FOR snivelling children and recalcitrant carnivores, requests that they should eat five portions of fruit and vegetables every day have mostly fallen on deaf ears. But those who did comply with official advice from charities, governments and even the mighty World Health Organisation (WHO), could remind themselves, rather smugly, that the extra greens they forced down at lunchtime would greatly reduce their chances of getting cancer. Until now, that is. Because a group of researchers led by Paolo Boffetta, of the Mount Sinai School of Medicine in New York, have conducted a new study into the link between cancer and the consumption of fruit and vegetables, and found it to be far weaker than anyone had thought.

In the past, veggie-associated reductions of cancer-risk rates as high as 50% had been reported. But it appears that some of these early investigations may have been biased by the use of “case-control” studies. Such studies try to identify the factors contributing to cancer by comparing people who have the disease with those who do not, but are otherwise similar. The problem is that they can easily be biased if researchers do not adequately establish that the two groups being compared are, indeed, otherwise similar. Walter Willet, at the Harvard School of Public Health, says it appears that earlier investigations were more likely to use health-conscious people as their controls. These types of people are, unsurprisingly, more likely to agree to be interviewed about their health than slobby couch potatoes.

Dr Boffetta and his colleagues have therefore carried out a different kind of study, known as prospective cohort study, which they report in the Journal of the National Cancer Institute. Their work follows a group of individuals over time and looks at how different factors contribute to different outcomes—in this case, the development of cancer. Analysis of dietary data from almost 500,000 people in Europe found only a weak association between high fruit and vegetable intake and reduced overall cancer risk.

Green with envy

According to Susan Jebb, of the British Medical Research Council's Collaborative Centre for Human Nutrition Research in Cambridge, the new study suggests that if Europeans increased their consumption of fruit and vegetables by 150g a day (about two servings, or 40% of the WHO's recommended daily allowance), it would result in a decrease of just 2.6% in the rate of cancers in men and 2.3% in women. Even those who eat virtually no fruit and vegetables, the paper suggests, are only 9% more likely to develop cancer than those who stick to the WHO recommendations.

On the face of it, that is quite a blow to the smug salad eaters, and the health lobby's spin-doctors were out in force in the wake of the paper's publication, to play down its conclusions. Before racing to the food-recycling bin with the contents of an ageing fruit bowl, they pointed out, there are a number of other factors that nutritionists would urge that you consider.

One is that this kind of study has attempted to adjust for every possible factor that might contribute to the relationship, and isolate only the contribution that fruit and vegetables make. This means that if people who turn away from fruit and vegetables end up eating more processed meats or foods high in fat instead, they probably will increase their cancer risk, even though the direct cause is not the consumption of less fruit and veg.

More importantly, there is still good evidence that fruit and vegetables protect against heart disease and strokes by reducing blood pressure. A separate investigation of the people involved in Dr Boffetta's study suggests that those who eat five servings a day of fruit and vegetables have a 30% lower incidence of heart disease and strokes than those who eat less than one and a half servings. It is also possible that some specific foods, such as tomatoes, broccoli and other cruciferous vegetables, do offer protective effects against particular kinds of cancer.

As a consequence, the best advice is probably still to eat your five a day. But for snivelling children and recalcitrant carnivores the fleeting thought that you might not have to was nice while it lasted.