"Interestingly, the risk was only apparent when we took into consideration other dietary factors," wrote lead author Alvaro Alonso in the Archives of Medical Research (Vol. 37, pp. 778-786).
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.
Dietary fibre and protein have long been linked to improved blood pressure by cross-sectional studies, but these either did not account for differences between animal and vegetable protein or did not account for minerals in vegetables, said Alonso.
And Alonso and his colleagues from the University of Navarra in Pamplona, also said that there has previously been no available prospective information about fibre and the risk of hypertension in populations outside the United States.
The researchers set about filling this knowledge gap by enrolling 5880 Spanish university graduates in the Seguimiento Universidad de Navarra (SUN) study. Dietary data was obtained at the start of the study with validated 136-item food frequency questionnaires.
After 28 months of follow-up, 180 subjects reported that they had been medically diagnosed with hypertension, defined as having a systolic and diastolic BP greater than 140 and 90 mmHg.
After adjustment for potential confounders such as age, sex, BMI, smoking, intakes of sodium, caffeine, magnesium, potassium, low-fat dairy and several other variables, the researchers found that people in the highest intake group of vegetable protein had an associated 50 per cent lower risk of hypertension that those in the lowest intake group.
No relationship between animal protein and raised blood pressure was observed.
Fibre from cereals was also associated with a protective effect - the highest intake was linked to a 40 per cent reduction in hypertension that the lowest cereal fibre intake group.
"We have observed in a prospectively followed-up Mediterranean population that vegetable protein and fiber from cereals are inversely associated with the risk of developing HT," wrote the researchers.
The researchers compared their results with US-based cross-sectional studies, such as the Chicago Western Electric Study (Hypertension, 2002, Vol. 39, pp. 1000-1006), which also reported an inverse association with vegetable protein and blood pressure.
Alonso argues however that this study did not account for potassium and magnesium intake, which are minerals linked to vegetable protein but can also directly affect blood pressure.
This study has several important limitations, namely the reliance on self-reporting of both dietary consumption and incidence of hypertension. This limitation is reduced, say the researchers, by the recruitment of university graduates who are well-educated and therefore more reliable.
The Navarra researchers noted that the mechanisms by which fibre or protein could reduce the risk of hypertension are not known, but suggest that the relationship between fibre and insulin resistance could, in turn, affect blood pressure, or that vegetable protein could result in higher serum concentrations of certain amino acids that are beneficial for blood pressure.
Previous studies have suggested that L-arginine, L-tryptophan, and tyrosine could reduce hypertension.
It is clear that significant further work is required, particularly in non-US populations to further support the relationship between intakes of cereal fibre and vegetable protein and decreased risk of hypertension.
Studies linking vegetable protein and reductions in blood pressure have been reported, such as a recent article in Archives of Internal Medicine (Vol. 166, pp. 79-86) by British researchers following 4680 people in China, Japan, the US and the UK.
Professor Paul Elliot from Imperial College London, lead researcher of this earlier study concluded: "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."