The vegetables are particularly high in a chemical called glutamic acid, which creates the fifth taste sense "umami" - known as "savory."

The flavor-enhancing compound is found in all vegetables and could be one reason why a "five-a-day" diet protects against heart disease.

Other common sources include whole grain foods like rice, breads and cereals, soy products such as tofu and durum wheat, which is used to make pasta.

In this study published in Circulation: Journal of the American Heart A ssociation, experts analysed the amount of five amino acids - the building blocks of protein - in the diets of participants from the U.S., the U.K., China and Japan.

Glutamic acid is the most common amino acid and accounts for almost a quarter of vegetable protein and nearly a fifth of animal protein.

The study of 4,680 people, aged 40-59, found on average that the more of the protein component they consumed the more their blood pressure fell. Preventive medicine specialist Professor Jeremiah Stamler of Northwestern University in Chicago, hopes his findings will further promote healthy eating. He said, "High blood pressure is a major cardiovascular disease risk factor and tends to rise with age, starting early. Most of the U.S. population aged 35 and older is affected by pre-hypertension or hypertension."

Co-author Dr Ian Brown, an epidemiologist at Imperial College, London, said, "Glutamic acid may partly explain the link between vegetable protein and lower blood pressure.

"However there is no 'magic bullet' for preventing high blood pressure, and vegetable protein and glutamic acid are individual elements of a broader healthy eating pattern."  

From the July 20, 2009, Prepared Foods E-dition