Summary
A paper in the British Journal of Cancer describes a literature review of the best studies to date investigating whether eating more fruit and vegetables reduces the risk of getting cancer. The author, Professor Tim Key of Oxford University, writes that the research suggests there is little or no benefit for relatively well-nourished, westernised populations. Even where studies have found a moderate reduction in risk, this may be accounted for by "confounding factors", namely alcohol consumption and smoking. In other words, the results are confused because people with poor diets tend to smoke and drink more than those with good diets.
The studies looked at overall fruit and veg consumption, and Key concedes that particular nutrients in certain fruits and vegetables may reduce the risk of specific cancers.
He concludes:
Nutritional principles indicate that healthy diets should include at least moderate amounts of fruit and vegetables, but the available data suggest that general increases in fruit and vegetable intake would not have much effect on cancer rates, at least in well-nourished populations. 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.
Contact us
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Key research
Boffetta et al (2010) Fruit and vegetable intake and overall cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). Journal of the National Cancer Institute; 102(8): 529–537.
George SM et al (2009) Fruit and vegetable intake and risk of cancer: a prospective cohort study. American Journal of Clinical Nutrition; 89(1): 347-353.
Other useful sources of information
Cancer Research UK statistics on lifestyle causes of cancer
Ed Yong: Fruit, vegetables and cancer – why it's still worth getting your five a day
Thursday 2 December 8.34am: The journal that published the study, the British Journal of Cancer, has put out a press release titled "Less booze not more veg is key to cut cancer risk":
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."
9.39am: The World Cancer Research Fund has issued this response:
Dr Rachel Thompson, Deputy Head of Science for World Cancer Research Fund, said: "The links between fruits and vegetables and cancer were reviewed by an expert panel of 21 scientists as part of our landmark 2007 Report. Their view was that fruits and vegetables probably reduce risk of several types of cancer, including stomach cancer and cancer of the oesophagus.
"This new review has not found anything greatly different. We would agree the evidence on fruits and vegetables and cancer is not absolutely convincing. But as part of our review, the panel judged that it is strong enough to recommend that people eat plenty of them. "Also, a big study published earlier this year suggested that if all of us ate an extra two portions a day, about 7,000 cases of cancer a year could be prevented in the UK.
"And even if there was no direct protective effect on cancer risk, people who eat plenty of fruits and vegetables are less likely to be overweight. Scientists now say that, after not smoking, maintaining a healthy weight is the most important thing you can do for cancer prevention. "So for people concerned about cancer risk, the advice remains that they should eat plenty of fruits and vegetables."
Our thanks to Richard Evans, head of communications at WCRF, for this statement.
10.36am: The World Cancer Research fund points to its general advice on plant foods and its stats page, where people can click on individual cancer types and see what percentage could be prevented by eating more fruits and/or veg.
1.11pm: The Nature press office draws our attention to this important sentence in the paper:
"Current advice in relation to diet and cancer should include the recommendation to consume adequate amounts of fruit and vegetables, but should put most emphasis on the well-established adverse effects of obesity and high alcohol intakes."
1.20pm: Cancer Research UK has published a blog analysing the literature review's findings and their implications, which concludes:
"General recommendations for people to eat plenty of fruit and vegetables will not, and should not, change in response to this study. There is plenty of evidence that fruits and vegetables play an important role in reducing the risk of high blood pressure and coronary heart disease. Also, fruit and vegetables contain many of the nutrients needed for general health."
However, the blog goes on to say that many unanswered questions remain:
"It may be that certain nutrients found in fruits and vegetables do affect cancer risk, but that the effect is restricted to certain groups of people or certain types of fruits or vegetable. This could explain why these effects haven't been seen in existing studies. Professor Key points out that more research into the basic nuts and bolts of how cancer develops could yield clues in this direction."
1.40pm: The World Cancer Research Fund has published a blog reiterating its conviction that "fruits and vegetables probably do reduce risk of cancer, although the evidence is not strong enough for us to be certain."
It says any direct protective effect seems to be limited to certain types of cancer such as those of the stomach, oesophagus, mouth, pharynx and larynx – and because fruits and vegetables are only linked to the risk of certain types of cancer, the percentage of overall cancer cases that could be prevented is quite small.
However, the blog continues: "because there are so many cases of cancer diagnosed every year, this still means thousands of cases could be prevented each year if everyone ate more fruits and vegetables".
It concludes:
"The media coverage of this review has been unhelpful. It gives the impression it has found something dramatically new, which it hasn't.
"And it is also likely to reinforce the impression that scientists are always changing their minds or cannot agree about cancer risk. Actually, the advice on how to reduce your risk has not changed that much over the last 10 years and in many areas the evidence behind our Recommendations has become stronger."
