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.
Is Red Meat Really The Culprit In Heart Disease?
The reason for the difference, the scientists believe, is that vegans don't have Acinebacter in large numbers in their colons, so their bodies don't receive toxic TMAO from their food. The researchers also note that mice given antibiotics to kill gut bacteria likewise don't have dangerous levels of TMAO in their blood.
The Truly "Toxic" Foods May Surprise You
Scientists have found that servings of certain foods cause the creation of less than 100 mM (a chemical measurement) of TMAO in the eight hours after the meal. These include:
- Soybean products, and
- Beef, beef leading to lower levels of TMAO than soy.
Servings some of certain other foods cause the creation of more than 100 mM but less than 200 mM of TMAO in the eight hours after the meal. These foods include:
- Cauliflower, and
- (Irish or white) potatoes.
In other words, eating 3-1/2 ounces (100 grams) of cauliflower releases more of the artery-clogging TMAO than eating 3-1/2 ounces (100 grams) of rib eye steak.
But there are other foods that cause the release of even more TMAO.
- Eating a 3-1/2 ounce (100 gram) serving of squid leads to the formation of over 6000 mM of TMAO.
- Eating the same sized serving of skate leads to the formation of over 5000 mM of TMAO.
- Eating the same sized serving of prawns leads to the formation of over 4000 mM of TMAO.
- Eating the equivalent serving of crab leads to the formation of over 2000 mM of TMAO.
But that isn't all that the headline stories leave out.
Even in Vegans, the Body Can Make TMAO
Even in vegans, the body makes L-carnitine. The authors of the study use that fact to point out (the references to figures are in the original article) that:
"In most subjects examined, despite clear increases in plasma d3-carnitine and d3-TMAO concentrations over time (Fig. 1e), post-prandial changes in endogenous (unlabeled) carnitine and TMAO concentrations were modest (Supplementary Fig. 5), consistent with total body pools of carnitine and TMAO that are relatively very large in relation to the amounts of carnitine ingested and TMAO produced from the carnitine challenge."
In plain language, a vegan eating steak experienced only about half the rise in TMAO as an omnivore eating steak, and the total rise in TMAO was very, very small, most of it coming from the fact that the steak was served with vegetables.
And the study's authors failed to address the previously established fact that more L-carnitine is converted to TMAO in the presence of estrogen. That is, foods that are high in L-carnitine, which is really sea foods and certain veggies, produce more of the artery-clogging chemical TMAO in women than in men.
The recent headlines about the toxic effects of red meat seem to be greatly overstated. If you choose not to eat meat because you respect animals, good for you. But don't refuse lean red meat because it's not good for your heart. Take a closer look at calamari, instead.