Overall meat consumption continues to rise in the U.S., and 58% of the meat consumed is red meat. People in the U.S. eat 5 ounces of meat per day on average.
Eating a lot of red meat is known to contribute to heart disease, presumably due to the large amount of saturated fats and cholesterol in the meat. Or that is what we used to think. New research published in Nature Medicine indicates that the real culprit is a chemical in the red meat called L-carnitine. In a series of experiments on humans and mice, researchers found that L-carnitine is broken down by gut bacteria to produce trimethylamine-N-oxide (TMAO), which previous research has linked to heart and artery damage. TMAO alters how cholesterol is metabolized so less is eliminated from the body, allowing more cholesterol to deposit and harden into the artery walls.
But the researchers also found that frequent meat eaters produced significantly more TMAO than vegetarians after consuming the same amount of L-carnitine. For instance, L-carnitine supplements (250 mg) were given to 74 healthy volunteers, including 23 who were long-term vegetarians or vegans. The lab tests showed that consuming L-carnitine increased the level of TMAO in the blood, but meat eaters made far more TMAO than vegetarians or vegans.
Fecal studies also showed that meat eaters and vegetarians had different types of bacteria in their guts, and the meat eaters had more of the bacteria involved in breaking down L-carnitine into TMAO.
“The bacteria living in our digestive tracts are dictated by our long-term dietary patterns,” explained the lead researcher Dr. Stanley Hazen in a press release. “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.”
The main food sources for carnitine are red meat and full-fat dairy products. It is also found in fish, poultry, tempeh, wheat, asparagus, avocados and peanut butter. L-carnitine is also commonly available as a dietary supplement, which is advertised as a weight loss and body building tool despite a lack of supporting scientific evidence. Plus L-carnitine is added to many energy drinks.
So eating all this L-carnitine is bad, right? Unfortunately it isn’t that simple.
Carnitine plays a vital and complex role in cardiac metabolism. Some people have diseases that cause a carnitine deficiency, so they need to take carnitine supplements. Studies have also shown that carnitine may help treat some heart conditions, such as angina, arrhythmias, heart attacks and heart failure. For instance, a meta-analysis review study in the Mayo Clinic Proceedings recently showed that taking L-carnitine supplements reduces your risk of ventricular arrhythmias by 65% and risk of angina by 40%, although no reduction in risk was seen for heart attacks and heart failure.
In general, scientific studies have shown both positive and negative effects on cardiac health from taking carnitine supplements. These discrepant findings may be due to differences in how the carnitine is administered and the dose. For instance, carnitine given by an intravenous or intramuscular injection would bypass the gut bacteria, so it may not form TMAO. Larger carnitine studies are needed, which also take into account the volunteers’ long-term diet and the bacteria ecosystem in their guts.
For more information about L-carnitine studies, check out my KQED Science blog.