( BBC ) - Drinking 500ml of beetroot juice a day can significantly reduce blood pressure, UK research suggests.
The key beneficial ingredient appears to be nitrate, which is also found in green, leafy vegetables.
The researchers found that in healthy volunteers blood pressure was reduced within an hour of drinking the juice.
The study, by Barts and the London School of Medicine and the Peninsula Medical School, could suggest a low-cost way to treat hypertension.
Previously the protective effects of vegetable-rich diets have been attributed to their antioxidant vitamin content.
While it took less than an hour to note a reduction in blood pressure in the beetroot juice tests, it was more pronounced after three to four hours and a degree of reduction continued to be observed for up to 24 hours, the report published on the online journal Hypertension said.
The researchers showed that nitrate in the juice is converted in saliva, by bacteria on the tongue, into nitrite.
This nitrite-containing saliva is swallowed, and in the acidic environment of the stomach is either converted into nitric oxide or re-enters the circulation as nitrite.
The peak time of reduction in blood pressure correlated with the appearance and peak levels of nitrite in the circulation.
No such drop in blood pressure was recorded in a second group of volunteers, who did not swallow their saliva while drinking beetroot juice, or for three hours afterwards.
More than 25% of the world's adult population are hypertensive, and it has been estimated that this figure will increase to 29% by 2025.
Hypertension causes around 50% of coronary heart disease, and approximately 75% of strokes.
In total, cardiovascular disease kills over 110,000 people in England every year.
Researcher Professor Amrita Ahluwalia said: "Our research suggests that drinking beetroot juice, or consuming other nitrate-rich vegetables, might be a simple way to maintain a healthy cardiovascular system, and might also be an additional approach that one could take in the modern day battle against rising blood pressure."
Professor Graham McGregor, of the British Hypertension Society, described the research as "interesting".
He said: "This shows that beetroot juice lowers blood pressure in the short term in volunteers with normal blood pressure.
"What we need now is research to see whether it has an effect on people with high blood pressure over a much longer period of time."
Professor McGregor said there was a growing body of work showing that a diet rich in fruit and vegetables had a beneficial impact on hypertension.
But he said previous research had suggested that potassium may be the key mineral.
Victoria Taylor, of the British Heart Foundation, said: "Whilst beetroot juice was used in this study, it is unlikely that people will be able to - or wish to - consume it in the quantities used in the research.
"Although we know that eating a diet rich in fruit and vegetables as part of a well balanced diet is beneficial to heart health, we do not know yet whether there are certain fruits or vegetables that are more helpful than others and so for now, people should continue to choose a wide variety in achieving their five a day."