activity - independent of weight - significantly reduces your odds of a variety of diseases including heart disease, stroke, diabetes and other cancers.
While the researchers in the latest study did not see a protective effect of physical activity in women with a family history of breast cancer, these account for less than five per cent of cases. So the finding remain relevant to the bulk of at-risk women.
The researchers, from University of Southern California's Keck School of Medicine compared reported physical activity levels between two groups of women aged 35 to 64 in Los Angeles: 567 who had been diagnosed with a breast cancer confined to the breast ( known as a breast cancer in situ or BCIS) and 616 BCIS-free women matched for age and race. Most BCISs are found through screening mammograms and, left untreated, some of these may develop into invasive breast cancer.
When the scientists adjusted for known breast cancer risk factors, they found the risk of BCIS was about 35 per cent lower in those who reported exercising compared to those who were inactive.
When they looked at only those women with no family breast cancer history - that is those without a mother or sister who had been diagnosed with the condition - the risk of BCIS appeared reduced with increasing hours per week of exercise. Those who did more than four hours a week had a 47 per cent lower risk than inactive women.
The findings are on par with those from previous international studies, says Ms Kathy Chapman, physical activity spokeswoman for the NSW branch of the Cancer Council of Australia.
Reviews by the International Agency for Research on Cancer suggest the reduction in breast cancer risk with exercise is on average between 20 and 40 per cent, Ms Chapman says. These figures are based on comparisons between the highest and lowest activity levels reported in populations.
"In the really big population studies, it's the people who do 60 minutes of activity a day who have the lower risk," she told ABC Science Online. "But the Cancer Council supports people aiming for at least 30 minutes. And it does look to be the case, that for cancer protection, some of that activity should be vigorous."
Previous research by the Cancer paper's authors and others has shown exercise reduces risk for invasive breast cancer, but relationships between physical activity and the more treatable early form of the disease are only poorly understood.
Exercise is known to alter oestrogens, the female hormones known to be linked to breast cancers.
"Although we presume physical activity works through hormonal means... this may not be an important mechanism for women who have a hereditary form of the disease," says senior author Professor Leslie Bernstein.
Exercise could also have a protective function by boosting the efficiency of the immune system or by increasing the body's sensitivity to the hormone insulin.