WASHINGTON — Our guts are awash in bacteria, and now a new study fingers them as culprits in heart disease. A complicated dance between the microbes and a component of red meat could help explain how the food might cause atherosclerosis. The work also has implications for certain energy drinks and energy supplements, which contain the same nutrient that these bacteria like chasing after.
Red meat is considered bad news when it comes to heart health, although studies aren't consistent about how much can hurt and whether it always does. Furthermore, it's not clear which components of meat are doing harm. Various studies have considered saturated fat or sodium but the results are inconsistent and sometimes depend on whether meat has been processed or not. Stanley Hazen, the section head of preventive cardiology and a biochemist at the Cleveland Clinic in Ohio, wondered whether another ingredient might be harmful: L-carnitine, a nutrient that helps transport fatty acids into the cell's energy powerhouses, the mitochondria. L-carnitine is a popular additive to energy drinks and supplements that claim to boost energy levels. In food, the highest levels of L-carnitine are in red meat.
Hazen's focus on L-carnitine was something of a wild guess based on earlier work he'd done. Two years ago, he and his colleagues published a paper in Nature identifying a compound in the blood called trimethylamine N-oxide (TMAO). It seemed to correlate with future heart disease risk and cause heart disease when fed to mice. TMAO is created when intestinal bacteria break down certain compounds in foods. Hazen wondered if the bugs might also convert L-carnitine to TMAO, which, in turn, could put the heart at risk.
To find out, Hazen, his Ph.D. student Robert Koeth, and their colleagues bought a George Foreman grill and started cooking steaks. "People lined up for the study," Hazen says, and participants "tended to be young, hungry students." Blood tests administered afterward revealed a rise in TMAO levels. This showed that something was converting L-carnitine to TMAO, but the researchers couldn't say yet that the culprits were bacteria. To pin this down, they gave five of their volunteers broad-spectrum antibiotics for a week to suppress gut microbes and then repeated the experiment. This time, there was virtually no TMAO in the blood or urine after the volunteers ate steak, suggesting the conversion couldn't happen without bacteria present.