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Recent headlines have been telling us yet another reason not to eat red meat. Citing a study conducted at the Cleveland Clinic that found that regular eaters of red meat accumulate higher levels of a substance called TMAO in their bloodstreams, health advocates have been advising that eating red meat can raise the risk of atherosclerosis. The scientific paper that made the headlines, however, does not tell the whole story.
TMAO and the Risk of Atherosclerosis
TMAO, which is an abbreviation for trimethylamine-N-oxide, is a naturally occurring compound that is particularly abundant in salt-water fish. In ocean fish, especially sharks, and also in mollusks, it stabilizes proteins so that they do not degrade with exposure to intense pressure. It also helps keep the blood from becoming too "watery" in the marine environment.
In humans, TMAO also changes the "wateriness" of the blood, but in this case by accelerating the transport of cholesterol out of the bloodstream and into the linings of arteries. In a study of 2,600 people, increased levels of TMAO are linked to increased incidence of hardening of the arteries, also known as atherosclerosis.
It's Not Just Fat That Causes Hardening of the Arteries
It is important to understand that TMAO isn't a fat, and it isn't cholesterol. Instead, it is a substance that changes the way the linings of the arteries store cholesterol.
The body uses L-carnitine to transport fatty acids. Every cell in the body has to have L-carnitine, and the muscles can't burn fat without it. There is L-carnitine, as its name suggests, in red meat, but the liver can make it from the amino acids lysine and methionine.
Just about everyone has encountered TMAO. This is the chemical that breaks down in fish to make trimethlamine, the source of "rotten fish" odor. In the human colon, a species of bacteria known as Acinebacter, which attach themselves to the lining of the colon and usually live in pairs, can transform the L-carnitine in meat into TMAO.
The Cleveland Clinic study attempted to show that meat eaters are at higher risk for atherosclerosis because they have Acinebacter in their colons that transform L-carnitine into TMAO. The scientists recruited five vegans and one omnivore (someone who eats both animal and plant foods), persuading the vegans to eat a steak for the benefit of science. After the volunteers ate their steaks, the scientists found that TMAO levels went up in the bloodstream of the regular meat eater but not in the bloodstreams of the vegans.