A substance found in tomatoes relieves impairment of blood vessels, which may explain why people who eat a Mediterranean diet have a notably reduced incidence of cardiovascular disease, according to a study.

A supplement of lycopene, a powerful antioxidant which is 10 times more potent than vitamin E, improved and normalised function of the endothelium (the inner lining of blood vessels) in volunteers with cardiovascular disease, researchers from the University of Cambridge found.

Dr Joseph Cheriyan, consultant clinical pharmacologist at Addenbrooke's hospital and associate lecturer at the University of Cambridge, said: "There's a wealth of research that suggests the Mediterranean diet – which includes lycopene found in tomatoes and other fruit as a component – is good for our cardiovascular health. But so far, it's been a mystery what the underlying mechanisms could be."

For the study, funded and sponsored by Cambridge University Hospitals NHS foundation trust, and published in the online scientific journal Plos One on Monday, researchers gave 36 cardiovascular disease patients, who were all on statins but had impaired function of the endothelium, and 36 healthy volunteers either Ateronon (an off-the-shelf supplement containing 7mg of lycopene) or a placebo treatment. Endothelial function predicts future events, so having a healthy endothelium is an important factor in preventing the evolution of heart disease.

It was conducted as a double blind trial, which meant that neither study participants nor the researchers knew which treatment was being provided.

They found that the lycopene supplement worked in the patients, improving the widening of the blood vessels by 53% after placebo correction, but not in the healthy volunteers. Constriction of the blood vessels is one of the key factors that can lead to heart attack and stroke. The supplement had no effect on blood pressure, arterial stiffness or levels of lipids.

Lycopene is found in tomatoes and its potency appears to be enhanced when it is consumed pureed, in ketchup or in the presence of olive oil. It is also found in other fruit and vegetables, such as grapefruit, watermelon, asparagus and carrots.

Cheriyan said the results reinforced the need for a healthy diet in people at risk from heart disease and stroke. "We've shown quite clearly that lycopene improves the function of blood vessels in cardiovascular disease patients," he said.

"A daily 'tomato pill' is not a substitute for other treatments, but may provide added benefits when taken alongside other medication." He said much larger trials were needed to establish whether lycopene did reduce heart disease.

Professor Jeremy Pearson, associate medical director at the British Heart Foundation, says: "Impaired endothelial function is a known predictor of increased risk of future heart disease. Further work is needed to understand whether the beneficial effects seen in this small study translate into clinical benefit for at-risk patients."