Nov. 7, 2014 -- Saturated fat in red meat has long been linked to heart disease. But new research suggests it might not be the only culprit.
Bacteria in the intestines convert carnitine, a protein building block that's especially plentiful in beef, lamb, and venison, into compounds that speed up hardening and thickening of artery walls, according to a new study.
Generally, the redder the meat, the more carnitine it has. Although pork is considered a red meat, it doesn't have as much carnitine as beef, lamb and venison, and chicken and fish have even less.
Scientists behind the study, published in the journal Cell Metabolism, say their work suggests new targets for drugs to prevent and treat heart disease. And, they say, it raises concerns about the safety of dietary supplements that contain carnitine and a related compound.
The name “carnitine” comes from the Latin word for “meat” or “flesh.” It’s not considered an essential nutrient in food, because “we make all the carnitine we need on our own,” says study researcher Stanley Hazen, MD, PhD, of the Miller Family Heart and Vascular Institute at Cleveland Clinic. “Every cell in our body needs that carnitine,” which is essential for converting fats into energy, he says.
Studies have repeatedly shown a strong link between the amount of red meat people eat and their risk of dying of heart disease, Hazen says. But “the amount of cholesterol and fat in the red meat is not enough for the increased risk that’s observed,” he says. “There’s more to it than just the fat.”
The trillions of microorganisms -- mainly bacteria -- that live in the intestines help with nutrition and the immune system. They're as varied as the people they inhabit, due mainly to diet and genetics.
“When we feed ourselves, we’re feeding our microbes, too,” Hazen says. The more carnitine you eat, the more likely you'll have microbes in your gut that will convert it to the compounds that promote hardening and thickening of artery walls, he said.
“The bacteria in our gut serve as a giant bioreactor,” says Jason Goldsmith, MD, PhD. He's a gastroenterology resident and post-doc fellow at the University of Michigan who was not involved in the new research. “They have an effect on us, and we have an effect on them.”
Another Potentially Heart-Breaking Finding
The new study, using mice, found that carnitine from the red meat is converted by gut microbes into a substance known as GBB. Then GBB is converted into compounds called trimethylamine (TMA) and TMAO, also by the gut microbes. All of that increases hardening of the arteries.
But give the mice antibiotics to wipe out their gut bacteria, and the level of GBB in their blood drops to near zero, Hazen’s team found.
“It’s not that the other stuff is not important,” says Andrew Mendelsohn, PhD, referring to other cardiovascular risk factors. “It’s just that this also is important.” Mendelsohn, who was not involved with Hazen’s study, is president and research director of the Regenerative Sciences Institute in Sunnyvale, CA.
Hazen says his new study isn’t about red meat, but about a compound that is more common in red meat than in chicken or fish. "A logical thing would be to cut back eating the nutrient,” he says of carnitine. Although, he says, "We haven’t directly tested yet whether lowering TMAO in humans will lower cardiac risk.”
He was direct about taking dietary supplements containing carnitine or GBB. “Our data should sound an alarm that we need to look at the safety of long-term supplement exposure,” Hazen says.
Hazen and his collaborators last year published the first paper to examine how gut bugs, together with eating red meat, might play a role in raising the risk of plaque buildup in the arteries.
In that study, his team found that supplementing the diet of mice with carnitine changed the types of bacteria in their gut, leading to increased production of trimethylamine, which then converted into TMAO.
The researchers also measured blood levels of carnitine and TMAO in about 2,600 people being checked for heart problems. They found that those numbers predicted heart disease risk better than cholesterol levels.