Scientists say that doing 10 hours of gentle exercise a week – which includes household chores – drastically cuts the likelihood of the illness for all age-groups.
American scientists believe that being active helps prevent the formation of fatty tissue, which in turn is known to trigger cancerous tumours.
They compared the lifestyles of 3 059 women aged 20 to 98, including 1 504 patients who had been diagnosed with breast cancer.
Each woman was asked to record how much physical activity she did a week, which included walking, household chores, cycling and jogging.
They found that women who did 10 to 19 hours of exercise a week were 30 percent less likely to have been diagnosed with breast cancer. This amounts to between an hour-and-a-half and nearly three hours daily.
Currently, the British Department of Health recommends that adults exercise for at least two-and-a-half hours a week, which ideally includes some intense activity.
But Lauren McCullough, of the University of North Carolina, insisted women did not necessarily need to go near a gym to reap the benefits.
She said: “The take-home message here is that it doesn’t need to be intense exercise.
Activity could include walking, housework or biking.”
But the study, published in the journal Cancer, found that the benefits of exercise in preventing breast cancer were cancelled out if women gained weight.
The scientists suspect that exercise helps inhibit tumour growth by reducing the number of fat cells and tissue.
These release certain hormones such as oestrogen and testosterone, which encourage tumour growth.
Previous studies have shown that women with high levels of these hormones in their bodies are two or three times more at risk of breast cancer.
Last year, Oxford University researchers claimed obesity was the main cause of breast cancer in older women. The academics, who had looked at the records of 6 300 women, said being fat was a more significant trigger than alcohol consumption or smoking.
Around one in eight women will develop breast cancer at some point in their lifetime, according to Cancer Research UK.
There are 48 000 new cases in Britain a year and it leads to 11 500 deaths.
But experts from Cancer Research UK played down the findings of this latest study.
Sarah Williams, health information officer at the charity, said: “There’s a substantial amount of evidence that breast cancer is less common among women who are more physically active. But just how active women need to be to reduce their risk is not yet clear.
“This study doesn’t make the picture any clearer, because the results aren’t statistically significant – in other words, the researchers can’t be sure that their results aren’t just down to chance.
“And rather than measuring things directly, the researchers asked women to remember how much exercise they had done outside work for every decade of their lives since their 20s, as well as how much they had weighed.” – Daily Mail