The nutrient L-carnitine, found in red meat and used as a dietary supplement, is associated with cardiovascular disease in people and causes vascular disease in mice, as reported in a study published online this week in Nature Medicine. The pathogenic effects of L-carnitine do not seem to be direct, but require its further metabolism by bacteria in the gut.

Stanley Hazen and colleagues report that in mice, the disease-promoting effects of L-carnitine require its metabolism by gut bacteria into the compound trimethylamine (TMA), which is further converted to trimethylamine N-oxide (TMAO). Likewise, in humans, L-carnitine is metabolized to TMA and TMAO. Vegetarians and vegans have a diminished ability to metabolize L-carnitine compared to omnivores and decreased amounts of specific types of gut bacteria suggesting that ingestion of red meat favors the growth of gut bacteria that can use L-carnitine as an energy source. Moreover, Hazen and colleagues report that high levels of L-carnitine in the blood are associated with cardiovascular disease, but only in those individuals who also have high TMAO levels, consistent with the idea that L-carnitine must be metabolized by gut bacteria to promote disease.

These results point to L-carnitine, rather than saturated fat and cholesterol, as explaining the link between red meat consumption and cardiovascular disease. Also, the findings have relevance to the widespread use of L-carnitine as a dietary supplement, and the authors suggest that the safety of this practice should be further investigated.