Doctors have long assumed that saturated fat and cholesterol in red meat are what raise the risk of heart disease. But a study in the journal Nature Medicine fingers another culprit: carnitine, a compoundabundant in red meat that also is sold as a dietary supplement and found in some energy drinks.

Carnitine typically helps the body transport fatty acids into cells to be used as energy. But researchers at the Cleveland Clinic found that in both humans and mice, certain bacteria in the digestive tract convert carnitine to another metabolite, called TMAO, that promotes atherosclerosis, or a thickening of the arteries.

The researchers, led by Stanley Hazen, chief of cellular and molecular medicine at the Cleveland Clinic's Lerner Research Institute, tested the carnitine and TMAO levels of omnivores, vegans and vegetarians, and examined records of 2,595 patients undergoing cardiac evaluations. In patients with high TMAO levels, the more carnitine in their blood, the more likely they were to develop cardiovascular disease, heart attacks, stroke and death.

Many studies have linked consumption of red and processed meat to cardiovascular disease and some cancers. The Harvard School of Public Health reported last year that among 83,000 nurses and 37,000 male health professionals followed since the 1980s, those who consumed the highest levels of red meat had the highest risk of death during study, and that one additional serving a day of red meat raised the risk of death by 13%.

The new findings don't mean that red meat is more hazardous than previously thought, but may help explain the underlying risk, which some researchers have long thought was higher than the saturated fat and cholesterol content alone could explain.

Dr. Hazen speculated that carnitine could be compounding the danger. 'Cholesterol is still needed to clog the arteries, but TMAO changes how cholesterol is metabolized