Consuming fruit and vegetables has no effect on reducing breast cancer risk, according to a large study that looks set to override previous evidence showing potential protective effects.
The results, published in this week's JAMA (293, pp183-193), come from the European Prospective Investigation Into Cancer and Nutrition (EPIC) study, the largest trial ever designed to investigate the relationship between diet and cancer.
It includes 519,978 individuals living in 10 European countries, spanning a wide range of vegetable and fruit consumption.
"This absence of a protective association was observed among almost all of the participating countries," said Carla H. van Gils from the Utrecht University Medical Center in the Netherlands and colleagues.
Many governments are currently trying to increase fruit and vegetable intake to help prevent chronic disease. The UK government advises that increasing fruit and vegetable consumption is the second most important cancer prevention strategy, after reducing smoking.
However a 1998 review of evidence commissioned by its health department only showed 'weakly consistent' evidence that higher fruit and vegetable consumption reduces the risk of breast cancer.
The new research is based on data from more than 285,000 women from the EPIC group, aged between 25 and 70 years old. Participants completed a dietary questionnaire in 1992-1998 and were followed up for incidence of cancer until 2002.
Researchers found no significant associations between vegetable and fruit intake and breast cancer risk, and no associations for six specific vegetable subgroups.
"A protective effect is supported by a vast number of case-control studies. It is possible, however, that the inverse relationships reported from case-control studies may have been overstated, because of recall bias and possibly because early symptoms in patients may have led to a change in dietary habits," write the authors.
"In addition, selection bias is a problem in situations where control participation is less than complete because those controls who participate are likely to be more health conscious and consume greater amounts of vegetables and fruits," they added.
They noted that the size of their cohort study and the wide range of vegetable and fruit intake, thanks to the inclusion of participants living in countries from the North to the South of Europe, add to the significance of their findings.
The study confirms the results of the largest pooled analysis to date, which found no large protective effects for vegetable or fruit intake in relation to breast cancer.
But the authors add: "This does not exclude the possibility that protective effects may be observed for specific nutrients or in specific subgroups of women, such as those with a family history of breast cancer or oestrogen-receptor positive tumours."
In an accompanying editorial on diet and cancer (pp233-234), Walter C. Willett of the Harvard School of Public Health, Boston, writes: "Although recent findings on fruit and vegetable consumption and cancer may be disappointing, reductions in blood pressure and epidemiological evidence for lower risks of cardiovascular disease provide sufficient reason to consume these foods in abundance."