By Natalie Obermeyer
A recent study published in Nature Medicine led by Stanley Hazen of the Cleveland Clinic has found another explanation for why red meat contributes to heart disease: red meat is rich in the amino-acid carnitine. The researchers demonstrated that when carnitine is ingested, bacteria in the small intestine convert it to trimethylamine (TMA), which is further converted to trimethylamine-N-oxide (TMAO). TMAO is believed to be responsible for accelerating atherosclerosis contributing to heart disease.
The link between red meat consumption and heart disease has been well established. Epidemiologic studies show that vegans and vegetarians have a significantly decreased risk of heart disease compared to omnivores. A combined analysis on data from the Nurses’ Health Study and Health Professionals Follow-up study on over 120,000 people estimates that eating just 1 serving (100 grams) of red meat per day increases the risk of cardiovascular death by 18%. Historically, people primarily blamed meat’s saturated fat and cholesterol content; however, we now recognize that saturated fat and cholesterol do not explain all of the increased heart disease risk. Carnitine, which is metabolized to TMAO, may play a vital role.
The researchers found that when people eat red meat, their blood levels of both carnitine and TMAO increase. And after looking at the blood levels of carnitine and TMAO in about 2,600 men and women, the researchers found that increased carnitine and TMAO levels were associated with increased risk of heart disease and major adverse cardiac events (such as heart attacks, strokes, and death) in a dose-dependent manner. These associations remained after the traditional heart disease risk factors, such as smoking, high blood pressure, and cholesterol, were removed. The researchers also found that meat eaters have much higher levels of carnitine in their blood than do vegetarians.
Interestingly, when vegetarians are given meat to eat they produce less TMAO than omnivores. Vegetarians do not have as much of the bacteria in their intestines that are responsible for metabolizing carnitine into TMAO. The regular consumption of carnitine-containing foods (such as meat) promotes the growth of the bacteria that metabolize carnitine into TMAO.
The researchers also found that feeding mice carnitine caused them to develop atherosclerosis. However, when the mice were given antibiotics that blocked the production of TMAO from carnitine, they no longer developed atherosclerosis. Therefore, the authors concluded that TMAO is the molecule responsible for the increase in heart disease. The researchers theorize that TMAO both inhibits the body from excreting excess cholesterol and increases the ability of cholesterol to become lodged in the arterial walls, leading to the build up of atherosclerotic plaque.
Carnitine is found primarily in animal products, especially red meat. Carnitine, like carnivore, is derived from the Latin word carnis, which means meat. Dairy, fish, and chicken also contain smaller amounts of carnitine.
L-Carnitine Content of Selected Foods
|Beef steak||3 ounces||81|
|Ground beef||3 ounces||80|
|Canadian bacon||3 ounces||20|
|Milk (whole)||8 fluid ounces (1 cup)||8|
|Fish (cod)||3 ounces||5|
|Chicken breast||3 ounces||3|
|Ice cream||4 ounces (1/2 cup)||3|
|American cheese||1 ounce||1|
|Whole-wheat bread||2 slices||0.2|
|Asparagus||6 spears (1/2 cup)||0.2|
This study has many implications. First, eating less meat can reduce heart disease risk. Fortunately, according to the USDA, beef consumption in the US peaked in 2002 and has has continued to fall since then. (See a post by Professor Parke Wilde in his U.S. Food Policy blog). However, Americans still eat much more red meat than is considered healthy, and it is recommended that we decrease our consumption.
Additionally, a number of supplements and sports drinks on the market contain carnitine. Body builders and athletes take carnitine because they assume it builds stronger muscles and increases fat metabolism. However, after the recent research on carnitine, consumers should think twice about ingesting the potentially toxic molecule.
Finally, some have suggested that rather than limiting meat consumption, researchers should look for antibiotics that might kill the bacteria that metabolize carnitine into TMAO. However, this direction of research is not very promising. An antibiotic would need to be found that only kills this one type of intestinal bacteria and not the others. Our gut hosts numerous different bacteria which maintain intestinal health, support the immune system, and activate nutrients and vitamins; certain bacteria may even play a role in regulating insulin and body weight. Rather than researching how to kill bacteria to stop TMAO synthesis, efforts should be focused on decreasing our overall meat consumption. In doing so, not only will we avoid the carnitine, we will also avoid the saturated fat and cholesterol.
Natalie Obermeyer is a first year student in the Nutrition Communication and Masters of Public Health programs. When she is not studying, reading, or writing, she loves to run, hike, ski, play outdoors in the sunshine, and experiment in the kitchen