Obesity is the biggest driving force behind the most common form of breast cancer in older women, say researchers.
Alcohol and then cigarettes are the next largest culprits, according to Cancer Research UK.
One in eight women in the UK develop breast cancer in their lifetime, data shows, and the majority of these tumours are “hormone sensitive” meaning their growth is fuelled by hormones.
Too much stored fat in the body raises the level of these “sex” hormones.
Studies show that post-menopausal women with high levels of oestrogen and testosterone have between two and three times the risk of breast cancer than women with the lowest levels.
Experts have known for some time that factors that influence hormone levels – like pregnancy, the oral contraceptive pill and the menopause – can change a woman’s breast cancer risk.
This latest work, published in the British Journal of Cancer, suggests obesity should go at the top of this list, not least because it is a lifestyle factor that women can have some control over.
The Oxford University team, funded by Cancer Research UK, studied the health records of nearly 6,300 post-menopausal women, looking for factors that might explain why some developed hormone sensitive breast cancer when others did not.
A woman’s weight had the greatest bearing on a woman’s sex hormone levels, shortly followed by smoking and alcohol consumption.
Women who are overweight or obese – meaning they have a body mass index of 25 or more – had high levels of hormones like oestrogen and progesterone.
So too did women who drank more than two and a half units of alcohol a day or smoked more than 15 cigarettes daily.
Experts say women should be made aware of these modifiable risk factors.
Dr Julie Sharp, of Cancer Research UK, said: “This is an important study as it helps to show how alcohol and weight can influence hormone levels. Understanding their role in breast cancer is vital and this analysis sheds light on how they could affect breast cancer risk.
“We know that the risk of the disease can be affected by family history and getting older, but there are also things women can do help reduce the risk of the disease. Maintaining a healthy body weight and reducing alcohol consumption are key to reducing breast cancer risk.”