HUMAN REQUIREMENTS FOR THE NUTRIENT CHOLINE

这个建议的主要前提是人类需要胆碱的饮食来源。 胆碱是一种对细胞膜的结构完整性和信号传导功能至关重要的膳食组分;它是膳食中甲基的主要来源，并且它直接影响胆碱能神经传递、跨膜信号传导和脂质转运/代谢。 医学研究所最近提出的关于膳食参考摄入量的建议包括对人类所需胆碱量的估计。 这份报告指出，胆碱对正常功能很重要，人类很可能需要饮食胆碱。 然而，他们指出，人类的证据不足以对这一要求作出准确的估计，而且“......能够提供这类人类数据的研究被赋予最高优先权。“我们建议进行研究，开始解决这些问题。 在该71天重复测量中，在分级饱食的受试者研究中，成年男性、成年绝经前和绝经后女性将入住综合临床研究中心，并喂食含有已知对照量胆碱的标准饮食（550 mg/胆碱/2500 Kcal饮食; 35 Kcal/kg体重），以及叶酸和甲硫氨酸的RDA，持续10天（基线）。 然后将它们置于实验饮食（无胆碱-低叶酸/甲硫氨酸;或无胆碱-正常叶酸/甲硫氨酸）21天。 将评估饮食胆碱状态的可能生物标志物，包括：胆碱及其代谢物的血浆浓度、RBC胆碱、肝胆碱代谢物、同位素稀释法测定的全身胆碱、蛋氨酸氧化率、尿胆碱和甜菜碱排泄、血清丙氨酸转氨酶、肝脂肪变性、血浆同型半胱氨酸、叶酸和蛋氨酸、肝脏质谱和体格检查。同位素标记研究将用于评估S-腺苷甲硫氨酸和磷脂酰乙醇胺的从头胆碱生物合成速率。 如果在21天时未观察到胆碱缺乏的功能影响，则将继续无胆碱饮食，直至出现此类影响，或直至再完成21天（雄性动物的经验表明不需要）。 在消耗期结束时，受试者将转换为含有25%对照量胆碱（和低叶酸/甲硫氨酸;或正常叶酸/甲硫氨酸）的饮食10天。 如果上述生物标志物指示耗尽的胆碱状态在10天后持续（生物标志物稳定且耗尽），则受试者将转换为含有50%对照量的胆碱（和低叶酸/甲硫氨酸;或正常叶酸/甲硫氨酸）的饮食10天。 如果生物标志物不指示饱食，那么受试者将被转换为含有75%控制量的胆碱（和低叶酸/甲硫氨酸;或正常叶酸/甲硫氨酸）的饮食10天。 如果没有吃饱，则受试者将被喂食含有100%对照量的胆碱（和低叶酸/甲硫氨酸;或正常叶酸/甲硫氨酸）的饮食，直到吃饱。 发生2级毒性的受试者将立即转为对照饮食，并进行随访，直至恢复正常。

The major premise for this proposal is that humans require a dietary source of choline. Choline is a dietary component essential for the structural integrity and signaling functions of cell membranes; it is the major source of methyl-groups in the diet, and it directly affects cholinergic neurotransmission, transmembrane signaling and lipid transport/metabolism. The recent recommendations on dietary reference intakes made by the Institute of Medicine included estimates of the amount of choline required by humans. This report noted that choline is important for normal function, and that it is likely that humans require dietary choline. However, they noted that evidence in humans is insufficient to make accurate estimations of this requirement, and that "...research that could provide such human data is assigned the highest priority." We propose studies that will begin to address these questions. In this 71 day repeated measure, within subject study with graded repletion, adult men, adult premenopausal and postmenopausal women will be admitted to the General Clinical Research Center and fed a standard diet containing a known control amount of choline (550 mg/choline/2500 Kcal diet; 35 Kcal/kg body weight) with the RDA for folate and methionine for 10 days (baseline). They then are placed on an experimental diet (no choline-low folate/methionine; or no choline-normal folate/methionine) for 21 days. Possible biomarkers for dietary choline status will be assessed including: plasma concentrations of choline and its metabolites, RBC choline, liver choline metabolites, total body choline by isotopic dilution, methionine oxidation rate, urinary choline and betaine excretion, serum alanine aminotransferase, hepatic steatosis, plasma homocysteine, folate and methionine, mass resonance spectroscopy of liver and physical examination. Isotopic labeling studies will be used to assess rates of de novo choline biosynthesis from S-adenosylmethioinine and phosphatidylethanolamine. If no functional effects of choline deficiency are observed at 21 days, the choline devoid diet will be continued until such effects occur, or until another 21 days has been completed (experience in males suggests that this will not be needed). At the end of the depletion period subjects will be switched to a diet containing 25 percent control amount of choline (and low folate/methionine; or normal folate/methionine) for 10 days. If the above biomarkers indicate that depleted choline status persists after 10 days (biomarkers are stable and depleted)- subjects would then be switched to a diet containing 50 percent control amount of choline (and low folate/methionine; or normal folate/methionine) for 10 days. If biomarkers do not indicate repletion, then subjects would then be switched to a diet containing 75 percent control amount of choline (and low folate/methionine; or normal folate/methionine) for 10 days. If not repleted, then subjects would be fed a diet containing 100 percent control amount of choline (and low folate/methionine; or normal folate/methionine) until repleted. Subjects who experience grade 2 toxicity will be immediately switched to the control diet and followed until they return to normal.