Plenty of vigorous exercise may cut a healthy, older woman's breast cancer risk by 30 per cent, researchers said on Friday.

A study of more than 30,000 post-menopausal women showed that strenuous activity — ranging from housework such as scrubbing floors to running — protected against breast cancer even among those who do not have a higher risk, the researchers said.

The effect was clearest among lean women.

"We know that being overweight puts women at increased risk of breast cancer," said Michael Leitzmann, who led the study while at the National Cancer Institute of the U.S. National Institutes of Health.

"What our study shows is that even among women without this increased risk, if they exercise they can get some benefit."

Breast cancer is the leading cause of cancer deaths among women worldwide, according to the American Cancer Society. The group estimates about 465,000 women died of breast cancer globally in 2007, and 1.3 million new cases were diagnosed.

A number of studies have shown that regular strenuous exercise can help people avoid heart disease, cancer and a range of other conditions.

Leitzmann and colleagues used questionnaires to determine how often the women exercised vigorously. All were healthy when the study began.

After 11 years the researchers found that overall the volunteers who exercised most were 13 per cent less likely to have developed breast cancer.

The reduced risk was even higher — 30 percent — when the researchers compared only women of normal weight, Leitzmann, now working at Germany's University Hospital in Regensburg, said in a telephone interview.

"The relationship was much stronger among leaner women," he added.

Interestingly, non-vigorous activity such as light housework, walking, hiking and easy jogging, did not seem to offer any protection against breast cancer, the team reported in BioMed Central's Breast Cancer Research journal.

The researchers did not look at why exercise may help but Leitzmann noted other studies have shown that working out reduces estrogen levels —a known risk factor for the disease — and protects the body's general immune system.