The study was led by researchers from the University of Exeter and is published in the 6 August issue of the Journal of Applied Physiology.
Professor Andy Jones of the University of Exeter's School of Sport and Health Sciences and colleagues found that drinking beetroot juice reduces oxygen uptake more than can be achieved by any other known means, including training.
Jones told the media that:
"Our study is the first to show that nitrate-rich food can increase exercise endurance."
For the double-blind, placebo-controlled, crossover study they recruited 8 men aged from 19 to 38.
The men consumed 500 ml a day of beetroot juice for six consecutive days; during the last three days they underwent a series of 'step' moderate-intensity and severe-intensity exercise tests involving riding an exercise bike.
Then on a separate occasion the men went through the same process except they consumed a blackcurrant cordial placebo instead of beetroot juice.
The men also gave blood samples and were monitored throughout the trial.
The beetroot juice contained 11.2 ± 0.6 mM of nitrate while the placebo blackcurrant juice had a negligible nitrate content.
Double-blind means neither the participants nor the administrators they interacted with knew which of the two agents was being tested, the beetroot juice or the blackcurrant cordial.
The results showed that:
- On days 4-6 plasma [nitrite] was significantly higher in the beetroot juice phase than in the placebo phase.
- Systolic blood pressure was also significantly reduced in the beetroot juice phase (other studies have shown that beetroot juice reduces blood pressure).
- During moderate exercise the oxygen uptake in the beetroot juice phase went down (the researchers estimated muscle fractional O2 extraction using near infra-red spectroscopy).
- The gain of increase in lung intake of oxygen (pulmonary VO2) after starting moderate exercise was 19 per cent lower when the men consumed beetroot juice than when they consumed placebo.
- Also, during severe exercise, the slow component of VO2 was lower and the time to exhaustion was higher when they took beetroot juice compared to when they took placebo.
"The reduced O2 cost of exercise following increased dietary nitrate intake has important implications for our understanding of the factors which regulate mitochondrial respiration and muscle contractile energetics in humans."
The men were able to cycle for an average of 11.25 minutes or 92 seconds longer when they took beetroot juice which translates to about 2 per cent reduction in the time taken to cover a set distance.
Jones and colleagues aren't sure how the nitrate in the beetroot juice boosts stamina, but they suspect it is because the nitrate turns into nitric oxide in the body (hence the measure for plasma nitrite), which in turn reduces the oxygen cost of exercise.
They now hope to do more tests to understand the effect of nitrate-rich foods on what happens in the body during exercise.
Jones said they were "amazed" by the effect of beetroot juice on oxygen uptake, especially as they know of no other means that can achieve this big a difference, even training.
"I am sure professional and amateur athletes will be interested in the results of this research," said Jones.
"I am also keen to explore the relevance of the findings to those people who suffer from poor fitness and may be able to use dietary supplements to help them go about their daily lives," he added.
"Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans."
Stephen J. Bailey, Paul Winyard, Anni Vanhatalo, Jamie R. Blackwell, Fred J. DiMenna, Daryl P. Wilkerson, Joanna Tarr, Nigel Benjamin, and Andrew M. Jones
Journal of Applied Physiology (August 6, 2009).
Additional source: University of Exeter.