WASHINGTON - The cholesterol and saturated fat in red meat may not the real culprit for heart disease as another compound -- carnitine -- could harden the arteries, according to US researchers.

The researchers said that gut bacteria metabolizes carnitine into trimethylamine-N-oxide (TMAO), a metabolite they previously linked to the promotion of atherosclerosis in humans.

Further, the researchers said 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.

"Carnitine metabolism suggests a new way to help explain why a diet rich in red meat promotes atherosclerosis," said study author Stanley Hazen, section head of Preventive Cardiology & Rehabilitation in the Miller Family Heart and Vascular Institute at Cleveland Clinic.

Prior research has shown that a diet with frequent red meat consumption is associated with increased cardiovascular disease risk, but that the cholesterol and saturated fat in red meat does not appear to be enough to explain the increased cardiovascular risks.

The researchers followed 2,595 patients undergoing elective cardiac evaluations and also examined the cardiac effects of a carnitine-enhanced diet in normal mice compared with mice with suppressed levels of gut microbes. Finally, they found a new connection between red meat and cardiovascular disease.

While carnitine is naturally occurring in red meats, including beef, venison, lamb, mutton, duck, and pork, it's also a dietary supplement available in pill form and a common ingredient in energy drinks. With this new research in mind, Hazen cautions that more research needs to be done to examine the safety of chronic carnitine supplementation.

"Carnitine is not an essential nutrient; our body naturally produces all we need," Hazen said. "We need to examine the safety of chronically consuming carnitine supplements.