Obesity is the biggest avoidable cause of breast cancer, a study claims.
Women who are dangerously overweight are at a far higher risk even than those who drink heavily or smoke.
Experts have long known that the excesses of a modern, unhealthy lifestyle can dramatically increase the likelihood of breast cancer.
But now scientists have shown that obese women have far higher levels of cancer-triggering hormones than those who smoke or drink to excess.
Breast cancer most commonly affects women just after the menopause and nearly half of all cases occur in those aged 50 to 70.
In the past 30 years rates of the disease have risen dramatically and 47,700 women are now diagnosed every year.
Over the same period, levels of obesity have also soared – nearly one in five females is now deemed so overweight that her health is at risk.
Researchers at Oxford University looked at levels of cancer-triggering hormones including oestrogen and testosterone in 6,300 post-menopausal women.
They wanted to establish how hormone levels are influenced by different factors in women most susceptible to being diagnosed with breast cancer.
The scientists found that obesity had a much greater influence on these hormones than alcohol consumption or very heavy smoking.
On average, obese women had oestrogen levels 50 per cent higher than those of normal weight. Testosterone levels were 16 per cent higher. Obesity is defined as having a body mass index greater than 30, which is equivalent to a 5ft 6in woman weighing 13.5 stone.
Experts believe that fat tissue secretes oestrogen and another hormone, insulin, which in turn triggers production of testosterone.
The researchers, who looked at 13 published studies, also found that drinking two and a half units of alcohol a day – one large glass of wine – increased levels of some cancer-triggering hormones by 18 per cent, but had no effect on oestrogen.
Smoking more than 15 cigarettes a day also raised the level of some hormones by a small amount, the study, published in the British Journal of Cancer, found.
A woman’s chance of developing breast cancer depends on a range of other factors including family history, age, time of menopause and whether she has had children.
But scientists say that obesity is the biggest avoidable cause of the disease.
Breast cancer is by far the most common form of cancer in women and they have a one in eight chance of being diagnosed at some point during their lifetime.
Dr Julie Sharp, Cancer Research UK’s senior science information manager, said: ‘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.’
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