A major study in the US has found a significant correlation between taking part in regular activities and lower rates of invasive and early-stage breast cancer.

Regular physical exercise can help protect women against breast cancer, new research has suggested.

A major study in the US found a significant correlation between taking part in regular activities and lower rates of invasive and early-stage breast cancer.

However, the results show a vigorous level of activity is needed to have an effect and the women who benefited most spent more than five hours a week running, swimming, going to aerobics classes, or engaging in other forms of strenuous exercise.

Moderate activity, such as brisk walking or playing golf, did not have an effect. Keeping fit also protected against non-hormone-sensitive invasive cancers only.

In 80% to 85% of cases, breast cancer is fuelled by the female hormone oestrogen. It is these patients who can successfully be treated with the drug tamoxifen, which blocks the hormone's activity.

However, other cases are not hormone-sensitive and are more difficult to treat. For these women, regular vigorous exercise may be part of the answer.

UK health experts agree that adults should accumulate at least 30 minutes of moderate activity on most days.

The latest findings, published today in the journal Archives of Internal Medicine, follow recent studies which claimed that regular exercise can help tackle a number of ailments including obesity, depression, cardiovascular disease, high blood pressure and diabetes.

At the start of the US study, in California in 1995, 110,599 women teachers and former teachers, aged 22 to 79, were questioned about their moderate and strenuous physical activity since leaving school, and for the past three years.

By the end of 2002, 2649 women had been diagnosed with invasive breast cancer - when the disease has spread beyond the milk-producing ducts - while 593 had early stage or in situ breast cancer.

The researchers compared the fate of women who did more than five hours, with those who did less than 30 minutes, of vigorous exercise each week. They found the former had a 20% lower risk of invasive cancer caused by tumours that were non-hormone-sensitive. They also had a 31% lower risk of in situ breast cancer.

Protection against invasive cancer was only seen when women participated in strenuous exercise over a long period. Vigorous activity in only the past three years was not associated with reduced risk.

For women with in situ breast cancer, it did not have to be non-hormone-sensitive to show an effect. But they had to take vigorous exercise and were not helped by moderate levels of activity.

Professor Leslie Bernstein, from the Keck School of Medicine at Southern California University, Los Angeles, who led the research, said: "This study confirms that breast cancer risk is influenced by consistent participation in strenuous exercise. However, it may require substantial commitment of time to achieve this lower risk."

Jason Gill, senior lecturer at the Institute of Biomedical and Life Sciences at Glasgow University, said: "This study adds to the large body of evidence which shows that physical activity has a number of health benefits.

"The suggestion that the reduction in breast cancer risk was maximised in women undertaking at least five hours of physical activity a week highlights the fact that current guidelines in Scotland of 30 minutes, five days a week, should be regarded as a minimum."

Lorraine Dallas, head of the charity Breast Cancer Care Scotland, said: "This latest study will be welcomed by many of the women we talk to who will feel reassured that while increasing age remains the biggest risk factor, they can take control and do more to try to protect themselves. However, much more research is needed on the type and amount of exercise needed. It is important to stress that regular exercise of any sort is beneficial in maintaining good health."

Breast cancer is the most common form of the disease in the UK, with more than 44,000 cases diagnosed each year.

In Scotland, where one woman is diagnosed with the disease every three hours, it accounts for 28.7% of all cancers. Although incidence of the disease is increasing, the mortality rate dropped nearly 14% between 1995 and 2005.