Red meat is always at the top of the list when it comes to foods to eat in moderation. Aside from its saturated fat and cholesterol content, I could never find a solid reason to avoid or moderate its intake. Now a new study from the Cleveland Clinic in Ohio and published online in Nature Medicine shows a new link between red meat and heart disease risk that involves gut bacteria, L-carnitine and a compound that promotes the clogging of arteries.
The large intestine is home to over 400 species of bacteria, collectively numbering more than 100 million microbial cells. Most are beneficial but some are pathogenic (disease-causing). The “friendly” bacteria help to control the growth of the pathogenic bacteria. They also make some vitamins, notably vitamin K and biotin (a B vitamin). They also aid lactose digestion and digest or ferment some of the fibers and starches not digested in the small intestine.
L- carnitine is a substance that helps the body turn fat into energy by carrying fatty acids in the cytosol to the mitochondria of a cell. Your body makes it in the liver and stores it in skeletal muscles, heart, brain, and sperm. Although abundant in red meat, it is also found in fish, poultry, wheat, and some vegetables. Carnitine is also a popular nutritional supplement. In healthy people, cells produce the carnitine needed and the supplements provide no additional benefits.
A previous study linked a compound called trimethylamine-N-oxide (TMAO) to the promotion of atherosclerosis and in the latest study it was found that gut bacteria turn L-carnitine into TMAO. It was also found that a diet high in L-carnitine encourages the growth of the bacteria that metabolize it, thereby producing even more of the artery-clogger. They also found that the gut of vegans are significantly less able to make TMAO from carnitine which may help explain the benefits of the vegan diet on heart disease risks.
In a study on mice, it was discovered that a diet high in L-carnitine changed the gut bacteria balance that greatly increased the synthesis of TMAO and thus atherosclerosis. When the researchers suppressed the bacteria living in the gut, the effects lessened.
In human studies, researchers studied 2, 595 heart patients and found that increased levels of L-carnitine were linked to a higher risk of cardiovascular disease, heart attack stroke and death, but only in those patients who had higher levels of TMAO. Specific gut bacteria were linked to levels of TMAO and the type of diet, and that people who included meat in their diets had higher levels of TMAO than did vegans and vegetarians.
The bacteria living in our gut are dictated by our long-term dietary patterns. Based on the authors’ assessment, a diet high in carnitine actually shifts our gut microbe composition to those that like carnitine and thus produce more TMAO, making meat-eaters more susceptible to its proposed effects. But wait, the plot thickens.
This study seems to simplify an obvious complex relationship between gut bacteria and carnitine. Before you decide to totally give up red meat, read this reasonable review of the study. Obviously, this hypothesis needs a lot more research.
Until then, keep red meat and processed meats in moderate amounts in your diet. Use meat as a condiment or for flavor, as they do in the healthier Mediterranean- type diets that limit specifically red meat to a few times a month.
This research, if supported, also indicates the need for the further study of carnitine supplements to determine their safety and make sure they are not promoting heart disease through this proposed mechanism.