Eating red meat introduces a potentially dangerous molecule into the body tissues, according to researchers.

Scientists from the University of California in San Diego believe it could cause heart disease and cancer by triggering a harmful immune response.

Humans cannot produce the molecule - a type of sugar - but it occurs at high levels in lamb, pork and beef.

The research is published in the Proceedings of the National Academy of Sciences.

Human volunteers

Several studies have associated red meat diets with cancer and heart disease.

But these have focused on saturated fats and chemicals produced during the cooking process.

The new research focuses on a sugar called N-glycolylneuraminic acid (Neu5Gc).

Tests on three human volunteers - all members of the research team - confirmed that if the sugar is present in the diet, it is absorbed into body tissues such as the blood vessels.

The tests also suggested that because the molecule is not naturally present in the body, it is viewed as an invader by the immune system, which is sparked into action.

Lead researcher Professor Ajit Varki said the molecule was almost certainly not immediately toxic, and it was possible that humans had built up a tolerance after hundreds of thousands of years of eating meat.

'It could be that the damage only builds up over years,' he said.

'However, we are now living longer and the question arises whether the gradual accumulation of Neu5Gc and the simultaneous presence of antibodies against could be involved in some diseases of later life.'

Animal transplants

Professor Varki said the molecule may be one of the main obstacles in the path of developing animal-to-human organ transplants.

Scientists are trying to find ways to stop the powerful immune response that occurs when a pig organ is put into a human.

The three scientists who were involved in the study drank a solution of Neu5Gc purified from pork.

Tests showed that most of the molecule was eliminated by the body, but small amounts were absorbed into the body.

About two days after ingestion, Neu5Gc levels were raised two or three-fold.

By four to eight days, levels had dropped almost to their original level.

Good evidence'

Dr Julie Sharp, from Cancer Research UK, said a third of all cancers were linked to diet.

'There is good evidence that a diet rich in fruit, vegetables and fibre and low in fat and red meat can reduce the risk of the disease.

'However these results are preliminary and were obtained from analysis of only three individuals.

'Large-scale population studies would be needed to prove if this molecule has any role in human disease including cancer.'

A spokesman for the British Dietetic Association told BBC News Online: 'We would encourage people to eat a balanced diet based mainly on starchy carbohydrates, fruit and vegetables with small amounts of protein from a variety of sources, including diary, vegetables, meat, fish and poultry.'