TMAO is TriMethylAmineOxide. It appears to be a major methyl donor, therefore involving the oxidation/reduction metabolism of the body. It is created by the liver by oxidizing TMA - TriMethylAmine. TMA is created by gut bacteria. And gut bacteria create TMA from 2 major precursors: choline & carnitine. These are key parts of omnivore diets, especially the animal part of an omnivore diet.

This is the landmark New England Journal of Medicine article of 2013.

It demonstrated that: TMAO levels in the blood clearly come from dietary choline. It also demonstrated that people with high TMAO levels heart much higher rates of heart attack & stroke.

Why, then would anyone ignore this? Obviously, the non-vegan community has done just that - ignored it. Or is there something else? Yes. It has to do with kidney function. There is a confounder in these studies. It has to do with kidney function. It appears that TMAO is cleared quickly enough to not cause damage in people with normal kidney function. With inadequate kidney function, it is not. That, assuming it's true, explains some things. But it leads to a whole new set of questions & problems.