Eating lots of fruit and vegetables will do little to reduce your risk of developing cancer, according to a review of a decade of research involving more than a million people. It concluded that maintaining a healthy weight and cutting down on smoking and drinking are far better ways to ward off the disease.

Vegetables and fruit are important for a healthy diet but the review says that eating increased amounts does not seem to offer much protection against cancer.

"There's strong scientific evidence to show that, after smoking, being overweight and alcohol are two of the biggest cancer risks," said Tim Key, an epidemiologist from Oxford University, who wrote the review.

In an article published today in the British Journal of Cancer, Key summarised the epidemiological evidence from more than a million people taking part in several dozen long-term research projects looking at the amount of fruit and vegetables people eat and their overall cancer risk. He also studied specific cancers of the gastrointestinal tract, lung and breast.

Key found little, if any, connection between eating lots of fruits and vegetables and the likelihood of developing cancer. "The conclusion implies that, at least in relatively well-nourished westernised populations, a general increase in total fruit and vegetable intake will not have a large impact on cancer rates," he wrote. "A certain level of intake is necessary to prevent nutrient deficiencies, but intakes above that level do not make the relavant tissues 'super healthy'."

The studies included data from the European Prospective Investigation into Cancer and Nutrition, the Pooling Project based at Harvard University, and the National Institutes of Health and American Association of Retired Persons Diet and Health Study.

The idea that fruit and veg might help reduce cancer rates was first postulated in the 1970s, when the results of a small-scale study showed that, after controlling for smoking, people with reduced intakes of vitamin A were at increased risk of lung cancer. By the 1990s, scientists were concluding that "for most cancer sites, persons with low fruit and vegetable intake experience about twice the risk of cancer compared to those with a high intake, even after control for potentially confounding factors."

But these "case-control" studies – where people with a disease are matched with controls who do not have the disease – still suffered from confounding factors. "While a lot of those [earlier] case-control studies do try and adjust for how much people smoke and how much people drink, there's always a worry that you haven't completely adjusted for that because smoking and drinking have such a massive impact on the risk of those cancers," said Ed Yong, head of health evidence and information at Cancer Research UK.

Key said case-control studies can suffer from two main types of bias. "One is that people with cancer may be under treatment so it may affect how they remember what they used to eat in the years before they developed cancer. They may be feeling ill or under strong medical treatment."

A bigger problem is with the selection of the control group, which might not be entirely random. "People who come forward are those interested in health and related behaviours," said Key. "The controls may well appear to have a healthy diet because the potential controls with an unhealthy diet may have stayed in the pub eating chips and beans and not volunteered to be studied."

A better way to analyse the relationship between diet and cancer is to conduct "prospective studies", which ideally follow hundreds of thousands of people who don't have cancer. "They tell you what they eat and you follow them until, inevitably, some of them do develop cancer," said Key. "But you made the measurements when they're healthy, so the biases don't apply. Those types of studies have been coming out in the last 10-15 years and have not supported the original findings [from case-control groups]."

Key's review supports work published in April in the Journal of the National Cancer Institute. Paolo Boffetta from the Tisch Cancer Institute at Mount Sinai School of Medicine in New York found that eating a lot of fruit and vegetables has only "a very modest" protective effect against cancer. That conclusion was based on a decade of research on almost 500,000 people in 10 European countries.

Despite the results of the studies, Yong said it was still a good idea for people get their minimum daily five portions of fruits and vegetables. "It's not a bad message because it could help people to lose weight, which is a massive cause of cancer, and it could displace other [unhealthier] things in their diet," he said. "There's no harm to eating lots of fruits and vegetables and there are benefits for other diseases as well, such as heart disease."

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