A new study has shown that girls and young women who take regular exercise can considerably reduce their risk of developing breast cancer before the menopause.
But a nurse specialist warned that more specific research is needed to look at other factors which may impact on breast cancer risk.
The study of nearly 65,000 women, by researchers at the Washington University School of Medicine in St Louis and Harvard University in Boston, found that those who were physically active had a 23% lower risk of developing breast cancer before the menopause than those who were not.
High levels of physical activity in those aged 12-22 contributed most strongly to the lower breast cancer risk, the study, published in the Journal of the National Cancer Institute, found.
Jackie Harris, a breast health nurse specialist at Breast Cancer Care, said the study did not take into account lifestyle factors or family and reproductive history, which may affect a woman's risk of developing breast cancer.
"More specific research is needed to understand the impact of different types and levels of exercise in reducing premenopausal breast cancer risk," she said.
"It is also important to remember that 80% of breast cancers occur in women aged over 50."