Vigorous exercise can reduce the risk of breast cancer by almost a third, a study has found.
Scientists made the discovery after following the progress of 32,269 women for 11 years.
They found that vigorous activities lowered the risk of breast cancer by around 30%, but not if women were overweight.
These included running, fast jogging, competitive tennis, aerobics, hill cycling and fast dancing.
"Heavy" house and garden work such as scrubbing floors, washing windows, digging, and chopping wood, were also classified as "vigorous activity".
Activities classified as "non-vigorous" did not offer the same protection. Around the home, these included vacuuming, washing clothes, painting and general gardening. They also included mild forms of exercise such as walking, light jogging, recreational tennis, and bowling.
Study leader Dr Michael Leitzmann, from the National Cancer Institute at the US National Institutes of Health said: "Possible mechanisms through which physical activity may protect against breast cancer that are independent of body mass include reduced exposure to growth factors, enhanced immune function, and decreased chronic inflammation, variables that are related both to greater physical activity and to lower breast cancer risk."
The findings are published in the journal Breast Cancer Research.
Maggie Alexander, director of policy and campaigns at the charity Breakthrough Breast Cancer, said: "This study adds to our increasing knowledge that regular exercise can help reduce the risk of breast cancer.
"There is still much to be learned about what type of activity you should do and how often in order to lower your breast cancer risk. However, we do know that regular exercise can help prevent weight gain and obesity, both of which are known to increase breast cancer risk.
"Breakthrough encourages all women to lead a healthy lifestyle by maintaining a healthy weight, eating a balanced diet, taking regular exercise and limiting alcohol intake."