Writing in the the March issue of the Journal of the American Dietetic Association Susie Nanney, acting director of the US-based Obesity Prevention Center at Saint Louis University, claims that although consumers recognise a healthy diet should include at least five fruits and vegetables, they are not making the most nutritious choices because messages about what to eat are unclear.
"They are not translating 'variety' in a way to capture health benefits, such as reducing their risk of developing chronic diseases," said Nanney.
She claims that the United States Department of Agriculture, the Department of Health and Human Services, the American Cancer Society, and the American Heart Association offer conflicting messages about which fruits and vegetables are most nutritious.
"You can see how the public gets confused by inconsistency in the messages. Until nutrition messages become more consistent and direct, we may not see improvements in powerhouse vegetable and fruit intake behaviours to any great extent," she said.
The fruit and vegetables that do the best job in reducing the health risk for chronic disease are dark green leafy vegetables, yellow/orange fruits, citrus fruits and cruciferous veg. But, says Nanney, even those guidelines can be confusing.
With obesity and heart disease rates rising - according to the WHO, heart disease kills more people around the world than any other disease - governments are finding themselves weighed down by rapidly mounting health bills.
Looking for strategies to improve the health of populations, many have launched a series of national health guidelines reaching out to the consumer to encourage them to consume '5 a day' fruit and vegetables.
But if, as Nanney suggests, the message is mixed, millions of euros may already have been wasted.
Advice to eat a specific quantity of fruit and vegetables has its roots in the UN agency, the World Health Organisation (WHO) that in 1991 started recommending a minimum intake of 400g fruit and vegetables a day. One portion of fruit and veg is 80g, so five portions add up to 400g.
The WHO advice - adopted by several national governments - is based on a wide range of international different studies that have shown consistently that populations with a high intake of fruit and vegetables have a lower incidence of heart disease, some cancers and other health problems.
In 1994, the Committee on Medical Aspects of Food and Nutrition Policy (COMA), which advises the UK government, examined the links between diet and heart disease. COMA concluded that fruit and veg help to protect against heart disease and that people in the UK should increase the amount they eat to at least five portions a day.