It's hard to have our steak and eat it too. Red meat was once implicated in a wave of studies and linked to heart disease and other maladies, before being absolved.

But the microbiome and the surge in advertising for probiotics to promote 'healthy' gut bacteria has implicated red meat again - this time by correlating a nutrient that the authors say is changed by gut bacteria into an atherosclerosis-causing metabolite, which means hardening of the arteries.

Writing in Cell Metabolism, Dr. Stanley Hazen, of the Cleveland Clinic and colleagues implicated a bacteria in the gut that converts L-carnitine, a nutrient abundant in red meat, into a compound called trimethylamine, which in turn changes to a metabolite named trimethylamine-N-oxide (TMAO), which promotes atherosclerosis. Now they believe another metabolite, called gamma-butyrobetaine, is generated to an even greater extent by gut bacteria after L-carnitine is ingested, and it too contributes to atherosclerosis. 

The researchers found that gamma-butyrobetaine is produced as an intermediary metabolite by microbes at a rate that is 1,000-fold higher than the formation of trimethylamine in the gut, making it the most abundant metabolite generated from dietary L-carnitine by microbes in the mouse models examined. Moreover, gamma-butyrobetaine can itself be converted into trimethylamine and TMAO. Interestingly, however, the bacteria that produce gamma-butyrobetaine from L-carnitine are different from the bacterial species that produce trimethylamine from L-carnitine.

That's not causation but they believe it is important that metabolism of L-carnitine involves two different gut microbial pathways, as well as different types of bacteria, and to them this suggests new targets for preventing atherosclerosis—for example, by inhibiting various bacterial enzymes or shifting gut bacterial composition with probiotics and other treatments. 

Marketing departments are rubbing their hands with glee.