Sports stars looking for a competitive edge over their rivals would do well to try some beetroot juice, according to new research.
Scientists have discovered why the juice from the red vegetable aids physical endurance - because it can boost stamina by making muscles more fuel efficient.
In 2009, scientists completed a study of men between the ages of 19 and 38 years. The men were able to cycle on exercise bikes 16% longer before getting tired out after drinking half a litre of beetroot juice a day for a week.
The same scientists have said they have worked out how the beetroot boosts endurance.
The new research shows that drinking beetroot juice doubles the amount of nitrate in the blood of volunteers, and reduces the rate at which muscles use their main source of energy.
Beetroot juice helped muscles work more efficiently and lowered their oxygen uptake. The same effect was seen during both low-intensity and high-intensity exercise.
Study leader, Professor Andy Jones from the University of Exeter's School of Sport and Health Sciences, said: "Since our first study came out we have seen growing interest in the benefits of drinking beetroot juice in the world of professional sport, and I expect this study to attract even more attention from athletes."
The scientists believe nitrate from beetroot juice leads to increased levels of nitric oxide in the body, which affects a range of biological functions including blood flow, hormone levels and cell signalling.
The new findings are published in the Journal of Applied Physiology.