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Reporting in the Feb. 26 issue of the Archives of Internal Medicine, researchers at the University of Southern California, Los Angeles suggested that Aerobic exercise conducted over the long term may reduce a woman's risk of breast cancer. 110,000  women were studied, aged 20 to 79, called the "California Teachers Study".

At the start of the study, the women were asked about their participation in moderate exercise (such as brisk walking, golf or volleyball) and strenuous exercise (such as swimming laps, aerobics, and running) from high school up until their current age and within the previous three years.

The study authors also collected information about the women's breast cancer risk factors, including race, family history and use of hormone replacement therapy.

Between the start of the study in 1995-96 and 2002, 2,649 of the women were diagnosed with invasive breast cancer and 593 were diagnosed with in situ (noninvasive) breast cancer.

Women with a long-term history of doing more than five hours per week of strenuous exercise were 20 percent less likely to develop invasive breast cancer and 31 percent less likely to develop in situ breast cancer than those who did less than 30 minutes of strenuous exercise per week, the researchers found.

These results provide evidence supporting a protective role for strenuous recreational physical activity on risk of  breast cancer. "The beneficial effects of moderate activity are less clear," the authors wrote.