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, which promotes atherosclerosis, or a thickening of the arteries.
The researchers 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.
The researchers speculated that carnitine could be compounding the danger. "Cholesterol is still needed to clog the arteries, but TMAO changes how cholesterol is metabolized and may explain why two people can have the same LDL level, but one develops cardiovascular disease and the other doesn't," said Stanley Hazen, lead author and Chief of Cellular and Molecular Medicine at the Cleveland Clinic's Lerner Research Institute.
The researchers also found that a long-term diet that includes meat affected the amount of TMAO-producing bacteria in the gut and thus magnified the risk. In the study, when longtime meat-eaters consumed an 8-oz steak and a carnitine supplement, their bacteria and TMAO levels rose considerably. But when a vegan ate the same combination, he showed no increase in TMAO or bacterial change.
The study didn't assess how little red meat people could consume and still have elevated TMAO. Nor did it look at how long someone had to abstain from red meat to end the process.
Trade groups for meat producers have questioned the link to cardiovascular disease, saying studies that ask people to recall what they ate over long periods are imprecise. In response to the study, the American Meat Institute released the following statement:
"Cardiovascular disease or CVD is a complex condition that appears to have a variety of factors associated with it, from genetics to lifestyle. Attempts to link cardiovascular disease to a single compound that is found at safe levels in red meat oversimplifies this complex disease. In fact, the study's authors themselves say red meat is not to blame, but rather argue that excessive supplementation with L-carnitine that is found at safe and healthy levels in red meat may be a concern. It is important to keep in mind that there are many other studies done on L-carnitine that do not show any adverse health effects at a variety of doses ... This study should not prompt any dietary changes and consumers who enjoy red meat should continue to do so with confidence."