Scientists say they have found that fats and cholesterol are not the main reason why red meat increases susceptibility to heart disease. According to scientists, a substance in red meat called L-carnitine is linked to the risk of heart disease.
In the study published online in the journal Nature on April 7, 2013, titled: "Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis," the researchers led by Dr. Stanely Hazen, head of preventive cardiology and chairman of the department of cellular and molecular medicine at the Lerner Research Institute of the Cleveland Clinic, found that the high incidence of heart disease among red meat eaters may be due to a relatively obscure substance called L-carnitine that helps transport fatty acids into the cell's mitochondria where energy is produced.
Although previous studies have linked heart disease with consumption of red meat, it remained uncertain which component of red meat was causing the damage. Researchers had suspected saturated fats and sodium in meat but the evidence was not conclusive. Hazen, investigating the possibly that an unsuspected substance was exposing red meat eaters to risk of cardiovascular disease, soon identified L-carnitine found in high levels in red meat, and a popular additive in energy drinks and supplements.
The discovery followed a previous study Hazen published in the journal Nature that identified a compound in the blood called trimethylamine N-oxide (TMAO) which seemed to be a good predictor of the risk of heart disease. The substance was also found to be linked to heart disease in mice. TMAO is formed when intestinal bacteria breaks down certain substances such as the dietary fat choline in the gut.
Hazen suspected that intestinal gut may also be converting L-carnitine in red meat to TMAO. Choline has a chemical structure similar to L-carnatine.
Hazen and his Ph.D student Robert Koeth, conducted a study in which they fed steak to volunteers, mostly "young, hungry students," who were happy to be used as guinea pigs in the study.
Blood tests administered on the students after a meal of steak found a rise in TMAO levels showing that something in the meat was being converted into TMAO. Further investigation showed that L-carnitine was indeed being converted into TMAO. The researchers were also able to demonstrate that intestinal gut was responsible for converting L-carnitine in TMAO. After giving the meat eaters antibiotics that wiped out their gut bacteria they found that they did not have TMAO in their blood.
According to Nature, the researchers also reported that mice fed a diet supplemented with L-carnitine had much higher levels of TMAO than control animals and that the animals had twice the incidence of atherosclerosis compared to control animals.
The authors reported that the intestine of mice fed L-carnitine was able to adapt by becoming enriched with bacteria that convert L-carnatine to TMAO. The study also found that people who eat a lot of red meat are more efficient at converting L-carnitine to TMAO and that limiting consumption of L-carnitine rich food could lead to reduced ability to convert L-carnitine to TMAO.
Nature reports the study found that vegetarians and vegans have lower levels of bacteria that convert L-carnitine to TMAO.
The study has implications for the use of L-carnatine as an additive in energy drinks. It also has implications for the use of L-carnitine capsules as a supplement used by body builders.
According to The New York Times, the study could also have implications for the treatment of heart disease. Doctors may be prescribing antibiotics that target gut bacteria which produce TMAO from L-carnitine to reduce the risk of strokes, heart attacks and atherosclerosis in high-risk individuals.
The study noted that more research is needed to determine safe levels of L-carnitine in the diet. A portion of red meat may contain up to about 94mg of L-carnitine, while cheese and milk contain only about 3mg per serving.
Nutritionists say that in spite of the study linking L-carnitine and gut microbes to cardiovascular disease, the nutrient plays a vital role in helping the body to convert fat to energy. Thus, rather than recommend abstinence, experts recommend limiting intake of red meat.
According to nutritionists, red meat contributes valuable nutrients such as zinc, iron, B vitamins and vitamin D.
Although biochemists do not yet understand how TMAO causes atherosclerosis, Hazen explains that it is known that long-term ingestion of L-carnitine changes the way the body metabolizes cholesterol, causing more of it to be deposited on artery walls and less being eliminated.