British middle distance runner Colin McCourt runs on a machine for research Tuesday in a lab at St. Mary’s University College in London. Some experts say adding beetroot juice to your diet could give you a performance boost.

London ? When Chris Carver ran an ultra-marathon in Scotland last year, which challenges athletes to run as far as possible within 24 hours, he ran 140 miles.

Determined to do better in this year’s race, Carver added something extra to his training regime: beetroot juice. For a week before the race, he drank the dark purple juice every day. Last month, Carver won it by running 148 miles.

“The only thing I did differently this year was the beetroot juice,” said Carver, 46, a professional runner based near Leeds, in northern England.

He said more exercise would have improved his endurance, but to get the same result he attributes to the juice — an extra eight miles — it would likely have taken an entire year.

Some experts say adding beetroot juice to your diet — like other foods such as cherry juice or milk — could provide a performance boost even beyond the blood, sweat and tears of more training.

In two studies conducted at Exeter University on 15 men, Stephen Bailey and colleagues found cyclists who drank a half-liter of beetroot juice several hours before setting off were able to ride up to 20 percent longer than those who drank a placebo blackcurrant juice.

By examining the cyclists under a scanner that analyzes how much energy is needed for a muscle to contract, Bailey and colleagues discovered beetroot juice allows cyclists to exercise using less oxygen than normal.

“The beetroot juice was effective even without any additional training,” Bailey said. “It reduces the energy requirements on your muscles so you can last longer.” While the beetroot juice was provided free by its manufacturer, Exeter University paid for the research.

Bailey said the high nitrate content of beetroot juice is responsible for its athletic benefits. Scientists aren’t exactly sure how it works, but suspect having more nitric oxide in your body, a byproduct of nitrate, helps you exercise with less oxygen. Bailey said the same effects might be possible if people ate more nitrate-rich foods like beetroot, lettuce or spinach.

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.

“Drinking beetroot juice is not going to turn a recreational runner into an Olympic champion, but it might make tolerating more exercise easier so you can train more,” said Dr. Andy Franklyn-Miller, a sports medicine expert at the Centre for Human Performance in London. He was not connected to the research and has not received any funding from beetroot juice makers.

Previous studies in Britain and the U.S. have found beetroot helps the heart by lowering blood pressure.