支链氨基酸（2）

支链氨基酸

支链氨基酸的功能

支链氨基酸包括异亮氨酸、亮氨酸、缬氨酸。它们是必需氨基酸，因为人类必需从食物中获得。

支链氨基酸用以维持肌肉组织，保存肌肉内的糖原（碳水化合物在体内的存储形式，可以转化成能量）^[1]。它们还阻止肌肉蛋白在运动时不被分解。^[2]

许多研究表明支链氨基酸补充剂(每天10～20克)既不能改变机体组分^[3]也不能提高运动性能，^{[4 5 6 7 8]}或增强体育训练的作用。可是，支链氨基酸补充剂对于一些特殊情况有一定作用，如在高海拔地区阻止运动员肌肉的丧失^[11]和在炎热环境下延长运动耐量。^[12]另有一项研究还证实支链氨基酸补充剂可以改善一些心理功能下的运动减退。^{[13 14 15]}

支链氨基酸可以激活谷氨酸脱氢酶――一种肌萎缩性侧索硬化症（ALS）患者缺乏的酶。在一项双盲试验中，每天服用26克的支链氨基酸补充剂可以帮助这些病人维持肌肉力量。^[16]可是，一项较大范围的研究表明支链氨基酸不仅不能改善这些病人的症状，而且会使死亡率增加。^[17]另外一些试验也证实支链氨基酸补充剂对于肌萎缩性侧索硬化症或其他神经肌肉性疾病没有效果，^{[18 19]}尽管其中一小部分患有脊髓小脑变性的病人服用支链氨基酸后有一定改善。^[20]

 一项对接受加强运动计划的糖尿病病人进行的研究发现，支链氨基酸对于降低过多脂肪或改善糖代谢没有效果。^[21]

患有肝病的人出现昏迷――称为肝性脑病――支链氨基酸水平降低而其他氨基酸水平升高。以前的试验证实支链氨基酸可以帮助这些病人改善其症状。双盲试验已经得出这些结论，^{[22 23 24]}且重新分析这些试验也发现支链氨基酸对于治疗肝性脑病有一定疗效。^{[25 ]}支链氨基酸的治疗作用在患有肝功能衰竭的儿童^[26]和肝硬化的成人^[27]也很明显。但任何关于肝功能衰竭的治疗需在医生的指导下进行。

支链氨基酸补充剂同样对患有慢性肾功能衰竭的病人有效。以前的试验发现在肾脏透析时静脉注射支链氨基酸可以改善呼吸和睡眠质量。^[28]

苯丙酮尿症是一种遗传性疾病，它会引起苯丙氨酸水平增高以及其终末产物在血液中聚积，从而引起神经系统的损害。一项对照试验表明患有苯丙酮尿症的儿童或成人常规服用支链氨基酸可以改善神经功能。^[29]这一结果表明支链氨基酸可以与苯丙氨酸竞争以减少其毒性作用。

在迟发性运动障碍病人中，苯丙氨酸水平也会升高。在一项研究中给予这些病人服用支链氨基酸（每2.2磅体重150～209毫克，每天早餐后及午餐、晚餐前1小时，维持两周）。^[30]支链氨基酸的配方为等剂量的缬氨酸和异亮氨酸加上比前两种氨基酸的量多33％的亮氨酸。9例接受治疗的病人中，有6例症状减轻至少58％，其他病人的症状减轻至少38％。

支链氨基酸的分布

所以含有蛋白质的食物均含有支链氨基酸。其中乳制品和红色肉类含量丰富。乳清蛋白和卵蛋白则是支链氨基酸的另一来源。支链氨基酸补充剂提供亮氨酸、异亮氨酸和缬氨酸。

支链氨基酸认为与以下疾病有关（以下所有信息适用于个体化健康情况）：

哪些人可能会缺乏支链氨基酸？

只有缺乏蛋白质的人才会缺乏支链氨基酸，因为大多数含有蛋白质的食物都可提供支链氨基酸。在西方国家很少有人会蛋白缺乏。

一般需要多少支链氨基酸？

大多数饮食都可提供足够的支链氨基酸，一般为每2.2磅体重25～65毫克。^{[31 32]}运动员加强训练时通常每天服用5克的亮氨酸、4克的缬氨酸和2克的异亮氨酸以减少肌肉丧失和增加肌肉组织。

是否有副作用和药物相互作用？

关于支链氨基酸的副作用目前还没有明确报道。据目前所知，患有肌萎缩性侧索硬化症的人应避免服用支链氨基酸补充剂。在一项试验中，服用大剂量的支链氨基酸（60毫克）会引起血中色氨酸、苯丙氨酸和酪氨酸水平改变。^[33]这些改变理论上会引起易感者发生抑郁症。因此，患有抑郁症的病人补充支链氨基酸前应先咨询一下医生。患有肾脏疾病或肝脏疾病的人没有医生的指导不应服用过大剂量的支链氨基酸。

到目前为止，还不知道支链氨基酸与其他药物间的相互作用。

参考文献

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