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 compound abundant 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 the study, and that one additional serving a day of red meat raised the risk of death by 13 percent.
The new findings don’t mean that red meat is more hazardous than previously thought. But they may help explain the underlying risk of eating red meat, 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—like the dimmer on a light switch,” he said. “It may explain why two people can have the same LDL level [a measure of one type of cholesterol], but one develops cardiovascular disease and the other doesn’t.”
One surprising finding, Dr. Hazen said, was how 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 eight-ounce 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.
“Vegans basically lose their ability to digest carnitine,” said Dr. Hazen.
The study, sponsored by the National Institutes of Health, 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. “We know it will be longer than one week, but shorter than one year,” Dr. Hazen said.
He and his colleagues have been exploring how altering gut bacteria might influence the risk of heart disease. “In the future, maybe there will be a heart-healthy yogurt, or a drug to block the formation of TMAO,” he said.