Chez les mangeurs de viande rouge les graisses saturées et le cholestérol ne contribueraient que de façon mineure au risque accru de maladie cardiaque, le vrai coupable étant un produit chimique qui est produit par des bactéries dans le tube digestif à partir de la carnitine de la viande (l’ingestion de pilules contenant de la carnitine a le même effet) puis est transformé par le foie en un autre produit chimique appelé TMAO qui pénètre dans le sang et augmente le risque de maladie cardiaque.
Ces bactéries, qui restent à découvrir, seraient en quantité bien moindre chez les végétariens et végétaliens.
...the investigators’ extensive experiments in both humans and animals, published Sunday in Nature Medicine, have persuaded scientists not connected with the study to seriously consider this new theory of why red meat eaten too often might be bad for people.
... the study’s findings indicated that the often-noticed association between red meat consumption and heart disease risk might be related to more than just the saturated fat and cholesterol in red meats like beef and pork.
Dr. Hazen began his research five years ago with a scientific fishing expedition. He directs a study of patients who come to the Cleveland Clinic for evaluations. Over the years, there have been 10,000. All were at risk for heart disease and agreed to provide blood samples and to be followed so the researchers would know if any patient had a heart attack or died of heart disease in the three years after the first visit. Those samples enabled him to look for small molecules in the blood to see whether any were associated with heart attacks or deaths.
That study and a series of additional experiments led to the discovery that a red meat substance no one had suspected — carnitine — seemed to be a culprit. Carnitine is found in red meat and gets its name from the Latin word carnis, the root of carnivore, Dr. Hazen said. It is also found in other foods, he noted, including fish and chicken and even dairy products, but in smaller amounts. Red meat, he said, is the major source, and for many people who eat a lot of red meat, it may be a concern.
The researchers found that carnitine was not dangerous by itself. Instead, the problem arose when it was metabolized by bacteria in the intestines and ended up as TMAO in the blood.
That led to [a] steak-eating study. It turned out that within a couple of hours of a regular meat-eater having a steak, TMAO levels in the blood soared.
But the outcome was quite different when a vegan ate a steak. Researchers had hypothesized that vegans would not have as many of the gut bacteria needed to make TMAO, and indeed virtually no TMAO appeared in the vegan’s blood after he consumed a steak.
“We did not expect to see such a dramatic difference,” Dr. Hazen said.
Then researchers gave meat eaters doses of antibiotics to wipe out almost all of their gut bacteria. After that, they no longer had TMAO in their blood either after consuming red meat or carnitine pills. That meant, he said, that the effect really was because of gut bacteria.
Researchers then tried to determine whether people with high blood carnitine or TMAO levels were at higher heart disease risk. They analyzed blood from more than 2,500 people, asking if carnitine or TMAO levels predicted heart attacks independently of traditional risk factors like smoking, high cholesterol and blood pressure. Both carnitine and TMAO did. But upon further analysis, they discovered that the effect was solely because of TMAO.
The researchers’ theory, based on their laboratory studies, is that TMAO enables cholesterol to get into artery walls and also prevents the body from excreting excess cholesterol.
But what is it about carnitine that bacteria like? The answer, Dr. Hazen said, is that bacteria use it as a fuel.
He said he worries about carnitine-containing energy drinks. Carnitine often is added to the drinks on the assumption that is will speed fat metabolism and increase a person’s energy level, Dr. Hazen said.
Dr. Robert H. Eckel, a professor of medicine at the University of Colorado and a past president of the American Heart Association, worried about how carnitine might be affecting body builders and athletes who often take it because they believe it builds muscle.
Those supplements, Dr. Hazen said, “are scary, especially for our kids.”