People should be warned that cancer is linked to obesity and alcohol, rather than urged to eat more fruit and vegetables to protect against the disease.
A review, published today (Wednesday) in the British Journal of Cancer, which looks at decade of evidence on the links between fruit and vegetables and the development of cancer, concludes that the evidence is still not convincing.
The only diet-related factors that definitely affect cancer risk are obesity and alcohol. But tobacco is still the single biggest cause of cancer.
Professor Tim Key, an epidemiologist from Oxford University, says that while there are undoubted benefits in eating fruit and vegetables there is little hard evidence that they protect against cancer.
But the evidence is indisputable that cancer is strongly linked to being overweight or obese, and drinking more alcohol than the recommended daily limits.
He said: "Fruit and vegetables are an important part of a healthy diet and a good source of nutrients. But so far the data does not prove that eating increased amounts of fruit and vegetables offers much protection against cancer.
"But there's strong scientific evidence to show that, after smoking, being overweight and alcohol are two of the biggest cancer risks."
Overweight people produce higher levels of certain hormones than people of a healthy weight and this can contribute to an increased risk of breast cancer.
Being overweight can increase your risk of other common cancers like bowel and also hard-to-treat forms of the disease like pancreatic, oesophageal and kidney cancer.
When alcohol is broken down by the body it produces a chemical which can damage cells increasing the risk of mouth, throat, breast, bowel and liver cancers.
In the UK 15,000 cases of cancer are caused by alcohol. And 19,000 cases of cancer are caused by being overweight or obese.
Sara Hiom, director of health information at Cancer Research UK, said: "Too few people know about the significant cancer risks associated with obesity and drinking too much alcohol. While stopping smoking remains the best way to cut your chances of developing cancer, the importance of keeping a healthy weight and cutting down on alcohol shouldn't be overlooked.
"Keeping alcohol intake to a maximum of one small drink a day for women and two small drinks per day for men and keeping weight within the healthy limits can have an enormous impact."
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