TMAO levels can be a signature of gut flora and choline/carnitine intake from meat, and fish contains more pre-formed TMAO than meat.

“it means I'm eating meat and it means the carnitine and the choline in that meat are being converted somewhat to tmao in my gut by bacteria I don't worry about this at all I've seen it high and low in humans it's just basically a signature of my gut flora and the fact that I'm eating choline sometimes tmao is high in vegetarians again this is one of the worst metrics I put it on there just to show you guys how much it is and I should point out that there's more pre-formed tmao in fish than there is in the corresponding amount of choline and carnitine in a similar amount of meat”