Americans eat much more red meat than most other people, and Americans get a lot more cardiovascular disease than most other people. Red meat has lots of saturated fats and cholesterol, which causes cardiovascular disease, right? Not so fast. As with many commonly held assertions, especially in the field of nutrition, this one sounds good but does not quite have the data to support it.

Intestinal microbiota, or gut flora, are a trendy health topic right now; their importance has only been acknowledged in the past twenty years or so, but they have already been shown to impact such vital physiological processes as immune function and the development of cancer, diabetes, and obesity. Dietary choices—like the choice to eat red meat or not—are known to influence the relative ratios of the different bacterial species residing in our guts.

So researchers at the Cleveland Clinic decided to check out how different intestinal microbiota metabolize the components of meat. They found that bacteria present in the intestines of omnivores, but not present in the intestines of vegans or vegetarians, generate a molecule that promotes atherosclerosis when they are fed red meat.

L-carnitine is a compound found in meat, hence the name carnitine. (Does it make you crave carne asada too?) Eukaryotes can produce it, but only prokaryotes can break it down. When fed L-carnitine, either in pills or in a steak (or both), omnivores metabolize it to generate the breakdown product trimethylamine –N-oxide (TMAO); vegans and vegetarians don’t.

But if the omnivores have been given broad spectrum antibiotics that kill the bugs in their guts, they don’t either. This suggests that TMAO is a product of these gut bacteria.

(Please note that no vegans were harmed in the course of this experiment, although one guy did decide to have an eight-ounce sirloin in the name of science after over five years of abstinence.)

In a cohort of 2,595 people undergoing elective cardiac evaluation, fasting plasma L-carnitine levels were associated with an increased risk of cardiovascular disease and its nefarious effects—heart attack, stroke, and death—in a dose-dependent manner. But this association only held true in people who also had high levels of TMAO. TMAO promotes atherosclerosis, but the mechanism by which it does so is not entirely clear. It seems to work at least partially by inhibiting reverse cholesterol transport—the transport of cholesterol from the peripheral tissues back to the liver.

L-carnitine is essential for transporting fatty acids into mitochondria. It has been given as an oral supplement to people on hemodialysis and as treatment for heart trouble, of all things. The authors of this paper suggest that, given their results, such supplementation might not be the best idea.

Nature Medicine, 2013. DOI: 10.1038/nm.3145 (About DOIs).