The five-a-day diet has been the advice of the World Health Organisation (WHO) since 1991 after a series of studies consistently showed that a diet rich in fruit and vegetables resulted in lower incidences of heart disease and some cancers.
"Our results are consistent with current recommendations that a diet high in vegetable products be part of a healthy lifestyle for prevention of high blood pressure and related chronic diseases," wrote lead author Professor Paul Elliot from Imperial College London.
However the study, which followed 4680 people in China, Japan, the US and the UK, did not confirm other results that a high total protein intake, from both vegetable and animal sources, was linked to lower blood pressure.
The cross-sectional epidemiological study measured dietary intake on four separate occasions based on 24-hour dietary recall. Blood pressure was measured eight times at four visits.
"Our main finding was an inverse relationship between individuals' vegetable protein intake and their blood pressure," said the researchers in the January issue of Archives of Internal Medicine (Vol. 166, pp. 79-86).
After adjusting for height and weight, the researchers found no link between intake of animal protein and blood pressure, nor was there a significant link between total protein intake and blood pressure.
The distinction between animal and vegetable protein is important since the amino acids of a diet rich in vegetables are significantly different than those of an animal diet.
"A potential mechanism is the action of constituent amino acids, several of which have been implicated," said Elliot.
Previous studies have suggested that L-arginine, L-tryptophan, and tyrosine could reduce hypertension.
However, the researchers could not say definitively that vegetable proteins were the reason behind the lower blood pressure.
"Other components of diets high in vegetable protein (e.g. magnesium) may interact with amino acids to lower blood pressure," they wrote.
The new research is part of the INTERMAP study on macronutrients, micronutrients, and blood pressure. Data collection is now complete for the study but funding until June 2009 will allow the scientists to fully analyse the results.
The European Heart Network (EHN) cautiously welcomed the new results, claiming that, although the study reached reasonably prudent conclusions, there were some limitations.
A spokesperson for EHN told NutraIngredients.com that an aspect, "which has been somewhat neglected, is the diverse amounts of various other bioactive compounds (such as polyphenols) that are present in a high versus a low vegetable diet."
"EHN believes that these aspects deserved to be taken in consideration and at least commented upon," they said.
The EHN called for more intervention studies to investigate directly the significance of the INTERMAP results.
Hypertension, defined as having a blood pressure higher than 140/90 mm Hg, affects an estimated 600 million people worldwide, and is the cause of over seven million deaths.