Those who drank the beet juice could ride up to 20 percent longer.
Scientists think that the high nitrate content in beets helped cyclists use less oxygen.
"The beetroot juice was effective even without any additional training," said Stephen Bailey with Exeter University. "It reduces the energy requirements on your muscles so you can last longer."
A company that makes the juice supplied it for free, but the university paid for the study.
Bailey and colleagues calculated beetroot juice could translate into a 1 to 2 percent better race time, a tiny improvement likely only to matter to elite athletes.
They are still tweaking the dosage but say athletes should consume the juice a few hours before training so their body has time to digest it.
Their latest study was published in June in the Journal of Applied Physiology.
The Associated Press contributed to this report.
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