Choline

Choline is a vitamin usually classed among the B-complex vitamins. It is, however, needed in considerably larger amounts than most of the rest of the B complex; around 500mg per day, although food sources of choline are reasonably plentiful. Choline is essential in many of the body's processes; it is needed as a very important part of cell membranes (in the form of phosphatidyl choline), as the main part of the substance acetylcholine, essential as a neurotransmitter in the brain and in the rest of the nervous system; and choline is a donator of methyl groups, essential in many reactions in the body involving the conversion of one biochemical molecule to another, and in the building of essential chemicals in the body.
Choline also has a strong effect on fat metabolism, indirectly. The reason for this is that choline forms a major part of the structure of lecithin, and lecithin is used by the body as part of the system that transports fat around the body. Accordingly, choline (and also lecithin) are often used for purposes that are concerned with fat mobilisation; these include slimming and cholesterol control.
Choline is almost devoid of side effects. One is that, because of its effect on brain and nerve function, it might have unpredictable effects if used by someone with epilepsy. Another one is that taken in large amounts (more than normally recommended) choline may make someone taking it smell of fish, because a breakdown product of choline (trimethylamine) has that smell. In fact, this chemical is the main component of the smell of fish that's past its best.
Some people have a mild metabolic defect called trimethylaminuria, that involves the body being unable to break down this substance. These people often smell fishy after eating high-choline foods (including fish). If you have this problem then taking choiline is probably not a good idea, but this problem, like most metabolic problems, is rather rare.
Choline is found in most B complex formulas, and it is also found as part of lecithin. Good food sources include soya beans, egg yolks and liver.