The high amounts of saturated fat and cholesterol in red meat have long been blamed for increasing people's risk of heart disease. But now, new research points a finger at another culprit in meat that may be more closely tied to this leading killer.
A new study reveals that a nutrient called l-carnitine, which is found in red meat and is also popular as a dietary supplement, may also play a role in the development of heart disease.
In a series of experiments in people and mice, scientists for the first time demonstrated that carnitine from foods as well as from supplements influenced cardiovascular risk.
"We now have an understanding of a new nutritional pathway that helps explain the long-standing recognition of a link between red meat and the development of heart disease," said study researcher Dr. Stanley Hazen, section head of preventive cardiology at the Cleveland Clinic in Cleveland, Ohio. The pathway involves the gut bacteria that metabolize carnitine in people who regularly eat meat, he said.
Hazen and his research team suspected there must be something else in red meat, besides its cholesterol and saturated fat, that explains its association with heart disease. "This study suggests carnitine may be a piece of this link," he said.
The findings were published online today (April 7) in the journal Nature Medicine.
A carnitine connection
Two years ago, Hazen and his research team discovered that microorganisms in the intestines can convert substances found in choline, a common dietary fat, to a by-product known as TMAO, trimethylamine-N-oxide.
This new study looked at l-carnitine, which has a similar chemical structure to choline.
Carnitine is a nutrient found at high levels in red meat, but fish, poultry, milk and other dairy products are also good food sources of it. Carnitine is also a popular over-the-counter diet supplement, often billed as helping to boost energy and bulk up muscle. It's found in some energy drinks and muscle milks.
The researchers looked at fasting levelsof blood carnitine in nearly 2,600 men and women. The findings showed that carnitine levels could quite strongly predict participant's risk of existing coronary artery disease, as well as the risk of having a major cardiac event, such as heart attack, stroke, or death over a three-year period, but only in adults who had high blood levels of TMAO.
Hazen's group also compared mice fed their normal chow, which is basically a vegetarian diet, with mice whose food was supplemented with carnitine.
"We saw that carnitine supplements doubled the rates of atherosclerosis in the mice," Hazen said. It did this by dramatically increasing levels of TMAO, which is produced by gut bacteria that metabolize l-carnitine.
As for how carnitine in red meat may be linked with heart disease, Hazen explained that chronic ingestion of carnitine fundamentally shifts the metabolism of cholesterol. "It's changing it in a way that will make you more prone to heart disease," he said. Eating carnitine causes more cholesterol to be deposited onto artery walls, and less to be eliminated from the body.
What to do
Besides looking at animal models, researchers also looked at what happens when people eat carnitine, comparing 51 people who normally eat meat to 23 people who were vegetarian or vegan (who consume no animal products). The researchers found that adults who avoid meat and eat fewer animal products produced much lower concentrations of TMAO in the blood compared with the meat eaters.
"If you're eating a lot of red meat, this study argues to consider cutting back," Hazen said. He recommended decreasing the frequency of eating red meat, and its portion size.
For people taking carnitine supplements, Hazen said he's unaware of a compelling study that shows a dramatic benefit from them. And taking the supplement could be influencing a person's long-term risk of heart disease, he suggested.
Pass it on: A compound called carnitine found in in red meat and supplements may increase the risk of heart disease.
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