Dr. Stanley Hazen of the Cleveland Clinic and Robert Koeth, a medical student at the Cleveland Clinic Lerner College of Medicine of Case Western Reserve University, said the bacteria living in the human digestive tract metabolize the compound carnitine, turning it into trimethylamine-N-oxide, or TMAO, a metabolite the researchers previously linked in a study to the promotion of atherosclerosis -- hardening of the arteries.
The researchers 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 TMAO altered cholesterol metabolism at multiple levels.
The researchers found 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, the study, published in the journal Nature Medicine, 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.