The study raises questions about government public health campaigns that recommend five or more servings of fruits and vegetables daily to reduce both risk of heart disease and cancer.
It also underlines the grey area between public health messages and the evidence required to support a claim for the health benefits of fruit and vegetables.
Cancer and coronary heart disease account for 60 per cent of all early deaths, according to the UK's department of health, which declares on its website that increasing fruit and vegetable consumption is the second most important cancer prevention strategy, after reducing smoking.
But researchers from the Harvard School of Public Health in Boston say the overall associations between fruit and vegetables and cardiovascular disease and cancer have rarely been evaluated in large cohort studies.
In a new analysis of data from more than 100,000 participants in two large cohort studies, the Nurses' Health Study and the Health Professionals' Follow-up Study, they found an inverse association between total fruit and vegetable intake and the risk of cardiovascular disease but no relationship with cancer incidence.
"Consumption of five or more servings of fruits and vegetables per day has been recommended in the National 5 A Day for Better Health Program for cancer prevention, but the protective effect of fruit and vegetable intake may have been overstated," the researchers write in the 3 November issue of the Journal of the National Cancer Institute (vol 96, no 21, 1577-1584).
In an analysis of different groups of fruits and vegetables, consumption of green leafy vegetables showed the strongest inverse association with both cardiovascular disease and major chronic disease (cancer and cardiovascular disease combined).
While the findings for cardiovascular disease "still support the recommendations of the American Heart Association of consuming at least five servings of fruits and vegetables per day", the evidence for a cancer-protective effect appears limited.
In an editorial, Arthur Schatzkin and Victor Kipnis of the National Cancer Institute discuss the possibility that substantial errors in measuring diet as well as other confounding factors in the study may have distorted true associations between fruit and vegetable intake and cancer.
"The evidence is simply inadequate at this time to determine whether fruit and vegetable intake confers modest protection against cancer. Researchers should recognize this uncertainty in nutrition and cancer epidemiology and do what it takes to move ahead, especially when it comes to improving exposure assessment in observational studies," they write.
The UK's department of health is also set to come under pressure from industry, confused by the discrepancy between public health advice and laws on health claims.
Last week the country's second biggest supermarket Asda was fined £5,000 for claiming that the antioxidant properties of mangoes could help to fight cancer. Asda said it is planning to write to the UK health minister John Reid for help in clarifying the law.
A spokesman from the department of health said the government's advice comes from recommendations by the Committee on Medical Aspects of Food Policy and Nutrition, which concluded in 1998 that a review of evidence showed that higher vegetable consumption would reduce the risk of colorectal cancer and gastric cancer. There was also weakly consistent evidence that higher fruit and vegetable consumption would reduce the risk of breast cancer.
"The World Health Organisation has also recommended the consumption of 400g (five portions) of fruit and vegetables a day within a balanced diet to help reduce the risk of chronic diseases," said the spokesperson.