A US study showing older women with higher levels of body fat are up to twice as likely to develop breast cancer - even if they're not overweight - was welcomed by British experts.
The study of almost 3,500 over 50s found the risk rose by more than a third for each every 11lb (five kilo) rise - despite them having a normal BMI (body mass index).
It underlines the importance of physical activity is staving off the disease, thee most commonly diagnosed cancer in British women.
More than 55,000 women are diagnosed with breast cancer each year in Britain, with almost 11,500 dying from it annually.
The discovery was unexpected as a higher BMI is linked to cancer as well as other conditions such as heart disease, high blood pressure and type 2 diabetes.
It adds to growing evidence that it is not the most accurate health indicator as it can't distinguish between fat and muscle, which tends to be heavier.
This can tip more toned individuals into overweight status, even if their fat levels are low.
Dr Tom Beattie, Health Information Officer at Breast Cancer Now, said: "This interesting study reinforces that having a bigger waist size may increase your risk of breast cancer, even if your BMI is within a healthy range.
"But crucially it also supports existing evidence that keeping physically active can reduce your breast cancer risk in other ways that are not actually linked to losing weight, which we hope will help many more women lower their chances of developing the disease.
"We know that the more fat tissue you have around your body, the higher your levels of oestrogen are likely to be, which can increase your risk of breast cancer.
"Our BMI and waist size are a very good proxy for estimating our body fat and its impact on our breast cancer risk - and it's important to know that a waist size over 31.5 inches is considered to be high and may increase your risk of breast cancer, even with a healthy BMI.
"While we cannot change some things that affect our risk, such as getting older, there are some things we can do something about.
"We'd encourage all women to keep physically active, to try to maintain a healthy weight and to reduce their alcohol intake to help keep their breast cancer risk as low as possible."
Medical oncologist Dr Neil Iyengar, of Memorial Sloan Kettering Cancer Centre in New York, added: "It is also notable the level of physical activity was lower in women with higher amounts of body fat.
"This suggests physical activity may be important even for those who are not obese or overweight."
The study, presented at the American Association for Cancer Research Special Conference Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes, involved 3,460 participants.