It may not be the saturated fats or cholesterol from a fatty steak that's triggering heart problems, but a chemical process involving gut bacteria and a compound found in meat called carnitine that may be to blame.

What is carnitine?

Besides being found in red meats, carnitine is also added to dietary supplements to boost weight loss, and is commonly found in another item linked to heart risks -- energy drinks.  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

What have previous studies found?

Several studies suggested that the fatty composition of meat alone accounted for only a portion of the risk increase, with meat's salt content, genetic risk factors or something about cooking itself possibly accounting for the remaining risk.

Another study found that a compound called trimethylamine-N-oxide (TMAO) may promote the growth of artery-clogging plaques.  TMAO is formed when bacteria from our digestive tract breaks down a compound found in meat known as carnitine.  Diets high in meat promote the growth of a gut bacteria that breaks down carnitine, which leads to more TMAO, which in turn leads to atherosclerosis.  Increasing carnitine levels increased risks for stroke, heart attacks and other cardiac events in subjects with high levels of TMAO.

This is why vegans and vegetarians have significantly lower baseline levels of TMAO than meat-eating omnivores.  Vegetarians and vegans given carnitine did not show major increases in TMAO levels, however, when compared with meat-eaters who consumed the same amount of carnitine, which suggests vegetarians may possess different gut bacteria.  TMAO prevents the breakdown of cholesterol, thereby raising risk for atherosclerosis.  Meanwhile, vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets

What do we do?

We need to examine the safety of chronically consuming carnitine supplements as we've shown that, under some conditions, it can foster the growth of bacteria that produce TMAO and potentially clog arteries.  Consuming meat, fish, dairy and alcohol in moderation, coupled with more vegetables, fruits, whole grains and mono-unsaturated fats, remains the nutritional blueprint for a healthy and healthful life.