By: Eddie Levin
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 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.
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.
The findings show that bacteria living in the human digestive tract metabolize carnitine — a naturally-occurring compound in red meat —into trimethylamine-N-oxide (TMAO), a metabolite previously linked in a 2011 study to the promotion of atherosclerosis in humans. Additionally, the study shows that a diet high in carnitine promotes the growth of bacteria that metabolize carnitine, compounding the problem.
“The bacteria living in our digestive tracts are dictated by our long-term dietary patterns,” said Dr. Stanley Hazen, lead researcher. “A diet high in carnitine actually shifts our gut microbe composition to those that like carnitine, making meat eaters even more susceptible to forming TMAO and its artery-clogging effects. Meanwhile, vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets.”
Carnitine is naturally occurring in beef, venison, lamb, mutton, duck, pork and other red meats. It is also used as a dietary supplement and a common ingredient in energy drinks.
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.”
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.”