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Molecule in meat may increase heart disease risk

Gut bacteria transform compound into artery hardener

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Drop that hamburger, put down the can of Monster Energy and back away from the body building pills.

A nutrient found in red meat and added to energy drinks and supplements may crank up people’s risk of heart disease, a new study suggests. Bacteria in the gut digest the nutrient, L-carnitine, and help turn it into an artery-hardening chemical — particularly in meat eaters, researchers report April 7 in Nature Medicine.

The intestinal microbes of vegetarians and vegans didn’t make much of the chemical, even when researchers fed them an 8-ounce sirloin steak.

“I always thought that what I ate mattered, but I never realized that my gut bacteria might matter more,” says biochemist Harry Ischiropoulos of the University of Pennsylvania in Philadelphia, who was not involved with the study.

What’s more, high blood levels of the bacterial by-product of L-carnitine, called trimethylamine N-oxide or TMAO, were an “astoundingly good” warning sign of impending heart attack, stroke and death, says study coauthor Stanley Hazen of Cleveland Clinic. A test for TMAO, which will become commercially available this year, could give physicians a new tool for gauging heart disease risk.

Scientists have long known that eating red meat jacks up a person’s chances of developing heart disease, but reliable biomarkers — blood-borne indicators of disease or health — have been hard to find. One way physicians gauge risk is with blood tests for cholesterol, a greasy molecule in meat and other foods, which gums up arteries. But tests for cholesterol and other molecules don’t wholly explain meat’s link to heart disease, Hazen says. “Cholesterol, saturated fat and salt only account for a tiny little piece of the risk.”

Gut bacteria might account for a bit more. Hazen’s team first linked intestinal microbes to heart disease in 2011, when they spotted TMAO in blood collected from people who later suffered heart attacks, had strokes or died (SN Online: 4/7/11).

For the new study, Hazen zeroed in on L-carnitine because the nutrient is structurally similar to a compound that gut microbes can convert to TMAO.

Volunteers — a mix of omnivores, vegetarians and vegans — ate steak and L-carnitine capsules, and then researchers measured TMAO levels in the blood. Only meat eaters could make TMAO from L-carnitine, Hazen’s team found, and they needed their gut bacteria to do it. TMAO production shut down when researchers wiped out volunteers’ intestinal microbes with antibiotics.

L-carnitine passed right through the guts of long-term vegans and vegetarians, leaving their blood practically TMAO-free. When researchers examined volunteers’ stool, they found different groups of bacteria in people who did and didn’t eat meat.

Hazen’s group also found that blood levels of TMAO and L-carnitine could predict heart disease risk, which they learned by collecting blood samples from 2,595 patients and tracking their health for three years.

The findings are new and exciting but need to be confirmed, says cardiovascular researcher Ishwarlal Jialal of the University of California, Davis Medical Center. Molecules proposed as biomarkers for heart disease often look promising in initial studies but fizzle out clinically. “We’ve been down this road so many times before.”

But one message is clear, Jialal says: “L-carnitine is not good for you. It’s not good as a supplement and it’s not good in red meat. That’s one thing you can take to the bank.”

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