A nutrient called l-carnitine, which is found in red meat and is also
popular as a dietary supplement, may play a role in the development of heart
disease.
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 in the journal Nature Medicine.
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.
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