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Compound in red meat and energy drinks linked to higher heart-disease risk

It’s not clear why TMAO appears to increase the risk of heart disease, but it may be because the compound makes it easier for cholesterol to form in arteries.

steak photo
The heart risks of red meat may be related to a compound called carnitine.

Meat consumption has long been associated with clogged arteries and a higher risk of heart attacks, strokes — and death. For decades the blame for that association was placed primarily on the saturated fat and cholesterol in the meat.

Recent research, however, has led many scientists to believe that saturated fat and cholesterol do not sufficiently explain why meat appears so hazardous to our health. The search was on for other possible explanations.

In a fascinating study published Sunday in the journal Nature Medicine, researchers led by Dr. Stanley Hazen of the Cleveland Clinic have uncovered another possible culprit: a compound in meat called carnitine.

This finding has significance not just for regular meat-eaters, but also for people who consume energy drinks. Carnitine is commonly added to those products.

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Carnitine is found other foods too, of course, but in much smaller amounts. Its job in the body appears to be to transport fatty acids into cells, where they are used for energy. In the digestive tract, carnitine is broken down by bacteria to form a metabolite called trimethylamine-N-oxide (TMAO). Earlier studies have shown that a high level of TMAO is linked to atherosclerosis, or hardening of the arteries, but the exact mechanism of that link is unknown.

A three-prong approach

For this new study, Hazen and his colleagues conducted three lines of research. First, they analyzed blood samples from 2,595 patients who were undergoing elective cardiac evaluation at the Cleveland Clinic. They found a strong association between high levels of carnitine in the blood and an increased risk of cardiovascular disease — but only in patients who also had high levels of TMAO.

Next, the researchers fed a group of omnivores, vegetarians and vegans a large amount of carnitine (either an 8-oz. sirloin steak or a carnitine capsule). Blood and urine tests taken after the meal revealed a significant rise in TMAO levels — but not among the vegetarians and vegans. Their blood and urine was almost TMAO-free.

Five of those volunteers were then given broad-spectrum antibiotics for a week to suppress bacteria in their intestines. The sirloin-steak experiment was then repeated. This time, no TMAO was found in the blood or urine, which suggests it isn’t converted in the gut from carnitine in the absence of bacteria.

In a third line of research, Hazen and his colleagues were actually able to cause heart disease in mice by feeding them a high-carnitine diet.

Reasons unclear

It’s not yet clear why TMAO appears to increase the risk of heart disease, but it may be because the compound makes it easier for cholesterol to form in arteries.

The link between carnitine ingestion and heart disease “has broad health-related implications,” write Hazen and his colleagues. One of those implications, they stress, is the need to examine the safety of adding carnitine to energy drinks and other supplements.

Of course, the findings also call into question the safety of consuming red meat.

“I used to have red meat five days out of seven,” Hazen told a BBC reporter, “but now I have cut it way back to less than once every two weeks or so.”