“bad” LDL cholesterol, which contributes to cardiovascular disease.
But, Hazen says, research has shown that any ill effects of saturated fat are not enough to explain the excess heart disease risks linked to heavy red-meat consumption. There had to be other mechanisms.
The new findings point to one, according to Lauri Wright, chair of nutrition and dietetics at the University of North Florida, in Jacksonville.
How to promote a healthy gut microbiome
There is still much to learn about the gut microbiome, says Wright, who is also a spokesperson for the Academy of Nutrition and Dietetics. But in general, she says, diets rich in foods like vegetables, fruits and high-fiber grains help “feed” beneficial gut microbes.
“It still goes back to food,” Wright adds.
Hazen, too, says he is a “big supporter” of using diet to change the gut microbiome, rather than adding certain bugs via probiotic supplements.
“Changing your diet changes the soil” that feeds gut microbes, he explained.
The latest findings build on earlier work by Hazen and his colleagues focusing on TMAO. The chemical is generated when gut bacteria break down carnitine, a nutrient particularly abundant in red meat.
The researchers had already shown that TMAO appears to raise the risk of heart disease and stroke. And in a 2019 study, they found that adding red meat to healthy people’s diets for a short time boosts blood levels of TMAO. Those levels went back down, though, when red meat was swapped for either white meat or vegetable proteins.
In the latest study, looking at both humans and lab mice, the researchers found that a cluster of gut bacteria — within a group called Emergencia timonensis — transform carnitine into TMAO. While meat-eaters harbor a decent amount of those microbes, longtime vegetarians and vegans have very few.
In the experiments with mice, the researchers found that introducing E. timonensis boosted TMAO levels and the blood’s propensity to form clots.
The researchers also analyzed stool samples from people who took part in the 2019 diet study. They found that when participants were eating a lot of red meat, their stool harbored more of the culprit E. timonensis microbes; when they switched to non-meat protein sources, those microbial levels dropped.
There are blood tests available to measure people’s TMAO levels. And Hazen said that these could potentially allow healthcare providers to give patients more personal diet advice: If someone’s TMAO levels were high, limiting red meat would be particularly important.
But what you take in, Wright notes, is as important as what you limit. She says that fermented foods like yogurt and kimchi, which contain certain microbes, can be good choices. But again, she stresses, overall diet is what’s key in supporting a healthy gut.