Intestinal microbiota may contribute to the well-established link between high levels of red meat consumption and cardiovascular disease risk. 

The study tested the carnitine and trimethylamine-N-oxide (TMAO) levels - a metabolite the researchers previously linked in a 2011 study to the promotion of atherosclerosis in humans - of omnivores, vegans and vegetarians, and examined the clinical data of 2,595 patients undergoing elective cardiac evaluations. The research finds that a diet high in carnitine promotes the growth of the bacteria that metabolize carnitine, compounding the problem by producing even more of the artery-clogging TMAO. 

Stanley Hazen, M.D., Ph.D.:

Carnitine metabolism suggests a new way to help explain why a diet rich in red meat promotes atherosclerosis.

The bacteria living in our digestive tracts are dictated by our long-term dietary patterns. 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.

Carnitine is naturally occurring in red meats, including beef, venison, lamb, mutton, duck, and pork. It is also a dietary supplement available in pill form and a common ingredient in energy drinks.