Physically active women are 25 per cent less likely to get breast cancer, but certain groups are more likely to see these benefits than others, finds a review of research published online ahead of print in the British Journal of Sports Medicine.
The type of activity undertaken, at what time in life and the woman's body mass index (BMI) will determine how protective the activity is against the disease.
Lean women who play sport or undertake other physically active things in their spare time, especially if they have been through the menopause, have the lowest risk of breast cancer.
The researchers reviewed the literature and analysed 62 studies looking at the impact of physical activity on breast cancer risk. They then examined the findings to find out how breast cancer risk appeared to be affected by type of activity, intensity of activity, when in life the activity was performed and other factors.
They found the most physically active women were least likely to get breast cancer. All types of activity reduced breast cancer risk but recreational activity reduced the risk more than physical activity undertaken as part of a job or looking after the house. Moderate and vigorous activity had equal benefits.
Women who had undertaken a lot of physical activity throughout their life had the lowest risk of breast cancer, and activity performed after the menopause had a greater effect than that performed earlier in life.
Physical activity reduced breast cancer risk in all women except the obese and had the greatest impact in lean women (BMI < 22kg/m2)
Women who were mothers, had no family history of breast cancer, were not white and had oestrogen receptor negative tumours also had a reduced risk of breast cancer.
The authors said the way in which physical activity protected against breast cancer was likely to be complex and may involve effects on sex hormones, insulin-related factors, the immune system and other hormone and cellular pathways.