Drinking half a litre of beetroot juice a day for a week enabled them to cycle 16 per cent longer before getting tired out.
Now the scientists believe they understand how the beetroot boost works.
They found that drinking beetroot juice doubled the amount of nitrate in the blood of volunteers, and reduced the rate at which muscles used their main source of energy.
Beetroot juice helped muscles work more efficiently and lowered their oxygen uptake.
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.
'I expect this study to attract even more attention from athletes.'
Drinking beetroot was found to increase blood nitrate levels and reduce muscle usage of adenosine triphosphate, the body's chief energy source.
Oxygen uptake by muscles was also lowered, said the study published in the Journal of Applied Physiology.
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