Tuesday June 29 2010
Beetroot is high in the chemical nitrate
Beetroot juice “could save your life” claimed the Daily Mail. It said that the juice contains nitrate, a chemical that reduces blood pressure and therefore cuts the risk of heart disease and stroke.
The research behind this story aimed to look at whether nitrates may be responsible for the blood pressure-lowering effects of beetroot juice. It found that drinking beetroot juice or taking nitrate capsules resulted in short-term reductions in blood pressure in healthy volunteers with normal blood pressure.
The study is limited in that it was in a small number of healthy volunteers (only nine people drank beetroot juice), who were only monitored for three hours. It did not look at long-term outcomes such as heart disease or stroke.
High blood pressure is a risk factor for cardiovascular disease and therefore reducing it is often assumed to reduce the risk of cardiovascular disease. However, whether this is the case will depend on if the effect is great enough, and if the reduction can be sustained over time. Whether drinking beetroot juice can reduce the risk of cardiovascular disease would therefore need to be tested in long-term studies that assessed outcomes such as heart disease or stroke.
Where did the story come from?
The study was carried out by researchers the Queen Mary University of London, University College London and the University of Exeter and Plymouth. The research was funded by the British Heart Foundation. Two of the researchers report that they are directors of Heartbeet Ltd, a company linked to commercial producers of organic beetroot juice. The study was published in the peer-reviewed medical journal Hypertension.
The BBC News and Daily Mail covered this story. The BBC News headline of, 'Nitrate content 'behind benefits of beetroot juice'' is a more accurate reflection of the aims and findings of the study than the Mail’s headline, ’Drinking beetroot juice dramatically lowers risk of heart disease and strokes’. The study has not looked at the effects of beetroot juice on either heart disease or stroke, so we cannot say whether it reduces the risk of these outcomes or saves lives. The Mail also suggested that the effects of nitrate tablets and beetroot juice were directly compared, which was not the case.
What kind of research was this?
This randomised crossover trial investigated whether taking nitrate, either within nitrate-rich food or as a supplement capsule, affects blood pressure. The researchers’ previous study found that drinking beetroot juice reduced blood pressure in healthy people. Beetroot is high in the chemical nitrate that, when mixed with saliva in the body, is converted into nitrite, a chemical that causes blood vessels to dilate.
The aim of this research was to test whether the nitrate content of beetroot was responsible for this blood pressure-lowering effect. The researchers say that, ‘determining how vegetables confer protection against [cardiovascular disease] and exploiting this to therapeutic advantage are likely to have considerable health and economic implications’.
The study design involves participants receiving different interventions in a random order. This is an appropriate design for looking at treatments that have only short-term effects. The researchers arranged a minimum break of seven days between each treatment. This was to reduce the chances that the treatment given first would still be having an effect when the second was given.
What did the research involve?
The researchers enrolled healthy volunteers and gave them capsules containing nitrate (potassium nitrate), capsules without nitrate (potassium chloride – to rule out an effect of potassium), beetroot juice, or water. The effects of each treatment on the levels of nitrite in the blood and blood pressure were then monitored for up to 24 hours.
The volunteers were 18 to 45 years old, non-smokers, with BMIs of 18 to 31kg/m2. They were not on medication to treat any medical condition and had normal blood pressure. They were asked to eat a diet low in nitrates during the study (no processed meat or leafy green vegetables).
There were three parts to the study. In each part, volunteers received two different treatments in a random order. The three parts of the study compared:
- potassium nitrate capsules (containing 1488mg nitrate) and potassium chloride capsules in 21 volunteers; participants and researchers did not know which type of capsule was being received
- a lower dose capsule of potassium nitrate and a higher dose capsule of potassium nitrate in six additional volunteers; participants and researchers knew which dose was being received
- 250ml of beetroot juice and 250ml water in nine different volunteers who were monitored for three hours after each drink; participants and researchers knew which drink was being received
There was a minimum of seven days between each treatment received.
Data was analysed by a person who did not know which treatment had been taken before each measurement of nitrite and blood pressure.
What were the basic results?
The researchers found that nitrate capsules were associated with increased levels of nitrite in the blood, and reduced blood pressure over a 24-hour period compared to potassium chloride capsules. Higher-dose nitrate capsules were linked with a greater increase in nitrite concentrations in the blood than lower-dose capsules.
Women had lower blood pressure and higher levels of nitrite in their blood at the start of the study (before any treatment) than men. Women showed a greater increase in nitrite in the blood after taking the nitrate capsules than men, but had smaller reductions in blood pressure.
Drinking beetroot juice also caused the levels of nitrite in the blood to increase over three hours, and systolic blood pressure to decrease by a maximum of 5.4 mmHg compared to drinking water.
How did the researchers interpret the results?
The researchers conclude that their findings showed dose-dependent decreases in blood pressure after taking a nitrate supplement or eating a food high in nitrate (beetroot). They say their study ‘suggests that a dietary nitrate approach to [cardiovascular disease] may have therapeutic use’.
This small study has shown some reduction in blood pressure with beetroot juice. This finding needs cautious interpretation however, as the study has several features that limit the conclusions that can be drawn from it. These include the fact that it was in only a small number of people (nine who drank beetroot juice) and that all participants were healthy and had normal blood pressure.
Another constraint is that the volunteers who drank beetroot juice were only monitored for three hours, so it is unclear how long this effect may last.
The puzzling result of this study - that more women absorbed nitrate and converted it to nitrite better but had a smaller blood pressure changes when compared with men - needs further explanation.
The researchers offer theories for why this may have happened. However, the fact that the fall in blood pressure in women taking the nitrate capsules appeared to be minimal compared to men, suggests that nitrates (and possibly beetroot juice) may not be effective for everyone, a point not made by the researchers or the newspapers.
High blood pressure is a risk factor for cardiovascular disease, therefore reducing it is assumed to reduce the risk of cardiovascular disease. However, whether drinking beetroot juice regularly can reduce the risk of cardiovascular disease or death would need to be tested in long-term studies. Such a study would ideally be a randomised controlled trial and look at the effects of different levels of beetroot consumption.