近年来,抗衰老行业得以飞速发展,离不开领域内多位学界大佬的硬核加持,就职于哈佛大学医学院的大卫·辛克莱(David A. Sinclair)教授便是个中翘楚,2014年,他还被美国《时代周刊》杂志评选为“全球最具影响力百大人物”。 作为业界标杆人物,辛克莱教授的抗衰心得一直为大众津津乐道。从转发最新研究动态,到日常生活分享,他在Twitter上人气可不低,算得上是位“流量博主”。早在2019年,他曾发布一条动态,劝告读者们“深思熟虑BCAA的摄入”。 这个本质上不过是类氨基酸的物质,怎得还能抗衰不成反折寿?考虑到BCAA对于不少读者也不是个新鲜事物,不少人说不定曾经或正在尝试。这期我们就一起来聊聊——“BCAA究竟是好是坏?”“我是否需要BCAA?”又或者“我应该如何正确服用BCAA?” BCAA体内代谢:不走寻常路 BCAA是亮氨酸、异亮氨酸和缬氨酸三种必需氨基酸的统称。它们无法由机体自主合成,需从外界直接摄取,大多数高蛋白食物,如肉类、乳制品、豆类等,均含有丰富的BCAA。 无论从饮食中直接获取,还是短期饥饿后肌肉蛋白质水解,BCAA在体内的代谢过程大致分为以下两步[1]。 起点:骨骼肌 与大多数氨基酸不同,由于BCAA分解代谢途径中的第一种酶BCAT(支链氨基酸氨基转移酶)的肝脏活性较低。因此,BCAA不走“寻常路”(从肝脏开始),转向骨骼肌作为自己的首发站。 在骨骼肌中,BCAA经过BCAT催化,转化为BCKAs(支链酮酸,包括KMV、KIV等),并产生谷氨酰胺和丙氨酸,共同进入血液。 下一站:多组织线粒体 进入血液循环后,BCKAs会到达肝脏、肾脏和脂肪等多个组织,在线粒体内膜上BCKDH(支链α-酮酸脱氢酶复合物)的作用下氧化脱羧,生成相应的支链酰基辅酶A酯类。这些底物可以进入三羧酸循环,或以糖原或甘油三酯的形式储存[2]。 图注:BCAA分解代谢的主要途径 值得一提是,BCKDH作为BCAA分解代谢途径的限速酶[3],在肝脏中的活性最高,并受到多种因素调控,如BCAA的含量与磷酸化水平[4]以及多种代谢因子(如IGF-1、TNFα、皮质醇)[1]等。 BCAA与机体代谢:时而有雨时而晴 仅从BCAA的代谢过程,就可以初步断言其对维持机体能量稳态的重要作用,或许这正是它在补剂市场上受到追捧的原因之一。 当前,不少研究已经发现,BCAA与生物体蛋白质合成、乳腺健康、胚胎发育、肠道及免疫功能均有显著关联[5]。尤其对于葡萄糖和脂质代谢,更是“掌控力”满满。 BCAA与葡萄糖代谢:爱恨交织的长诗 维持机体正常血糖水平,对保证能量稳态意义深远。BCAA对葡萄糖转运过程的调控已经得到证实[6],例如亮氨酸能通过上调胰岛素水平,增强GLUT4和GLUT1(葡萄糖转运蛋白)易位[7],并激活PI3K和PKC信号通路[8],增加骨骼肌中葡萄糖的摄取。 然而,“福兮祸所伏”,BCAA的血糖调节作用也不是一路长红。若过量摄入BCAA,会持续激活mTORC1,使胰岛素受体与信号介质IRS-1分离,诱导胰岛素抵抗[9],使得葡萄糖代谢受损[10]。 但BCAA与胰岛素间的逻辑关系又似乎不那么简单。另一种观点认为,BCAA可能是胰岛素抵抗的标志,而不是原因,因为胰岛素抵抗会抑制BCKDH活性[11],导致BCAA代谢异常积累,引发一系列线粒体功能障碍和应激信号[9]。 BCAA与脂质代谢:背后一刀的盟友 “高蛋白低碳水”、“一天十几个鸡蛋白”,诸如此类的减重方法,可能不少读者在控制体重时都曾尝试。但不加限制狂吃蛋白质,对我们的脂肪代谢可能并不友好。 虽说,适当补充BCAA能增加细胞中乙酰辅酶A的水平,并抑制丙酮酸脱氢酶活性,将能源偏好由碳水化合物转移到脂质,帮助控制肥胖[12]。 但在一项小鼠试验中却发现,长期补充BCAA不仅让它们吃得更多、长得更胖,还缩短了这些小鼠10%的寿命,而限制BCAA摄入后,其健康状况得到扭转[13]。同时,2016年一项针对2000余名我国汉族人群的大规模研究也表明,体内BCAA水平与血脂代谢异常(表现为总胆固醇高、高密度脂蛋白低)呈现正相关[14]。 为何多吃了些氨基酸,反而影响代谢、更易长肥肉? 目前推测限制BCAA后,一方面可能通过AMPK-mTOR-FoxO1途径,影响肝脏脂肪相关基因(如ACCα与SCD-1)表达[15],并激活GCN2通路[16],帮助控制食欲、减少脂肪合成、加快脂肪酸氧化;另一方面,还可能通过调控限速酶BCKDH及下游信号,影响脂质代谢[17]。 BCAA与延年益寿: 小酌怡情,贪杯伤身 长寿通路对生物体寿命的影响及其调控机制,是无数学者与极客们的奔赴之地。对于BCAA,好像无论走哪条路,mTOR这个点是如何也绕不过去的。 一直以来,根深蒂固刻在我们脑海中的“铁律”是——抑制mTOR通路,有助于延长寿命。这也是多种“不老药”(如雷帕霉素)的作用原理。然而,到了BCAA这儿,情况却好像有点不同。 全网大搜索后,派派总结了目前BCAA对模式物种(有广泛研究,对其生物现象有深入了解[18])寿命影响的试验(见下表)。这些结果虽然很难直接类比人类,但借助这些探索,我们也许能为未来可能的人体临床试验找到方向。 这些外表看似毫无关联的试验结果,实则一番仔细分析后,都指向同一点:限制BCAA是有必要的,过高或过低的BCAA都可能对机体能量稳态、神经系统与肠道健康等方面造成损害,并最终影响寿命。 《BCAA服用建议》 BCAA不能少,但也绝不可多。在进行严谨的科学检测之前,大家或许想知道:我如何预判自己是否需要额外补充BCAA?又该如何正确去补充? 如果你有此类疑问,不如联系时光派专属小助理Hebe,发送暗号、领取为你精心整理的“《BCAA服用建议》”,一睹为快吧~ —— TIMEPIE —— 参考文献 [1] Holeček, M. 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