长期以来,乳酸都被认为是无氧代谢的产物,且被视为导致运动能力下降,运动疲劳和肌肉酸痛的元凶,被称之为代谢废物甚至代谢毒物。然而近些年一系列研究表明:乳酸不但不是影响运动能力和导致肌肉酸痛的原因,甚至有助于运动能力的提高并对人体具有不可替代的积极作用。乳酸是一种有机酸,我们在运动领域常说的乳酸本质上可以理解为血乳酸,即一种体内糖代谢的中间产物。在体内葡萄糖代谢过程中,如糖酵解速度增加,剧烈运动、脱水时,可引起体内乳酸升高。
过去认为当人体运动的时候,肌肉需要能量的支持。因此肌肉通过不同的代谢途径产生细胞能量(三磷酸腺苷)。代谢途径基本上是一系列化学反应。我们最重要的代谢途径之一是糖酵解,即将葡萄糖分子(我们吃的食物中的单糖)分解成丙酮酸。这种化学物质随后被用作人体的能量来源--但是它只能在氧气存在的情况下作为能量来源来获取。当高强度运动时,氧供应满足不了需要,肌肉就会利用三磷酸腺苷(ATP)、磷酸肌酸(CP)的无氧分解和糖的无氧酵解生成乳酸,并释放出能量。当前研究表明:乳酸在有氧环境中产生。
新研究显示:乳酸阴离子的史氏环氧化反应催化剂对映体(L-enantiomer)在完全有氧条件下形成并连续地在不同的细胞中使用。实际上,作为一个代谢途径(糖酵解)和下游途径(线粒体呼吸)的底物的乘积,乳酸可以视为糖酵解和有氧途径之间的联络物。根据乳酸穿梭假说,乳酸在细胞内与细胞间穿梭移动,能维持能量稳态,发挥信号传导作用,调节运动代谢过程。
另外,对于传统的糖酵解通量仅通过线粒体网摄取丙酮酸而指向线粒体呼吸的观点,并没有坚实的实验支持。也没有实验支持糖酵解通量只有在缺氧时才指向乳酸的产生。同样,也没有证据表明乳酸氧化的第一步(即转化为丙酮酸)发生在任何组织的细胞质中,包括跳动的心脏或工作中的骨骼肌,该组织具有乳酸-丙酮酸浓度比(L/P)>100,并且是净葡萄糖消费者。相反,有强有力的证据表明,葡萄糖分解代谢和糖原分解代谢继续产生乳酸。对完整的动物、动物组织制剂和人类的研究中,葡萄糖和糖原分解代谢在完全有氧条件下进行乳酸生产。对人体的研究结果清楚地表明,乳酸的生产是在完全有氧条件下发生的。在静止的健康人的肌肉和动脉血液中,乳酸浓度约为1.0 mM,而丙酮酸浓度约为0.1 mM,乳酸/丙酮酸(L/P)为10;结果表明,在完全有氧条件下,糖酵解进展为乳酸。此外,在健康个体的静息肌肉中,肌内氧分压(PO2 )约40 Torr,远高于临界线粒体PO2以获得最大的线粒体呼吸(1-2Torr )。显然,当氧气充足时,净乳酸产生和释放发生。在运动过程中,大约是最大耗氧量的65%(VO2 工作肌肉床的乳酸产量和净乳酸释放增加,L/P上升超过一个数量级(达到500),但肌肉内的PO2保持在3-4Torr,远高于临界线粒体O2水平因此,可以得出这样的结论:在健康的人类中,糖酵解在完全有氧条件下进入乳酸。重要的是,大多数(75%-80%)的乳酸立即在组织内处理或随后被工作肌肉释放和再摄取,心脏显著吸收和氧化,肝脏用于糖异生。
↑ 糖酵解与氧化代谢之间联系的乳酸穿梭示意图 ↑
糖酵解产生乳酸而不是丙酮酸
研究表明:红血球在体内漫游时也会产生乳酸。许多种类的细菌也会呼吸缺氧,产生乳酸作为代谢废物。
从休息到高强度运动时,骨骼肌细胞的能量消耗可能会增加100倍。这种高能量需求超过了肌肉细胞的有氧能力,所需的ATP的很大一部分将来自厌氧代谢。高强度运动还导致收缩功能迅速下降,称为骨骼肌疲劳。因此,很多人认为厌氧代谢的某些后果会导致收缩功能下降。而糖原的厌氧分解会导致无机酸的细胞内积累,其中乳酸是数量上最重要的。另外,由于乳酸是浓酸,因此将其分离为乳酸盐和H+。乳酸盐对肌肉收缩几乎没有影响;但是,H+(即pH降低或酸中毒)的增加是骨骼肌疲劳的经典原因。 然而,现在挑战pH降低作为疲劳的重要原因的作用正在受到挑战,最近的一些研究表明,在生理温度下,pH值减少可能对哺乳动物肌肉的收缩几乎没有影响。还有一个有力证据就是乳酸的代谢速度很快,几个小时就可以让血乳酸浓度下降到静息状态,但是我们的疲劳感和肌肉酸痛却不会在短时间内消失。而根据最近的发现,在高强度运动中,无机磷酸盐(Pi)而不是酸中毒似乎是疲劳的最重要原因。运动过程中除酸中毒外,骨骼肌中的厌氧代谢还涉及肌酸磷酸盐(CRP)的水解为肌酸和无机磷酸盐(Pi)。肌酸对收缩功能几乎没有影响,而在某些机制中,Pi增加反而可能会降低收缩功能。除此之外,肌肉的轻微撕裂以及无菌性炎症也被认为是运动后酸痛的原因之一。
乳酸帮助人体正常工作,是人体功能不可或缺的物质。它是细胞,组织和器官功能的重要组成部分,而不仅仅是运动的副产物。
人体在全身水平上使用乳酸,在人体内有以下主要用途:
当细胞需要的氧气多于身体提供的氧气时,就会产生能量来为细胞提供动力。
转化为新的葡萄糖,您的身体可以在需要更多能量时使用。
向您的细胞发出信号,针对受伤和感染产生免疫反应。
另外,通过其转运蛋白(MCT)和受体(GPR81),乳酸在多种细胞过程中起关键作用,包括能量调节,免疫耐受性,记忆形成,伤口愈合,缺血性组织损伤以及癌症生长和转移。乳酸可以在许多生理和病理条件下发挥多种调节功能和信号传递作用。
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文章整理:裴超,国家认证康复治疗师,中国田径协会运动健康专家智库秘书,英国Brunel University 康复治疗硕士
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