Good food sources of choline are animal foods, especially eggs, poultry, seafood, and beef liver.

Just saw:
Nutrient In Meat And Eggs May Play Role In Blood Clotting, Heart Attack Risk, CBS News, 25 April 2017

Researchers found that when they gave 18 healthy volunteers choline supplements, it boosted their production of a chemical called TMAO.

That, in turn, increased their blood cells’ tendency to clot.

TMAO is short for trimethylamine N-oxide. It’s produced when gut bacteria digest choline and certain other substances.

Past studies have linked higher TMAO levels in the blood to heightened risks of blood clots, heart attack and stroke, said Dr. Stanley Hazen, the senior researcher on the new study.

These findings, he said, give the first direct evidence that choline revs up TMAO production in the human gut, which then makes platelets (a type of blood cell) more prone to sticking together.

Choline is found in a range of foods, but it’s most concentrated in animal products such as egg yolks, beef and chicken.

The amount of choline in 2 or 3 eggs (450 mg) increased their TMAO 10-fold in just a month.

Here’s the study:

Gut Microbe-Generated Trimethylamine N-Oxide From Dietary Choline Is Prothrombotic in Subjects, Circulation, 25 April 2017

Choline has three methyl groups attached to a nitrogen, so it provides the trimethylamine part of TMAO. According to Wikipedia, all you have to do is oxidize that trimethlyamine to create TMAO. That oxidation takes place in the liver. Carnitine in the diet can also provide that trimethylamine structure, but this study just looked at choline.

Choline is also found in plants, like broccoli, cauliflower, and other cruciferous vegetables; and in beans and whole grains. But plant sources contain much lesser amounts than animal sources.

There are several variables here: the ingestion of choline (or some other TMAO precursor like carnitine), the action of colonic bacteria, oxidation of TMA in the liver, and systemic action of TMAO. You can affect what’s going on by changing any of these variables. Not eating foods high in choline would reduce TMAO. But you also need a certain gut microbiome, a particular set of organisms that can metabolize choline. Vegan diets select against that set of organisms.