Pinotti L, Baldi A, Dell'Orto V
Department of Veterinary Sciences and Technology for Food Safety, University of Milan, Via Celoria 10, 20133, Milano, Italy.
Nutr Res Rev. 2002 Dec;15(2):315-32. doi: 10.1079/NRR200247.
The present review examines the importance of choline in dairy cow nutrition. Choline is an essential nutrient for mammals when excess methionine and folate are not available in the diet. The requirement for choline can be met by dietary choline and by transmethylation reactions. Two types of functions for choline are known: functions of choline per se; functions as a methyl donor. The two principal methyl donors in animal metabolism are betaine, a metabolite of choline, and S-adenosyl-methionine, a metabolite of methionine. Choline and methionine are interchangeable with regard to their methyl group-furnishing functions. In adult 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. In dairy ruminants, the dietary availability of choline is still low, but the output of methylated compounds in milk is high, and precursors from the tetrahydrofolate pathway are limiting, especially at the onset of lactation. Therefore choline may be a limiting nutrient for milk production in high-yielding dairy cows.
本综述探讨了胆碱在奶牛营养中的重要性。当日粮中缺乏过量的蛋氨酸和叶酸时，胆碱是哺乳动物必需的营养素。胆碱的需求可通过日粮胆碱和转甲基反应来满足。已知胆碱有两种功能：胆碱本身的功能；作为甲基供体的功能。动物代谢中的两种主要甲基供体是胆碱的代谢产物甜菜碱和蛋氨酸的代谢产物S-腺苷甲硫氨酸。胆碱和蛋氨酸在提供甲基的功能方面是可互换的。在成年反刍动物中，胆碱在瘤胃中被大量降解；因此，日粮胆碱对胆碱体库的贡献微不足道，甲基代谢通常较为保守，甲基分解代谢率相对较低，通过四氢叶酸系统从头合成甲基的速率较高。在奶牛反刍动物中，胆碱的日粮可利用性仍然较低，但牛奶中甲基化化合物的产量较高，四氢叶酸途径的前体有限制作用，尤其是在泌乳初期。因此，胆碱可能是高产奶牛产奶的限制营养素。
