A recent study has allegedly linked L-carnitine, a nutrient found in red meat and commonly used as an additive in energy drinks, to an increased risk of cardiovascular disease (CVD). Robert Koeth, “Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis,” Nature Medicine, April 2013. According to the study, L-carnitine, like the trimethylamine-containing compound choline, forms a proatherogenic compound known as trimethylamine-N-oxide (TMAO) when metabolized by intestinal microbiota. Given the “markedly” increased ingestion of L-carnitine in industrial societies, researchers apparently set out to examine the effects of the nutrient on CVD risk using isotope tracer studies in humans as well as animal models.

In particular, the study’s authors reportedly “tested the carnitine and TMAO levels of omnivores, vegans and vegetarians, and examined the clinical data of 2,595 patients undergoing elective cardiac evaluations,” in addition to examining “the cardiac effects of a carnitine-enhanced diet in normal mice compared to mice with suppressed levels of gut microbes.” The results evidently showed that not only did increased TMAO levels predict CVD risk in humans who also exhibited high carnitine levels, but that the metabolite caused CVD in mice by altering “cholesterol metabolism on many levels.” The findings also suggested that a diet high in carnitine primes intestinal microbiota to produce TMAO, whereas vegans and vegetarians included in the study apparently lacked the microbiota to produce much TMAO even after consuming red meat.

“The bacteria living in our digestive tracts are dictated by our long-term dietary patterns,” said the study’s lead author, Stanley Hazen, in an April 7, 2013, Cleveland Clinic 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. Meanwhile, vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets.”

Hazen also warned about the potential danger of supplementing the diet with additional carnitine. “Carnitine is not an essential nutrient; our body naturally produces all we need,” he noted. “We need to examine the safety of chronically consuming carnitine supplements as we’ve shown that, under some conditions, it can foster the growth of bacteria that produce TMAO and potentially clog arteries.”