Professor Tim Key, an epidemiologist from Oxford University, reviewed evidence from previous studies on whether eating fruit and vegetables prevents cancer.
He concluded that although there are undoubted benefits from eating fruit and vegetables, there is little hard evidence that they protect against cancer.
He found much stronger evidence that being overweight or obese and drinking more alcohol than recommended daily limits raised cancer risk.
Why was the issue re-examined?
Professor Key said that, although the possibility that fruit and vegetables may reduce the risk of cancer has been studied for over 30 years, no protective effects have been firmly established.
He therefore looked at evidence from 12 recent studies, involving over 100,000 patients with cancers of the mouth, throat, stomach, bowel, lung, breast and prostate and calculated overall cancer risk rates.
In well-nourished populations, increases in fruit and vegetable intake would not have much effect on cancer rates, Professor Key concluded.
'Currently, advice in relation to diet and cancer should include the recommendation to consume adequate amounts of fruit and vegetables, but should put more emphasis on the well-established adverse effects of obesity and high alcohol intakes on cancer risk,' he said.
What do other researchers say?
Sara Hiom, director of health information at Cancer Research UK, said the links between cancer and both obesity and excessive drinking were poorly recognised by the public.
'Too few people know about the significant cancer risks associated with obesity and drinking too much alcohol,' she said.
'While stopping smoking remains the best way to cut your chances of developing cancer, the importance of keeping to a healthy weight and cutting down on alcohol shouldn't be overlooked.'
She added: '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.'