A compound abundant in red meat and added as a supplement to energy drinks may increase the risk of atherosclerosis - or hardening of the arteries - a new study has warned.
According to research led by Cleveland Clinic in the US, bacteria living in the human digestive tract metabolise the compound carnitine, turning it into a metabolite trimethylamine-N-oxide (TMAO).
It has been previously linked in a 2011 study to the promotion of atherosclerosis in humans.
Further, the study found that a diet high in carnitine promotes the growth of the bacteria that metabolise carnitine, compounding the problem by producing even more of the artery-clogging TMAO.
The study tested the carnitine and TMAO levels of omnivores, vegans and vegetarians, and examined the clinical data of 2,595 patients undergoing elective cardiac evaluations.
They also examined the cardiac effects of a carnitine-enhanced diet in normal mice compared to mice with suppressed levels of gut microbes, and discovered that TMAO alters cholesterol metabolism at multiple levels, explaining how it enhances atherosclerosis.
The researchers found that increased carnitine levels in patients predicted increased risks for cardiovascular disease and major cardiac events like heart attack, stroke and death, but only in subjects with concurrently high TMAO levels.
Additionally, they found specific gut microbe types in subjects associated with both plasma TMAO levels and dietary patterns, and that baseline TMAO levels were significantly lower among vegans and vegetarians than omnivores.
Remarkably, vegans and vegetarians, even after consuming a large amount of carnitine, did not produce significant levels of the microbe product TMAO, whereas omnivores consuming the same amount of carnitine did.
"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 synthesise TMAO from carnitine, which may explain the cardiovascular health benefits of these diets," said lead researcher Stanley Hazen, Vice Chair of Translational Research for the Lerner Research Institute and section head of Preventive Cardiology & Rehabilitation in the Miller Family Heart and Vascular Institute at Cleveland Clinic.
The study was published in the journal Nature Medicine.
