Wednesday February 6 2008
Nitrates are also found in green, leafy vegetables, including spinach and lettuce
Drinking a glass of beetroot juice daily could help beat high blood pressure, reports the Daily Mail. “Drinking half a litre – just under a pint – led to dramatically lower readings”, the newspaper says. The effect was linked to the nitrate in the beetroot that, the researchers suggest, “reacted with bacteria in the mouth… which resulted in blood vessels dilating to increase blood flow”, the Mail adds.
The newspaper story is based on a study conducted in healthy volunteers that looked at the short-term effect of beetroot juice on blood pressure. The volunteers who drank the beetroot juice had a drop in blood pressure between two-and-a-half and three hours after drinking. If this research can be replicated with other vegetables and in people with high blood pressure, this would further confirm the benefits of a healthy, vegetable-rich diet.
Where did the story come from?
Dr Andrew Webb from the William Harvey Research Institute at Barts and the London School of Medicine, London and colleagues from other institutions around the UK, conducted this research. The study was supported by fellowships from the Wellcome Trust and the British Heart Foundation. It was published in the (peer-reviewed) medical journal of the American Heart Association: Hypertension.
What kind of scientific study was this?
This was a randomised trial of 14 healthy subjects using a crossover design in which participants were given both 500ml of beetroot juice or 500ml of water in a random order, seven days apart. There were three parts to the study, which aimed to test the researchers’ theory that beetroot juice, which has high nitrate content, might be converted to nitric oxide by bacteria in saliva and that this chemical could dilate blood vessels and cause a drop in blood pressure.
In the first part of the study, the 14 volunteers were allocated to two groups, one of which received the beetroot juice first and the water second, and the second group received the drinks in the reverse order. All participants knew which drink they were receiving (open-label). Blood pressure was measured with an automated machine, every 15 minutes for an hour before and three hours after drinking, then every hour until hour six with one final reading at 24 hours. The average of the second and third readings was used as the blood pressure for analysis. The researchers also took blood samples to measure the nitrite and nitrate concentration before and during the study.
The second part of the study tested whether it was necessary for the volunteers to swallow their saliva for the effects on blood pressure and nitrate and nitrite levels to be shown. In six volunteers, who either swallowed their saliva or spat it out after the beetroot drink, the researchers assessed the potassium levels in the blood and the extent to which platelets – cells involved in blood clotting – clumped together.
In the third part of the study, in 10 volunteers received beetroot or water and the researchers assessed how well an artery in the arm contracted and then expanded (dilated) after the blood flow had been temporarily obstructed with a tight band. The volunteers were randomised in a similar crossover way to the first part of the study with seven days between the two phases.
What were the results of the study?
The researchers state there were no significant differences between the individuals recruited at the start of the study, and that, apart from developing red urine and red “stools”, the beetroot juice was well tolerated.
There were no changes in the levels of nitrate or nitrite in the blood after the volunteers drank water, but after they drank beetroot juice the level of nitrate rose significantly (by about 16 times) compared with water, with levels peaking 90 minutes after drinking. The nitrate level in the blood was still slightly elevated, though this was not statistically significant, at 24 hours. The level of dietary nitrite in the blood doubled in a similar pattern, peaking at three to five hours after drinking and returning to normal at 24 hours. The potassium level in the blood peaked at one hour and had returned to normal by three hours.
Blood pressure measurements measured in millimetres of mercury (mmHg) – a normal reading is 120/80mmHg – represent how the blood pressure changes each time the heart beats. Systolic pressure (the higher figure) is the pressure that occurs as the heart beats and diastolic (the lower figure) is the “resting” blood pressure between beats. The researchers found that both systolic and diastolic blood pressure readings dropped after drinking beetroot juice. The lowest systolic blood pressure occurred at 2.5 hours after ingestion with a drop of about 10 mmHg and the lowest diastolic blood pressure (about 8mmHg drop) was seen at three hours after the beetroot juice drink. There were no significant differences in blood pressure at 24 hours between the two groups, though in the group that drank beetroot juice systolic blood pressure was still lower 24 hours after drinking than it had been at the start.
Drinking beetroot juice but spitting out all the saliva, blocked the rise in nitrite levels in the blood and also the reduction in systolic blood pressure but had no effect on plasma nitrate levels, potassium levels or the clotting of platelets. This supports the researchers’ theory that the conversion of nitrate to nitrite by bacteria in saliva is an important part of the mechanism.
In the third part of the study, the researchers looked at the effect of a beetroot juice on the reaction of the blood vessels in the forearm to an experimental disruption of blood flow and oxygen. The results support the theory that it is through the action of nitrite, or the products of its breakdown, that beetroot juice protects the function of the artery wall.
What interpretations did the researchers draw from these results?
The researchers claim that all these findings taken together suggest that the ingestion of dietary nitrate underlies the beneficial effects of a vegetable-rich diet, by the conversion of nitrate to nitrite. They say that “bioactive nitrite substantially decreases blood pressure, inhibits platelet aggregation, and prevents endothelial dysfunction in healthy volunteers”. Furthermore, they go on to say that this highlights the “potential of a ’natural‘ low cost approach for the treatment of cardiovascular disease”.
What does the NHS Knowledge Service make of this study?
This well-conducted study aimed to investigate further the theories about how vegetables exert their beneficial effects and protect against heart disease. The timing of the changes in the measurements of blood pressure and nitrate and nitrite levels is in keeping with the theory that the nitrate content of beetroot juice is important in determining blood pressure.
The authors of this study also say that “it is possible that the blood pressure effect of dietary nitrate, evidenced in our study of people with normal blood pressure, will be heightened in hypertensives”. They call for the promotion of a “natural” diet containing vegetables with a high nitrate content.
There is nothing in this study that contradicts the accepted healthy dietary policy, however the consumption of large amounts of nitrate by people at risk of heart disease may need further research for a number of reasons:
- The study was very small and therefore should be repeated in more people.
- It was conducted in healthy volunteers and should be repeated in people with hypertension or at increased risk of heart attacks.
- Long-term beneficial effects of beetroot juice were not investigated, nor were any potential harms measured.
The dramatic effect of beetroot juice on blood pressure in this study certainly justifies the need for more investigation.
Sir Muir Gray adds...
I think I would wait until they had identified the active ingredient, made a palatable concoction and showed the concoction was safe and effective before using this as a blood pressure treatment.