Fighting fit: Exercising during pre-reproductive and reproductive years is essential

The number of women with breast cancer has risen by 81 per cent in the past 33 years in Britain. Although breast cancer death rates are falling, the statistics are worrying for women, because scientists say it’s hard to pin down the exact cause of the rise.

However, there is something women can do to reduce the risk of breast cancer: exercise. Prof Leslie Bernstein, the chair in cancer research at the University of California, draws on 20 years’ research into the effect of exercise on breast cancer rates and concludes that young girls can significantly reduce their risk of developing breast cancer as they get older if they exercise regularly in their teens. And both pre-menopausal and post-menopausal adults can improve their odds of staying clear of the disease by keeping active.

Her research indicates that exercising over a lifetime seems to have the strongest protective effect; young women who exercise for just four hours a week over their entire reproductive years experience more than a 50 per cent reduction in breast cancer risk. But exercising in adolescence may be particularly crucial; breast cancer risk is reduced by 30 per cent among women who exercised for two hours or more every week during their teens.

New data indicates that physical activity may have a special importance in preventing a type of breast cancer called oestrogen-receptor negative (ORN) breast cancer, with women who exercise five times or more a week experiencing a 50 per cent reduction in risk of these types of tumour.

In women who have already had the other main kind of breast cancer, oestrogen-receptor positive (ORP), hormone treatments such as tamoxifen can protect against recurrence. This doesn’t work with ORN tumours, so exercise is potentially one of the best preventive measures.

For years, the risk factors for breast cancer have been accepted as gender, family history, early periods, late menopause, not having children and age. That still applies, says Prof Alan Ashworth, director of the Breakthrough Research Centre, London. “But, to reduce her risk, exercising is a good strategy. You could also lose weight, but that only seems to be a risk factor in post-menopausal women,” he says.

As with all emerging areas of scientific knowledge, however, frustrations surround what we don’t yet know. Prof Bernstein cannot be prescriptive on what type of exercise and how much has a protective effect, and research has not yet revealed how long it takes for the protection to build up. Prof Bernstein believes, however, that it is exercise in the pre-reproductive and reproductive years that is most significant: in post- menopausal women, it still seems to help but partly because it counters obesity, another risk factor.

Many studies have shown that strenuous physical activity reduces the production of female hormones. Oestrogen seems to play a key role in the development of breast cancers because it regulates the rate of reproduction of certain types of cells in the breast. This is why hormone replacement therapy, not having children and a late menopause are all risk factors. It’s all to do with how much oestrogen the breast is exposed to.

The cumulative number of ovulatory cycles a woman has in a lifetime is another influence on this. Prof Bernstein believes that how early a girl starts her periods may be an especially important part of this equation. Exercising from an early age can have a beneficial effect because it tends to delay the first period.

“In a study of 14 to 18-year-olds that we did in a high school, we gathered a perfect record of menstruation and how much exercise the girls had done, from going up stairs at home to organised activities at school,” she says. “When you compared the active girls with the inactive girls, they were two to three times more likely to experience, instead of full ovulation, a type of immature menstrual cycle where they don’t ovulate. That means they don’t release oestrogen and progesterone in the same way. So their exposure to the hormones will be lower and their chances of breast cancer in later life will be lower.”

Prof Bernstein says that of 40 reputable studies that have looked at the impact of exercise on breast cancer risk, all but two show a reduction in risk. This reflects a growing acknowledgement of the importance of exercise in preventing cancers generally.