People who eat red meat every day have a higher risk of heart disease. Why?
“Probably a combination of half a dozen different compounds and nutrients lead to the ill-health effects of meat,” suggests Adam Bernstein, formerly research director at the Cleveland Clinic’s Wellness Institute.
One of those nutrients is a culprit you may not have heard of: carnitine, which ferries fuel into our cells’ furnaces called the mitochondria. It’s found mostly in red meat, even lean red meat, and in some dietary supplements.
“We believe that carnitine ingestion leads to accelerated atherosclerosis, or hardening of the arteries,” says Stanley Hazen, chair of cellular and molecular medicine at the Cleveland Clinic’s Lerner Research Institute.
Food for your microbes
“Both in animals and in humans, when you ingest carnitine, microbes that live in your intestines digest the carnitine,” explains Hazen. “And as a byproduct, the microbes make something that gets converted in the liver into a compound called TMAO.”
And TMAO, which stands for trimethylamine-N-oxide, may be the true troublemaker for your heart, says Hazen.
How much TMAO your microbes make is influenced by your everyday diet.
“In omnivores, who are constantly eating meat, the microbes that like to eat carnitine are more abundant, and they’re more likely to make more TMAO from carnitine,” notes Hazen.
Omnivores vs vegans
Hazen and his team fed a large dose of carnitine to omnivores (who reported eating beef, pork, lamb, or other meat almost every day) and to six long-term vegans (who for at least one year had eaten no meat, fish, dairy, or eggs).
The omnivores had a rise in blood levels of TMAO, but the vegans had virtually none. When Hazen put the omnivores on a cocktail of oral antibiotics (which wipe out gut bacteria) for a week, they made no TMAO, either. Three weeks after stopping the antibiotics, they once again did, confirming that gut microbes have a role in forming TMAO.
Effect on the heart
The damage that TMAO can do to arteries is seen in laboratory animals.
Hazen and his colleagues fed chow with carnitine to mice from the time they were weaned. The results: They had twice as much plaque in their aortas as mice that got no carnitine and they had accelerated heart disease, says Hazen.
What’s more, he could see how the animals’ arteries got clogged. The cells that deposit cholesterol in the artery walls were more active, and the animals’ livers made fewer bile acids, which shuttle cholesterol into the gut and out of the body.
“The net effect was increased depositing of cholesterol in the artery wall and decreased removal of cholesterol from the artery wall,” explains Hazen.
Does the same damage occur in people?
Researchers can’t feed carnitine to people for years to see if it clogs their arteries. Instead, they look at whether people with high carnitine or TMAO levels have a higher risk of heart disease.
First, Hazen and his team studied roughly 2,600 people who had gone to the Cleveland Clinic for a cardiac catheterization—a procedure that enables doctors to see if the arteries feeding the heart muscle are clogged.
“We found that increased blood carnitine levels in patients strongly predicted increased risks for cardiovascular disease and major adverse events like heart attack, stroke, and death,” says Hazen. “But that was only true in subjects who also had high TMAO levels.”
In a second study, the researchers reported on roughly 4,000 stable patients who were undergoing a heart evaluation. Those who had higher blood levels of TMAO were about 50 percent more likely to have a heart attack, stroke, or other cardiovascular event over the next three years (once the researchers took into account risk factors like high blood pressure, smoking, and high LDL cholesterol).
“TMAO was a strong and independent predictor of future heart attack, stroke, and death,” concludes Hazen.
His second study showed that gut bacteria can also make TMAO out of choline, a nutrient that’s especially high in egg yolks and liver, but it’s also found in meat, poultry, fish, grains, vegetables, and some dietary supplements. “You can’t get away from choline,” says Hazen. “It’s in everything, and you don’t want to completely eliminate it from your diet.”
TMAO’s role in heart disease is not a done deal. “Our results have to be replicated by others,” says Hazen. “We need to look at other populations.”
For example, does TMAO predict heart disease in people who have no symptoms or known risk factors for heart disease? Even more important, does lowering TMAO reduce the risk of heart disease?
“There’s more that needs to be done,” says Hazen. But “these studies identify a new target involved in heart disease.”
What to do
Until we know more, you can hedge your bets by cutting back on red meat and ditching supplements of carnitine, choline, or choline’s most common form, phosphatidylcholine (lecithin).
“Unless you’ve been instructed by your physician to take carnitine, which can happen with mitochondrial disorders or with long-term dialysis, you probably don’t need it,” says Hazen. “Your body can make all the carnitine it needs even if you’re a vegan or never eat any meat.”
You don’t need choline supplements either. “Deficiencies are exceedingly rare,” says Hazen. “We just don’t see it in the United States. You would have to be very malnourished.”
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