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British nutritionists threw down the gauntlet to dietary guidelines in April by declaring seven daily portions of fresh fruit and vegetables, rather than the recommended five, were the key to health.

But a new foray into the arena of sound eating says the famous five-a-day recommendation made by the UN's World Health Organisation (WHO) in 2003 should be fine.

Researchers in China and the United States trawled through 16 published investigations into diet and health involving more than 830,000 participants, who were followed for periods ranging from four and a half years to 26 years.

Every additional daily serving of fruit and vegetables reduced the average risk of premature death from all causes by five percent, the scientists found.

Over the period of the studies, 56,000 of the participants died, researchers said.

In the case of death from a heart attack or a stroke, each additional serving curbed risk by four percent.

But there was no evidence of an additional fall in risk beyond five portions, according to the review, published online Tuesday by the British Medical Journal (BMJ).

"We found a threshold of around five servings a day of fruit and vegetables, after which the risk of death did not reduce further," said the investigators, led by Frank Hu of the Harvard School of Public Health in Boston, Massachusetts.

High consumption of fruit and veg did not translate into a significant reduction in the risk of death from cancer, the study also found.

In addition to advising patients about the virtues of healthy eating, doctors should also push home the message about risks from obesity, inactivity, smoking and excessive drinking, said the paper.

In April, researchers at University College London found that eating seven daily portions or more could reduce the risk of cancer by 25 percent and of heart disease by 31 percent, compared to people who consumed less than one portion a day.

The study was based on the eating habits of more than 65,000 people in England between 2001 and 2008.

The London researchers admitted to being surprised by what they found and cautioned the results may not be applicable to other countries.

Britain has one of the highest rates of heart disease in Europe, a fact blamed in part on a diet high in fat and sugar.

The WHO guidelines are based on the equivalent of five 80-gram (three-ounce) portions. One portion is roughly equivalent to a medium-sized apple, a bowl of mixed salad or three dessert spoonfuls of raw, cooked, canned or frozen vegetables.