Choline metabolism in high-producing dairy cows: metabolic and nutritional basis

Choline, the beta-hydroxyethyltrimethylammonium ion, is a strong base containing a trimethylated quaternary nitrogen. Choline occurs widely in biological materials as the compound itself, as acetylcholine and as various phospholipids. In feed ingredients and crude unprocessed fat sources, most choline is present as phosphatidylcholine (lecithin). Although soybean, soybean meal, rapeseed meal, fish meal and dried yeast are relatively rich sources of choline, the bioavailability of choline in these feedstuffs is only moderate. In dairy ruminants, choline is extensively degraded in the rumen. For this reason, dietary choline contributes insignificantly to the choline body pool, and methyl group metabolism is generally conservative with a relatively low rate of methyl catabolism and an elevated rate of de novo synthesis of methyl groups via the tetrahydrofolate system. Gluconeogenic precursors are the primary sources of the new methyl groups. Depending on energy balance and physiological state, such precursors can be deficient in ruminants. In situations of glucose imbalance, for example at the onset of lactation in the dairy cow, choline may become a limiting nutrient for milk production, owing to the extra demand for methyl groups. Furthermore, choline is a lipotropic substance and it may optimize the balance between fat retained and fat metabolized by the liver, thereby improving lipid metabolism in general. The data reviewed in this paper are consistent with these assumptions and support the fact that choline supplementation for dairy cows, in a form protected from ruminal degradation, may improve methyl group metabolism and the status of other nutrients, including vitamin E.