Dr Leslie Bernstein, of the Department of Preventive Medicine at the University of Southern California, told the Lancet breast cancer conference in Bruges yesterday that younger women who did four or more hours of exercise a week reduced their risk by more than half compared with inactive women.
Her findings were based on a study of nearly 1,000 women under 40, half of whom were breast cancer patients.
Her work is drawn from the effects of exercise on ovulation. It is known that female athletes in training often miss periods or fail to ovulate in their monthly cycles, and that early onset of periods increases the risk of breast cancer in later life. Doctors have found much evidence to suggest that circulating female hormones influence the risk of breast cancer.
'Physical activity leading to cycle disturbance reduces exposure to oestrogens. Exercise is a natural mechanism by which women can take control,' Professor Bernstein said.
In another study her team has looked at menstrual cycle disturbance in college students who jogged two to three hours a week. It found that the earlier the girls began their periods the more likely they were to have established regular cycles in which they ovulated.
'You don't have to start exercising in adolescence particularly, except that if you are physically active when you are young you are more likely to stay active when you are older,' she said. The team is now studying exercise in post-menopausal women to see if the effects of exercise on breast cancer are still beneficial in middle age.
Dr Gordon McVie, scientific director of the Cancer Research Campaign, one of the conference chairmen, said that US research in the 1970s had first suggested that exercise could reduce the risk of lung, bowel and breast cancer. 'Lack of exercise came out as a strong factor. It may well be linked to diet as well, but I think we now have to look at breast cancer and exercise carefully in the UK.'
A study by Dr Clair Chilvers, of Nottingham University, has found no evidence that smoking significantly increases the risk of breast cancer, but found a slightly increased risk among women who drank alcohol daily compared with those who never drank.Reuse content