Women of all ages can reduce their risk of breast cancer by exercising regularly, according to researchers.
Swimming is just one of the ways to reduce the risks
Previous studies have shown that regular exercise can protect against the disease.
But now a study in the United States involving over 74,000 women shows that it is never too late to start.
Writing in the Journal of the American Medical Association, they said that even post-menopausal women can reduce their risks by exercising.
Dr Anne McTiernan and colleagues from the Fred Hutchinson Cancer Research Center in Seattle based their findings on a survey of each of the women.
They were all asked how often they exercised and whether they had exercised when they were younger.
The researchers found that women who exercised regularly reduced their risk of developing breast cancer by 20%.
Moderate exercise, such as walking, cycling or swimming, five times a week had the greatest impact.
They found that physical activity also reduced the risks for women who may have a greater chance of developing the disease.
This includes those with a family history of breast cancer and those taking hormone replacement therapy.
The study suggests that the benefits are greatest for women of low to normal weight and even those who are slightly overweight.
They found that regular exercise did not help overweight or obese women.
However, Dr McTiernan urged these women to start exercising too.
"This finding certainly shouldn't be an exercise disincentive for obese women," she said.
"There are many benefits for women of any weight to start exercising, like reducing their risk of heart disease and diabetes.
"But in terms of breast cancer risk, obese women will see most benefit once they start getting their weight down."
She suggested that exercise reduces the risk of breast cancer by reducing fat levels in the body.
"We think that exercise works to lower cancer risk by lowering body fat, which in turn lowers the levels of circulating cancer-promoting hormones.
"So even if a woman is exercising, if she's overeating and her body fat stays high, she's not going to get the same cancer-fighting protection as a women with less body fat."
She said women should aim to do moderate exercise 30 minutes each day, five days a week. Women who do not exercise regularly should start off slowly, increasing their physical activity over time.
Breast Cancer Care welcomed the study.
Anna Wood, its policy analyst, said: "Age is the strongest risk factor for breast cancer, and this research shows that moderate exercise such as walking or swimming is something older women can do to help reduce their risk of breast cancer.
"Women often want to know what they can do in order to reduce their risk of breast cancer and this research suggests a simple, positive and empowering lifestyle change."