HUMAN REQUIREMENTS FOR THE NUTRIENT CHOLINE

这一建议的主要前提是人类需要从饮食中获取胆碱。胆碱是一种对细胞膜结构完整性和信号功能至关重要的膳食成分；它是日粮中甲基的主要来源，直接影响胆碱能神经传递、跨膜信号和脂质转运/代谢。美国医学研究所（Institute of Medicine）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。该报告指出，胆碱对正常功能很重要，人类很可能需要从饮食中摄取胆碱。然而，他们指出，在人类身上的证据不足以对这一需求做出准确的估计，并且“……能够提供此类人类数据的研究被赋予了最高的优先级。”我们建议开展研究，着手解决这些问题。在这项71天的重复测量中，在受试者的分级补充研究中，成年男性、成年绝经前和绝经后女性将被纳入普通临床研究中心，并被喂食含有已知控制量胆碱的标准饮食（550毫克/胆碱/2500千卡饮食；35千卡/公斤体重），叶酸和蛋氨酸的RDA为10天（基线）。然后，他们被放置在一个试验性饮食（无胆碱-低叶酸/蛋氨酸；或无胆碱-正常叶酸/蛋氨酸）21天。评估饮食胆碱状态的可能生物标志物包括：血浆胆碱及其代谢物浓度、红细胞胆碱、肝脏胆碱代谢物、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、肝脏质磁共振谱和体格检查。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从头合成胆碱的速率。如果在21天内没有观察到胆碱缺乏对功能的影响，则将继续进行无胆碱饮食，直到出现这种影响，或者再持续21天（男性的经验表明，这是不需要的）。在消耗期结束时，受试者将切换到含有25%控制量胆碱（低叶酸/蛋氨酸；或正常叶酸/蛋氨酸）的饮食10天。如果上述生物标志物表明10天后仍处于胆碱耗尽状态（生物标志物稳定且耗尽），那么受试者将被转换为含有50%控制量胆碱（低叶酸/蛋氨酸；或正常叶酸/蛋氨酸）的饮食10天。如果生物标志物没有显示饱足，那么受试者将被切换到含有75%控制量的胆碱（低叶酸/蛋氨酸；或正常叶酸/蛋氨酸）的饮食10天。如果没有补充，那么受试者将被喂食含有100%控制量的胆碱（和低叶酸/蛋氨酸；或正常叶酸/蛋氨酸）的饮食，直到补充。出现2级毒性的受试者将立即切换到对照饮食，并进行随访，直到恢复正常。

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.