There was a recent article published in Nature Medicine that was picked up by the wire services and made its way into the mainstream media, including both the NY Times and the Wall Street Journal. Many readers have asked me to comment on the report. In what follows, again please remember that correlation or association is not causation.
The article cited studies of 2950 patients undergoing cardiac evaluation, and found that increasing levels of carnitine correlated positively with the risk for stroke and heart attacks. Gut bacteria can convert carnitine to a compound called TMAO, and levels of this compound have been correlated with atherosclerosis in the past.In mice, TMAO inhibits the breakdown of cholesterol. Due to the absence of meat in their diet, vegetarians have much lower levels of carnitine than do meat eaters.
Based on other studies, TMAO may promote the growth of plaques in arteries, but the data is not solid. At this point, we do not know if there is a third molecule derived from TMAO that is also correlated with cardiovascular events.
L-carnitine (carnitine is a stereo isomer and comes in both an l (left-handed) and d (right-handed) form, but as is usual with the human body, our enzymes recognize only the l form of amino acids. L-carnitine is a facilitator for the cell's use of fatty acids to obtain energy with the help of vitamin C via the citric acid cycle. The body can make carnitine as well as obtain it from food, which is not surprising since it is a necessary part of the energy cycle, and children born without the ability to use carnitine suffer severe metabolic problems.
The body has other uses for carnitine, and I will discuss a few. It has been shown in clinical studies that administering l-carnitine to patients with angina reduces the amount of cardiac medicine they need, and increases their chest pain-free exercise ability. The amount of carnitine in cells decreases with age, which thereby diminishes osteocalcin which in turn promotes osteoporosis.
Oddly enough, one study suggested that carnitine can improve sperm quality in infertile men, and can also reduce the size of a variocele.
Carnitine reduces fat mass and increases muscle mass, which is why bodybuilders take so much of it. Carnitine supplements appear to improve neurotransmitter functioning in the brain of elcerly patients. It was also found that l-carnitine levels were lower in children with asthma, and administering l-carnitine to these asthmatic children improved their pulmonary function tests. L-carnitine is also a peripheral antagonist of thyroid hormone action.
To summarize, increased l-carnitine levels in patients with increased TMAO levels was positively correlated with cardiovascular events, and further studies are definitely needed.