Choline, unlike most vitamins, can be synthesized by most species, although in many cases not in sufficient amounts or rapidly enough to satisfy all the animal's needs. The choline requirement for growing poultry of various species ranges from 750 to 2,000 mg per kg (341 to 909 mg per lb) of diet. Generally, adult species of poultry can probably synthesize the vitamin in adequate quantities, with one exception being breeding quail, which require 1,500 mg per kg (682 mg per lb) of diet (NRC, 1994).
Dietary factors such as methionine, betaine, myo-inositol, folic acid and vitamin B12 or the combination of different levels and composition of fat, carbohydrate and protein in the diet as well as the age, sex, caloric intake and growth rate of animals, all have influence on the lipotropic action of choline and thereby requirement of this nutrient (Mookerjea, 1971). Dietary betaine can spare choline, since choline functions as a methyl donor by forming betaine. It is suggested that the effects of betaine may be to spare methionine by providing labile methyl groups for the synthesis of methylation products, and to reduce abdominal fat by increasing carnitine synthesis and beta-oxidation of fatty acids as well as providing energy (creatine phosphate) for cell metabolism (Kidd et al., 1997; Xu et al., 1998).
Studies have shown that vitamin B12 and folic acid reduce requirement for choline in chicks and rats (Welch and Couch, 1955). Folic acid and vitamin B12 are required for the synthesis of methyl groups and metabolism of the one-carbon unit. Biosynthesis of a labile methyl group from a formate carbon requires folic acid, while B12 plays a role in regulated transfer of the methyl group to tetrahydrofolic acid. Therefore, marked increases in choline requirement have been observed under conditions of folic acid and (or) vitamin B12 deficiency. It is concluded that chicks fed practical ingredient-based diets require 1.3 mg per kg (0.59 mg per lb) of folic acid with low levels of choline, but 1.2 mg per kg (0.54 mg per lb) of folic acid when choline is offered near the NRC recommended level of 1,300 mg per kg (590.9 mg per lb) (Ryu et al., 1995).
The two principal methyl donors functioning in animal metabolism are choline and methionine, which contain "biologically labile methyl groups" that can be transferred within the body. This phenomenon is called transmethylation. Therefore, dietary adequacy of both methionine and choline directly affects requirements of each other. Other than exogenous sources of methyl groups from choline and methionine, methyl group formation from synthesis of formate carbons is reduced with folic acid and (or) vitamin B12 deficiencies.
Most animals can synthesize sufficient choline for their needs provided enough methyl groups are supplied. As an example, methionine in the pig can completely replace that portion of the choline needed for transmethylation. Young poultry, on the contrary, are unable to benefit from methionine or betaine as a dietary replacement for choline unless methylaminoethanol or dimethylaminoethanol is in the diet, as young poultry appear unable to methylate aminoethanol when fed a purified diet (Jukes, 1947). Later studies showed that the chick can synthesize microsomal methylaminoethanol and choline from S-adenosylmethionine, but, unlike the pig, at an insufficient rate in relation to needs (Norvell and Nesheim, 1969).
The metabolic needs for choline can be supplied in two ways: either by dietary choline or by choline synthesis in the body that makes use of labile methyl groups. For selected species, body synthesis sometimes cannot take place fast enough to meet choline needs for rapid growth and thus clinical signs of deficiency result. Since choline functions in prevention of fatty livers, hemorrhagic kidneys and perosis, it does not act as a true vitamin since choline is incorporated into phospholipids (via cytidine disphosphocholine). Therefore, unlike a typical B vitamin, the choline molecule becomes an integral part of the structural component of liver, kidney or cartilage cells (Scott et al., 1982).
Excess dietary protein increases the young chick's choline requirement. Ketola and Nesheim (1974) observed that over three times as much choline was needed for maximum growth of chicks when fed a diet containing 64% protein than when fed 13%. Diets high in fat aggravate choline deficiency and thus increase requirement. Fatty liver is generally enhanced by fats containing a high proportion of long-chain saturated fatty acids (Hartroft et al., 1952). Choline deficiency develops to a greater degree in rapidly growing animals, with deficiency lesions more severe in these animals. Need for supplemental choline is greatest with the starting bird because all facets of use are likely to be maximal. As growth diminishes, the necessity for choline supplementation disappears (NRC, 1994).
