SUNDAY, April 7 (HealthDay News) -- A compound found in red meat and added as a supplement to popular energy drinks promotes hardening and clogging of the arteries, otherwise known as atherosclerosis, a new study suggests.

Researchers say that bacteria in the digestive tract convert the compound, called carnitine, into trimethylamine-N-oxide (TMAO). Previous research by the same team of Cleveland Clinic investigators found that TMAO promotes atherosclerosis in people.

And there was an another twist: The study also found that a diet high in carnitine encourages the growth of the bacteria that metabolize the compound, leading to even higher TMAO production.

"The [type of] 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," study leader Dr. Stanley Hazen, head of preventive cardiology and rehabilitation in Cleveland Clinic's Heart and Vascular Institute, said in a clinic news release.

Hazen's team looked at nearly 2,600 patients undergoing heart evaluations. The researchers found that consistently high carnitine levels were associated with a raised risk of heart disease, heart attack, stroke and heart-related death.

They also found that TMAO levels were much lower among vegetarians and vegans than among people with unrestricted diets (omnivores). Vegetarians do not eat meat while vegans do not eat any animal products, including eggs and dairy.

Even after consuming a large amount of carnitine, vegans and vegetarians did not produce significant levels of TMAO, while omnivores did, according to the study in the current issue of the journal Nature Medicine.

Although the new study could not prove any cause-and-effect relationship between carnitine and heart damage, the findings may provide a new understanding of the benefits of vegan and vegetarian diets, the researchers said.

"Vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets," said Hazen, who is also vice chair of translational research for the clinic's Lerner Research Institute.