蛋氨酸和胆碱缺乏饲料
蛋氨酸和胆碱缺乏(MCD)饲料已被用于研究肝脏疾病超过40年。喂食蛋氨酸和胆碱缺乏饲料的啮齿目动物将在2-4个星期内发展出显著脂肪肝然后迅速演变为的炎症和纤维化 (6,7)。蛋氨酸和胆碱缺乏饲料诱导脂肪肝的机制在于磷脂酰胆碱合成不足引起的极低密度脂蛋白的分泌障碍(8)。重要的是,不像人类和其他膳食诱导非酒精性脂肪肝啮齿目动物模型,喂食MCD饲料的啮齿目动物会减去体重(由于大大降低的热量摄入量),不发生胰岛素拮抗(9,10)。这与人类典型的非酒精性脂肪性肝炎患者的情况相反,他们不但肥胖而且对胰岛素拮抗。蛋氨酸和胆碱缺乏饲料中的脂肪来源可以改变表型。通过使用多不饱和脂肪膳食来源,肝脏脂肪氧化、炎症和炎症基因的诱导可被增加(相对于含更饱和脂肪膳食),虽然这并不必然导致肝脏损伤的增加(11)。在另一个不同的研究中,橄榄油减少肝脏的TAG的积累而鱼油则降低肝脏胆固醇水平(12)。
胆碱缺乏饲料
胆碱缺乏饲料提供的潜在优势在于:他们增加肝脏脂肪水平,增加体重,导致血脂异常和胰岛素拮抗 (13),在这些方面,它们并不像蛋氨酸和胆碱缺乏饲料。肝脂肪的积累所涉及的机制可能与蛋氨酸和胆碱缺乏饲料的也不一样(14),肝脏脂肪堆积、肝脏损伤和炎症并没有蛋氨酸和胆碱缺乏饲料引起的严重(13)。有趣的是,胆碱缺乏在高脂膳食中能改善小鼠的葡萄糖耐受(15)。
高脂饲料
众所周知高脂饲料可以为啮齿目动物模型增加体重、体脂肪和导致胰岛素拮抗。高脂饲料也可以相当迅速地(以天计算)在周边脂肪沉积发生显着上升前增加肝脏脂肪水平(16)。如此迅速的肝脏脂肪堆积是与肝胰岛素拮抗相关联的(16)。长期来说,高脂饲料诱导的肝脏脂肪堆积可能不遵循线性进展而且肝脏脂肪含量实际上可能减少,然后在长时间的高脂喂养下增加(17)。如果喂食的时间一致,高脂饲料喂养比蛋氨酸和胆碱缺乏饲料喂养得到10倍更低的肝脏脂肪水平(18)累积。一般来说,高脂饲料喂养与蛋氨酸和胆碱缺乏饲料喂养相比不产生肝纤维化,且只有轻微脂肪肝(3)。
在最近的糖尿病的研究方面,Raubenheimer等人用胆碱缺乏和高脂饲料组合来调查肝脏脂肪过剩对伴随膳食诱导肥胖症的胰岛素拮抗和糖耐量的影响(15)。C57Bl/6 小鼠被喂食有胆碱或无胆碱的高脂或低脂饲料。胆碱缺乏并不影响体重增加或脂肪组织的重量,但在高脂和低脂组都提升了肝脏甘油三酯的水平。对照高脂饲料增加了实验动物体重以及空腹胰岛素和葡萄糖水平(相对于对照低脂饲料喂养动物来说),这表明小鼠正逐渐呈现胰岛素拮抗。有趣的是,喂食胆碱缺乏高脂饲料的小鼠与喂食含胆碱高脂的相比具有更低的胰岛素水平。此外,喂食胆碱缺乏高脂饲料的小鼠比喂食含胆碱高脂的小鼠改善了糖耐量。这些研究人员还发现,胆碱缺乏在高脂膳食的条件下诱导涉及将游离脂肪酸酯化为三酰甘油的肝酶基因的表达,而脂肪酸合成和氧化有关的酶基因的表达则没有改变。
作者们的结论是,脂肪酸转化为肝脏甘油三酯储存可能只是一个初步的保护机制,以降低肝细胞内的脂肪酸含量。由于细胞内的脂肪酸升高被认为是肝胰岛素拮抗的部分原因,它们作为甘油三酯储存将有助于维持肝胰岛素敏感性。长期喂食胆碱缺乏高脂饲料最终是否会导致胰岛素拮抗则仍然未知。
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