"When you measure things in people's blood, you don't think of [them] as coming from bacteria," Hazen says, but in this case that appears to be what's happening. The finding is the latest in a series of studies that have shown that the population of bacteria in our guts-collectively known as the gut microbiome-can influence everything from weight loss to brain chemistry.
As the researchers described Sunday in Nature Medicine, mice fed a diet supplemented with L-carnitine for 15 weeks had much higher levels of TMAO than control animals. Animals getting extra L-carnitine had about double the burden of atherosclerosis in their arteries compared with mice eating a normal diet.
And the intestines of the mice getting extra L-carnitine also adapted, becoming enriched for various classes of bacteria that could more readily convert L-carnitine to TMAO. This hinted that people who eat lots of red meat might be especially efficient at converting L-carnitine to TMAO and that consuming the food in moderation could be less harmful because the conversion might be more sluggish. Twenty-three vegans and vegetarians given L-carnitine supplements were, Hazen's group found, less able to synthesize TMAO than those who regularly ate red meat.
It's still unclear why TMAO seems to promote atherosclerosis. Work by Hazen's group hints that TMAO seems to make it easier for immune cells in the arteries to accumulate cholesterol. Another mystery is how other foods containing L-carnitine might impact TMAO levels. For example, fish, which is thought to lower cardiovascular risk, also contains much lower levels of L-carnitine, as do chicken and milk, notes Dariush Mozaffarian, an epidemiologist at the Harvard School of Public Health in Boston who has studied meat and heart disease, in an email. Inconsistencies also remain about how harmful red meat really is. That said, although more work needs to be done to tease apart the L-carnitine-microbe connection in people, "these findings may turn out to be seminal in the field," Mozaffarian writes.