Saturated fat? Cholesterol? Sure, red meat has plenty of those, but it also contains a compound that toys with gut bacteria and can lead to clogged arteries.
When it comes to explaining exactly why steaks and hamburgers and other red meats can be so harmful to the heart, the saturated fat that the body breaks down and sequesters in blood vessel walls where they can form dangerous plaques is an easy and obvious culprit. But the high rates of heart disease in the developed world suggest that these fats may not be working alone, say a group of researchers from the Cleveland Clinic who study how microbes and bacteria in our gut influence heart disease.
Our gut is full of bacteria — good strains that don’t cause disease — and recent studies show that these microbes can have a significant impact on our health, affecting our propensity for obesity, asthma, inflammatory diseases and even cancer. Not surprisingly, what we eat can influence which populations of bacteria are more common at any given time, so the researchers of the new study, published in the journal Nature Medicine, focused on how these gut microbes responded to a diet that included meat. Specifically, they looked at a compound called carnitine, which is abundant in meats like beef, lamb, duck and pork, but is also a popular dietary supplement in energy drinks.
In previous work on mice, the scientists found that gut bacteria can transform choline, a vitamin-B-group nutrient, from the diet into a compound called trimethylamine N-oxide (TMAO) that transports cholesterol to arteries where it forms potentially heart-stopping plaques. Carnitine, it turns out, is structurally similar to choline, so the researchers set out to document whether carnitine is metabolized by human gut bacteria in a similar way to gum up heart vessels and cause atherosclerosis.
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To better understand the relationship between carnitine and TMAO, the researchers conducted a series of experiments with meat eaters and a vegan willing to consume meat for the sake of the study. In the first phase, they documented the boost in TMAO produced after the meat-eating volunteers ate an 8-oz. steak and downed a capsule that would attach to and label the carnitine for easy detection. Consuming high amounts of carnitine from the steak was only associated with a higher level of TMAO in the blood of the five meat eaters, however, and not in the vegan who hadn’t consumed meat in at least a year. That suggests that eating meat can promote larger numbers of bacteria that break down carnitine into TMAO, thus generating more heart-harming cholesterol and establishing a cycle of damage to the heart.
This was confirmed when the researchers then looked at the levels of TMAO and carnitine in the blood of 2,595 patients undergoing heart-disease evaluations who were either omnivores, vegans or vegetarians. Meat eaters tended to harbor higher levels of carnitine and had a higher risk of heart disease, stroke or heart attack compared with the vegans or vegetarians. The bacteria in the gut, then, are heavily influenced by long-term-diet patterns, adding another layer to the understanding of how food can affect our risk for developing certain diseases. “A diet high in carnitine actually shifts our gut microbe composition to those that like carnitine, making meat eaters even more susceptible to forming TMAO and its artery-clogging effects. Meanwhile, vegans and vegetarians have a significantly reduced capacity to synthesize TMAO from carnitine, which may explain the cardiovascular health benefits of these diets,” said study leader Dr. Stanley Hazen, of the Cleveland Clinic, in a statement.
In fact, when the meat eaters were given antibiotics for a week to cull some of the intestinal bacteria, levels of TMAO dropped significantly. That finding hints that it may be possible to control some of the heart-harming effects of red meat by suppressing certain populations of bacteria in the gut, although more studies need to be done to confirm exactly which bacterial populations are responsible for breaking down carnitine, and how direct the association between carnitine and TMAO is.
And then there are questions about carnitine supplements. Some energy drinks contain the compound, which is often added to rev up metabolism and increase energy, but if it also promotes the growth of bacteria that contribute to atherosclerosis, then people consuming energy drinks may not be aware that these products may be associated with an increased risk of heart disease.
The findings certainly set the stage for more detailed studies on how red meat may contribute to heart disease, but in the meantime, it’s probably not necessary to entirely cut out red meat from your diet. Hazen’s own strategy should serve as a model: once a meat eater who enjoyed about 12 oz. several times a week, he told the New York Times that he now limits himself to eating 4 to 6 oz. once every two weeks. Moderation, it seems, is the best approach until more information becomes available.