Friday August 7 2009
“Drinking beetroot juice can increase stamina, allowing keep-fit enthusiasts and athletes to exercise for longer,” according to The Daily Telegraph. The newspaper said that a daily glass of beetroot juice helped men work out for 16% longer than when they had a regular fruit drink.
The effects are being credited to the nitrates in the juice which, according to scientists, help the body to use less oxygen. This means that people feel less tired when they are exercising. The results of this small randomised study in eight healthy men suggest that nitrate supplementation can improve exercise tolerance at moderate levels. Larger studies are needed to confirm these findings and to investigate the longer-term effects of dietary nitrate supplementation.
Where did the story come from?
Dr Stephen Bailey and colleagues from the University of Exeter carried out this study. It is unclear who funded this study, which is published in the Journal of Applied Physiology.
What kind of scientific study was this?
In this study the researchers investigated the effects of nitrate supplementation on the ability of people to perform high-intensity exercise and their use of oxygen during exercise.
Previous research suggests that dietary supplementation with sodium nitrate reduces the oxygen cost of sub-maximal exercise (it improves the way the muscles use oxygen). The researchers behind this new study theorised about how this may happen, suggesting that the supplements may help to ensure a more even distribution of oxygen in the muscles. They hypothesised that if nitrate had this effect then supplementation with food stuff containing high nitrate levels (such as beetroot juice) would increase exercise tolerance during severe-intensity exercise.
The researchers conducted a double-blind, randomised crossover study in which eight males consumed 500ml of either beetroot juice (which has a high nitrate concentration) or blackcurrant cordial (which has negligible nitrates) each day for six days.
The men were an average of 26 years old and were generally active people with normal blood pressure. None were smokers and none used dietary supplements. The men’s fitness was assessed at entry into the study through cycling tests. They completed a series of moderate-intensity and severe-intensity exercise tests on the last three days of the six-day consumption period. The exercises increased in intensity, beginning with a low-work session at the start of the study, one session of moderate cycling and one session of severe cycling on day six.
Throughout the study period the men received instructions to sip the juice at regular periods during the day. After the six days the men had a 10-day washout period and switched to the opposite juice. The participants were also asked to refrain from eating certain foods rich in nitrates over the course of the study period.
The researchers then compared a range of measures taken during exercise, comparing the two groups’ blood nitrate concentration, blood pressure, muscle oxygenation and VO2, which is a measure of metabolic rate (use of oxygen).
What were the results of the study?
Supplementation with beetroot juice increased the level of nitrate in the blood by 96%. Beetroot juice reduced systolic blood pressure (when the heart exerts pressure to push the blood around the body). There was no change in diastolic pressure (when the heart is at rest) or mean arterial pressure.
Following beetroot juice consumption the oxygen cost of sub-maximal exercise was reduced. During moderate-intensity cycling work there was a 19% reduction in pulmonary VO2 (oxygen uptake by the lungs). As there was no significant difference in heart rate or ventilation, the researchers say that the reduction in oxygen uptake was probably due to differences in the way the muscles were using oxygen. There was no notable difference between the groups in oxygen use during the more severe exercise sessions.
What interpretations did the researchers draw from these results?
The researchers conclude that a short period of dietary supplementation of nitrates improves muscle oxygenation during moderate exercise. They say that this improvement cannot be achieved by any other known means, including long-term endurance exercise training. For certain groups of people, for example the elderly and those with cardiovascular, respiratory or metabolic disease, activities are difficult and nitrate supplementation may improve quality of life for these people through improvement of their oxygen metabolism.
What does the NHS Knowledge Service make of this study?
Diets rich in fruit and vegetables are associated with reduced blood pressure, and this may in part be due to the effects of the nitrates they contain. This small randomised trial has demonstrated some beneficial effects of dietary nitrate supplementation through beetroot juice on exercise tolerance in healthy young men. These benefits were only seen during moderate exercise levels, and not when exercise was severe.
The researchers say that the reduction in use of oxygen they observed was greater than could be achieved by any other known means including training. However, they have not actually compared training versus beetroot juice consumption in this study, so this claim remains to be tested.
While athletes will be interested in these findings, this is a very small study and the results need to be replicated in larger trials. These should ideally look at long-term benefits and more closely investigate potential harms associated with consuming high levels of nitrates.