Choline is more comparable to the amino acids than to the vitamins because, unlike most vitamins, it is synthesized by most species, although in some cases, not in sufficient amounts to meet all demands. Choline also resembles the amino acids in the nature of its metabolic functions. Dietary sources and in vivo synthesis from labile methyl groups donated by methionine, creatine or homocysteine meet choline requirements under normal conditions.
Choline and methionine are the two primary methyl group donors in metabolism via a process called transmethylation. Therefore, the choline requirement is related to methionine status, and vice versa. Folic acid and vitamin B12 status also affect methyl group metabolism because of their role as enzyme cofactors in methyl group transfer and activation.
Calculations from lactating dairy goats on rates of methyl group transfer revealed that only 6% of methionine methyl groups were derived from choline, while 28% of choline methyl groups were derived from methionine (Emmanual and Kennelly, 1984). This suggests that considerable dietary methionine is used for choline synthesis, but that choline is not a major direct precursor of methionine, although it may spare methionine.
Typical choline requirements for monogastric animals range from 1,000 to 2,000 mg per kg of diet. In contrast to monogastric animals, no requirements for choline have been established in ruminants except for milk-fed calves, where 260 mg of choline per liter of synthetic milk prevented choline deficiency signs. The NRC (1989) suggests that milk replacers for calves should contain 0.26% choline.