A recent study has got many people talking about the potential benefits—or detriments, depending which side you’re on—of L-carnitine for heart health. The study, by Cleveland Clinic and published in Nature Medicine, found a link between increased blood levels of TMAO (trimethylamine N-oxide, a metabolite of L-carnitine produced in the gut by certain bacteria) and an increased risk of cardiovascular disease and related events.1

L-carnitine is found in highest concentrations in red meat and dairy. High consumption of red meat has been linked to heart disease risk, and is thought to be due to red meat’s high saturated fat and cholesterol content. The myth that saturated fat and cholesterol are bad for you has recently been eroding, however, so researchers are looking for other qualities in meat that may be causing disease. This recent study may offer one explanation. The gut bacteria involvement is particularly interesting.

L-carnitine is produced in the body from the amino acids lysine and methionine (obtained from dietary proteins). L-carnitine is considered a conditionally-essential nutrient, however, because under certain conditions a deficiency can occur. In the body, L-carnitine is particularly active in skeletal and heart muscles, playing an important role in processing fatty acids for transport into the mitochondria—the energy powerhouses of the cell.2 Tissue levels of L-carnitine have been found to decline with age.3 For all these reasons, L-carnitine supplementation has been widely recommended.

But this recent Cleveland Clinic study questions the value and safety of L-carnitine intake in certain people—specifically, in meat eaters. The researchers found that L-carnitine was only converted by gut bacteria into TMAO in omnivores. Vegans were protected against this potentially harmful conversion.

“The present studies suggest that the reduced ingestion of L-carnitine and total choline [another TMAO-converting amino acid] by vegans and vegetarians, with attendant reductions in TMAO levels, may contribute to their observed cardiovascular benefits. Conversely, an increased capacity for microbiota-dependent production of TMAO from L-carnitine may contribute to atherosclerosis, particularly in omnivores who consume high amounts of L-carnitine,” stated the researchers. The study concluded, “Our results reveal a new pathway potentially linking dietary red meat ingestion with atherosclerosis pathogenesis [development]. The role of gut microbiota in this pathway suggests new potential therapeutic targets for preventing cardiovascular disease.”

This research builds on a previous study that Brenda blogged about a couple years ago by the same researchers. These studies have struck a nerve with other experts, however. They are not quite ready to write off L-carnitine just yet. And as a matter of fact, another meta-analysis of 13 studies involving over 3,600 patients was published a week later that found increased L-carnitine intake was linked to a 27 percent reduction in death, a 65 percent reduction in heart arrhythmias, a 40 percent decrease in chest pains, and a reduction in heart attack severity.4

Other experts point out the fact that seafood, in addition to red meat, is also a high-TMAO food, which questions the suggestion that TMAO is the culprit in the development of atherosclerosis, since fish are considered one of the most heart-healthy foods available. Obviously, the jury is still out.

I certainly find it interesting, and quite plausible, that the gut bacteria are responsible for producing metabolites that contribute to atherosclerosis. Our gut bacteria are constantly fermenting the foods that pass through our digestive tract, producing an array of metabolites that can be beneficial or harmful, depending on our bacterial composition. We are only scratching the surface of understanding how our health is affected by our microbiota. These studies are further proof of that. But as the story unfolds, its becoming glaringly clear that our gut microbes have a huge impact on our health.

Hopefully scientists will work to tease out the specific gut bacteria that are protective against the production of TMAO, and perhaps even develop a probiotic that could be taken along with L-carnitine and L-carnitine-containing foods as a protective measure. Carnitine is an important nutrient for optimum mitochondrial function and energy production, so a companion probiotic would be ideal.

Until we know more about TMAO and L-carnitine, I recommend—as I always have—that you eat red meat only sparingly, and that you consume a diet high in non-starchy vegetables and fruits, healthy fats, lean proteins, nuts, and seeds. And keep your gut balanced with probiotics and prebiotics.



  1. RA Koeth, et al., “Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis.” Nat Med. 2013; Published online 07 April 2013.
  2. CJ Rebouche. “Carnitine.” In: ME Shils, et al., eds. Modern Nutrition in Health and Disease. 10th ed. Philadelphia: Lippincott, Williams & Wilkins; 2006:537-544.
  3. M Costell M,et al., “Age-dependent decrease of carnitine content in muscle of mice and humans.” Biochem Biophys Res Commun. 1989;161(3):1135-1143.
  4. JJ DiNicolantonio, et al., “L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis.” Mayo Clin Proc. 2013 June;88(6):1–8.