Leading the list of tips for starting a heart healthy diet is usually the advice to limit red meat. Most experts cite saturated fat and cholesterol as being the primary reason for avoiding burgers and steaks, but researchers have found another reason why you may want to shun meat.
Researchers with the Cleveland Clinic have found that L-carnitine, a molecule found in red meat may be responsible for arterial clogging. L-carnitine is converted within the gut to trimethylamine N-oxide (TMAO), a chemical associated with an increased risk of atherosclerosis and cardiovascular disease.
A team led by Stanley Hazen MD PhD and Robert Koeth tested the carnitine and TMAO levels in more than 2,595 patients. Those who ate the most meat or other foods containing carnitine had the highest levels of TMAO due to a greater amount of gut bacteria that metabolized the nutrient. Vegetarians and vegans did not make as much TMAO because of the lower amount of bacteria they synthesized in response to limited meat intake.
The research team also studied mice that were given L-carnitine and found that they produced more TMAO and had twice the level of atherosclerosis as those whose carnitine sources were limited. In addition to meat, carnitine can also be found in energy drinks and body building supplements where it claims to improve endurance and increase fat metabolism.
Stanley Hazen, a cardiologist at the Cleveland Clinic says that TMAO changes our cholesterol metabolism and contributes to the accumulation of cholesterol within the artery wall. He says that there is a significant association between carnitine intake and risk for heart attack, stroke, and death.
"Carnitine is not an essential nutrient; our body naturally produces all we need," adds Dr. Hazen. "We need to examine the safety of chronically consuming carnitine supplements as we've shown that, under some conditions, it can foster the growth of bacteria that produce TMAO and potentially clog arteries."
A TMAO blood test for patient use is slated to hit the market later in the year, Dr. Hazen says. “It will show who’s at increased risk independent of existing risk factors, and it may also help to personalize your diet,” he says.
What foods contain carnitine? Here is some nutrient information from a variety of foods:
Beef steak (100g) - 95 mg
Ground beef (100g) - 94 mg
Pork (100g) - 27.7 mg
Bacon (100g) - 23.3 mg
Tempeh (100g) - 19.5 mg
Cod (100g) - 5.6 mg
Chicken breast (100g) - 3.9 mg
American cheese (100g) - 3.7 mg
Ice cream (100 ml) - 3.7 mg
Whole milk (100 ml) - 3.3 mg
Eggs (100g) - 0.0121 mg
Robert A Koeth, Zeneng Wang, et al. Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature Medicine, 2013; DOI:10.1038/nm.3145