A US firm says it has created a chocolate bar that's good for you. Heart doctors have doubts

The Aztecs thought it was a life-lengthening gift from the gods. Spanish conquistadors brought it to Europe as a cure for high fevers and haemorrhoids. Then, chocolate developed a health problem.

A favourite treat for women spending the night on the sofa, chocolate is accused of stoking acne, obesity and heart disease. But now one of the world's leading confectioner claims to have put the nutrition back into chocolate.

Chocolate eaters, Mars claims, will look better, feel better and live longer thanks to a new formula that could prevent cancer and heart disease. For the company's new CocoaVia bar is loaded with vitamins and antioxidant compounds that promise "real chocolate pleasure, real heart health benefits".

According to Mars, eating two of the dark chocolate bars each day could help to lower cholesterol and combat numerous diseases. CocoaVia was launched in the United States last week and is expected in Britain later this year. It is, Mars says, the first chocolate product to migrate from the sweet shelves to the health food shops. And that, say industry observers, could herald a confectionery revolution.

However, chocolate remains perceived as a high-sugar, high-fat treat. Dr Charmaine Griffiths of the British Heart Foundation (BHF), said: "Chocolate is part of the problem for heart health. We are certainly not suggesting people never eat chocolate, but there are much better ways of looking after your heart."

But CocoaVia is now challenging that view. Harold Schmitz, Mars's chief scientist, said that 15 years of research had gone into perfecting the recipe. "This is the first cocoa-based product designed specifically for cardio-vascular health," he said. "For hundreds of years, cocoa was used as a medicinal product. Now we're rediscovering why."

Cocoa beans are rich in flavanols, a naturally occurring antioxidant, which doctors think may act against viruses, allergies and cancer. These compounds are normally destroyed during the production of chocolate, but by modifying cultivation and processing methods, Mars has created a patent cocoa powder with a flavanol concentration more than three times as high as that of normal chocolate.

One study in 2000 showed that a flavanol-rich cocoa drink had a blood-thinning effect similar to aspirin, helping to prevent the blood clots that cause heart attacks. Last month, Professor Norman Hollenberg of Harvard Medical School unveiled research showing that a flavanol-rich diet may reducecancer and heart disease.

But although research suggests a relationship between a dark chocolate diet and a healthy heart, it does not prove that eating more of it will prevent disease, said Bridget McKiveth of the BHF. "Studies suggest some kind of link between flavanols and heart health but there is no proof of cause and effect," she said.

There are easier - and cheaper - ways to obtain flavanols, which are also found in other food and drinks, such as red wine, tea, cranberries, peanuts, strawberries and apples, she added.