膳食硝酸盐对人体运动健康的研究进展
2022-05-28陈琦朱永博
陈琦 朱永博
摘 要:目的与方法:对近年来国外有关膳食硝酸盐改善运动表现及可能生理机制的相关研究进展进行综述。结果与结论:大部分研究认为,补充膳食硝酸盐可以显著提高运动能力,膳食硝酸盐作为运动补剂的应用前景广阔。
关键词:膳食硝酸盐;一氧化氮;运动健康
一直以来,饮食中的硝酸盐对人体健康的影响具有较大争议,但是近几年,大量国外临床实验研究证实,从饮食中摄取适宜膳食硝酸盐对人体运动健康有益。因此,本研究综述国外关于膳食硝酸盐提升人体运动健康的生理机制,为膳食硝酸盐作为一种营养补剂的可能性提供科学依据。
1 膳食硝酸盐概况
20世纪50—60年代,人们发现(亞)硝酸钠可形成致癌化合物二甲基亚硝胺[1-3],自此腌肉中使用(亚)硝酸盐是否安全开始成为有争议的话题[4-6]。对膳食硝酸盐的改观始于20世纪80年代末,科研人员首次发现一氧化氮是人体内一种重要的生物活性信号分子,参与调节大量的人体生理过程[7-10]。研究表明,除了L-精氨酸途径以外,硝酸盐也是人体合成一氧化氮的重要途径[11-12]。人体吸收硝酸盐后,在口腔中,舌表面兼性厌氧细菌将其还原为亚硝酸盐[11,13],进而在胃中转化为一氧化氮[14],在人体生理上有重要意义[12,15-16]。大量临床研究也证实,从饮食中摄取硝酸盐对人体有益。
1.1 膳食硝酸盐的来源
人体硝酸盐的主要来源仍然是饮食,因此近年来硝酸盐的饮食来源及其对健康的影响越来越受到关注[17-20]。调查显示,膳食硝酸盐是蔬菜中的一种天然无机盐成分,在绿叶蔬菜和根茎类蔬菜中含量尤其丰富,通常蔬菜中硝酸盐含量为2 000~3 000 mg/kg[21]。大多数常见的蔬菜其硝酸盐含量在中间范围(100~1 000 mg/kg),含硝酸盐较高(>1 000 mg/kg)的蔬菜多为十字花科(芝麻菜)、藜科(甜菜根、菠菜)、菊科(生菜)、伞形科(芹菜)等,而洋葱和西红柿中硝酸盐含量却非常低(<100 mg/kg)[22-23]。研究显示[24],成人每天的硝酸盐摄入量为93 mg,通常是通过土豆(33%)、绿色蔬菜(21%)、其他蔬菜(15%)、饮料(8.5%)、肉制品(4.2%)、新鲜水果(3.5%)、乳制品(3.1%)、牛奶(2.9%)、杂粮(2.1%)、面包(1.6%)和其他食物(5.1%)摄入的。对553名荷兰运动员的调查显示,运动员膳食硝酸盐摄入量略高于普通人,平均为106 mg/d,其中来源主要是蔬菜、土豆和水果摄入,占总量的74%,生菜和菠菜比例最大,女性运动员膳食硝酸盐摄入量显著高于男性运动员[25]。
1.2 膳食硝酸盐的代谢途径
研究表明,当膳食硝酸盐被摄入时,上消化道迅速将其吸收,吸收率几乎达100%[11]。有证据表明,在补充硝酸盐后30 min内,血浆硝酸盐水平迅速上升,在1.5 h时达到峰值,半衰期约为5 h[26-27]。大多数被吸收的硝酸盐最终通过尿液排泄出去,但血浆中仍有25%的硝酸盐由唾液腺吸收,经唾液腺分泌至唾液中,被口腔中的兼性厌氧菌还原为亚硝酸盐[11,13]。通过这一途径,人体内亚硝酸盐含量显著升高[11,27]。研究表明,当人体摄入富含硝酸盐的饮料后3 h内不吞咽唾液,这一循环被打断,血浆中亚硝酸盐含量不升高[27]。亚硝酸盐在适当生理条件下,可以在胃中转化为一氧化氮和其他氮氧化物[12,14]。所以这种硝酸盐—亚硝酸盐—一氧化氮通路与经典的L-精氨酸—一氧化氮通路分别是人体产生一氧化氮的不同调控系统,与L-精氨酸途径不同的是,硝酸盐转化为一氧化氮的机制会在组织缺氧[28]及酸中毒(低pH值)[29-30]条件下增强,而L-精氨酸途径这种条件下形成一氧化氮的能力反而受损[11]。所以由于硝酸盐途径的存在,膳食硝酸盐成为机体组织缺氧状态下一氧化氮的重要来源,可以调节机体在低氧状态下的血管舒张及细胞对低氧和缺血的恢复能力,从而调节心血管功能及其他机体恢复能力。
2 补充膳食硝酸盐对运动表现的生理机制
2.1 补充膳食硝酸盐可以降低运动过程中机体的耗氧量
在运动过程中,缺氧对骨骼肌能量代谢以及疲劳发生影响非常大,运动性缺氧比环境性缺氧更影响人体机体功能,会导致肌肉中磷酸肌酸以及糖原的加速消耗,并快速积累与疲劳相关的代谢物(如ADP、Pi、H+),这些代谢物会导致运动耐受性变差[31-32],同时当缺氧严重或训练量加大时,在停止运动后机体磷酸肌酸恢复速度也会减慢[33-35]。因此如果能有效降低在运动过程中机体的耗氧量对改善人体运动性缺氧有非常重要的意义。根据经典的运动生理学理论,机体在一定的运动量下,无论是训练情况、年龄、体质、饮食还是药物干预,每个人的耗氧量波动都不会太大[36-37]。可是令人惊讶的是,近年来多项研究证明,饮食中添加硝酸盐,会有效降低机体运动过程中的耗氧量[18,26,38-43]。Larsen等[39]对9名健康男性的饮食中添加硝酸钠(0.1 mmol/kg),并让他们进行一定强度的运动,试验进行了3 d,结果表明,与对照组相比,摄入硝酸钠的试验组耗氧量显著降低,而两组人员其血乳酸浓度、肺通气量却没有显著差异,说明同样的运动量,试验组摄入硝酸盐会有效降低机体耗氧量,提高机体氧气的利用率,有氧代谢能力加强,一定程度提高了试验组的有氧运动能力。Bailey等[18]在膳食中添加甜菜根汁也达到了同样的效果,试验组摄入含5.5 mmol/d膳食硝酸盐的甜菜根汁连续6 d,可以降低耗氧量达20%,其他研究也证实了这一点[38,42,44]。以上研究均表明,同样的运动量,摄入硝酸盐可以使机体肌肉活动耗氧量更少,有氧代谢率有所提升,能量生成更加有效。
目前有几种假说试图解释膳食硝酸盐如何降低机体运动中的耗氧量。有研究认为,摄入的硝酸盐在缺氧环境下转化为一氧化氮,直接作用于线粒体。研究人员通过骨骼肌活检的方法,检测到肌肉细胞线粒体P/O比值升高(线粒体测定氧化磷酸化效率的经典方法,即每产生1 mol的ATP消耗的氧气量)[45],线粒体呼吸明显改善[46],因此运动过程中耗氧量的降低可能与摄入硝酸盐后氧气消耗成本的降低有直接关系。还有研究认为,一氧化氮可能会增强机体对氧气利用的有效性,同时增加氧气与肌肉代谢的局部匹配作用[47-49],这些作用均使肌肉更加有效利用氧气,从而降低运动过程中的耗氧量。
2.2 补充膳食硝酸盐对心肺功能的作用
研究表明,在低氧环境下,硝酸盐补充也能提升血氧饱和度。研究人员通过使用近红外光谱法监测15名健康男性肌肉组织氧合指数,证明无论在休息还是最大运动量条件下,口服硝酸盐补充剂可以增加动脉和肌肉中血氧饱和度[50]。同样,研究人员在低氧环境中(氧气含量为14.5%,约为海拔4 000 m),让受试者24 h内摄入750 mL甜菜根汁(约含9.3 mmol硝酸钠),能够减少肌肉代谢紊乱,显著恢复运动耐受性,机能状态与常氧下基本一致[51]。膳食硝酸盐的补充还可以在不影响心率的基础上,有效延长潜水运动员呼吸暂停时间[52]。研究显示,高原常驻人群体内亚硝酸盐水平更高,是平原地区人群的好幾倍[53],而且有研究表明,体内亚硝酸盐水平越低或呼气一氧化氮含量越低,高原地区人群高原疾病发生率越高[54-55]。因此有研究人员建议,膳食硝酸盐可以作为一种平原人群进入高原的饮食策略,以确保高原反应的减缓[56]。以上这些均表明,膳食硝酸盐以及其代谢物质,是低氧运动及其恢复过程中肌肉中氧气输送的重要调节因子。
膳食硝酸盐还能影响人体的心肺功能。研究证明,摄入膳食硝酸盐可以影响钙处理蛋白在心脏细胞中的表达,导致心肌细胞钙信号通路增加,从而进一步改善左心室的收缩功能[57],增加心血管病人运动时心血管舒张和心输出量储备,减少动脉波的反射,重构有功能障碍的左心室的舒张功能[58]。研究认为,这些机制不仅与一氧化氮有关,还可能与膳食硝酸盐转化通路中的亚硝酸盐及其他氮氧化物有关[58-59]。由于一氧化氮是一种强效的血管扩张剂,近年来,膳食硝酸盐多次被科研人员提出可以作为一种辅助治疗高血压的营养补充剂[19-20,27,60-61],有数据表明,在正常的饮食中添加富含硝酸盐的蔬菜汁可以显著降低血压,扩张血管,且效果能维持15 d以上,具有重要的临床学意义[17,27,63-66],对肺功能影响也是通过扩张肺动脉,降低肺动脉压[67-68],从而可能提高肺动脉中血氧含量。因此,膳食硝酸盐的摄入能够有效提高人体心肺功能。
2.3 补充膳食硝酸盐对肌肉功能的作用
补充膳食硝酸盐还可以提高运动员或普通人的肌肉力量表现。研究人员让受试者摄入膳食硝酸盐,结果显示,试验组膝关节最大伸展速度提高11%,最大伸肌力量增加6%,说明膳食硝酸盐可以提高健康人群的肌肉速度和力量[69]。因此采用浓缩甜菜根汁补充膳食硝酸盐,可显著提高运动员多关节向心运动的最大力量和收缩速度。膳食硝酸盐对高强度间歇运动表现也有提升,补充富含硝酸盐的甜菜根汁后,专业赛艇运动员在运动后期最大力量的桨频增加[60]。对小鼠的研究同样证明,连续7 d在饮水中加入1 mmol硝酸钠,小鼠快速收缩肌的收缩能力增强[75]。其他研究也有类似结果[71-74]。虽然膳食硝酸盐对人体肌肉力量影响的确切生理机制尚不清楚,但不同证据显示有以下几种可能:其一,膳食硝酸盐转化产物——一氧化氮的生理靶点主要为线粒体,摄入足够的硝酸盐能够提高骨骼肌线粒体的氧化效率,减少运动时肌肉的耗氧量,在一定程度上提高肌肉细胞中ATP合成效率[45,50]。其二,有研究表明,膳食硝酸盐的摄入可能会减少机体ATP维持肌浆钙离子稳定的支出量[9,75],从而可以降低收缩肌纤维所需要的ATP成本[38],增加ATP的使用效率。其三,在低氧状态下膳食硝酸盐的补充可以降低磷酸肌酸的降解速度,减少代谢物的积累,从而减缓肌肉疲劳,增加肌肉耐受性,然而在常氧状态下补充膳食硝酸盐并不会加速磷酸肌酸的恢复速率,说明在肌肉氧气输送功能减弱的情况下,补充膳食硝酸盐才可以有助于改善肌肉功能[76-77]。其四,还有人提出,膳食硝酸盐的补充可能通过促进运动过程中的肌肉血流量来刺激运动表现[78],有研究证明,健康人群在补充硝酸盐后,使用近红外光谱法可以检测到其骨骼肌血容量明显增加,改善健康人体肌肉微血管功能,从而加快肌肉组织氧循环,提高肌肉有氧能力[50,79-80]。最后,研究人员对大鼠进行研究显示,膳食硝酸盐的摄入,由于一氧化氮的作用,直接加强了乙酰胆碱在肌肉神经处的作用,提高神经兴奋性,提高了肌肉最大收缩速度[81]。总而言之,适量补充膳食硝酸盐可以改善人体肌肉功能,增加运动过程中的肌肉能力表现。
Breese等[82]研究认为,膳食硝酸盐对肌肉的作用主要是针对Ⅱ型肌纤维的靶向作用。相比Ⅰ型肌纤维,Ⅱ型肌纤维有不同的肌纤维蛋白以及肌浆钙离子处理方式,并且其中细胞线粒体和毛细血管密度较低,因此Ⅱ型肌纤维更加依赖以ATP为基础的能量消耗方式[83]。从运动生理上看,高强度间歇运动需要动用大量的Ⅱ型肌纤维,因此,膳食硝酸盐的摄入对运动员肌肉能力提升作用较明显。
2.4 补充膳食硝酸盐对认知功能的作用
膳食硝酸盐的摄入还可能会增加机体脑血流量,提升认知功能。研究表明,一氧化氮在脑血管舒张、脑血流量、神经传递以及神经活动与局部脑血流的耦合中起关键作用[84]。Presley等[85]研究了富含硝酸盐的饮食对75岁左右老年人脑灌注的影响,结果显示,这些老年人额叶白质区域脑灌注增强,提示膳食硝酸盐可能具有增强执行功能和对抗认知衰退的潜力。在针对运动员的研究中也有同样的结果,16名男性集体项目的运动员在补充膳食硝酸盐后,可以提高重复短跑的运动表现,并且减缓长时间间歇运动中运动员可能出现的认知功能(特别是反应时间)下降[86]。因此,膳食硝酸盐的补充有可能改善普通人甚至运动员脑血管生理机能,增强认知功能。
3 膳食硝酸盐在运动营养中的应用
除了对普通人运动能力有提升作用外,越来越多的研究证明,膳食硝酸盐的补充可以在耐力项目中延长运动员的疲劳时间,提高运动员的计时赛成绩[18,42,44,87-89]。最近一项双盲试验发现,与服用安慰剂相比,连续6 d补充富含硝酸盐的甜菜根汁(约8 mmol/d)后,自行车运动员完成10 km的骑行时间缩短12 s[90]。对赛艇运动员的研究中也显示,在训练前2 h摄入较高剂量(8.4 mmol),可以有效提高其2 km的赛艇比赛成绩[91]。对于肌肉力量及高强度间歇运动,摄入膳食硝酸盐也对运动员运动表现有明显提升[72-74]。研究表明,膳食硝酸盐的摄入还能增加普通自行车运动员在多次冲刺时的输出功率[92]。研究人员连续3 d对运动员补充富含硝酸盐的甜菜根汁后,运动员高强度间歇运动能力提升170%,并且间歇跑的重复次数明显高于安慰剂组[71]。而36名运动员在摄入甜菜根汁(含6.4 mmol硝酸盐)5 d后,提高了20 m冲刺的能力以及YO-YO测试成绩,提高集体项目优秀运动员短跑和高强度间歇跑的成绩[93]。
雖然大部分研究均认为,运动员在补充膳食硝酸盐后运动表现有所提高[44,74,87,94,96],但有部分研究认为,与一般运动员相比,优秀运动员摄入膳食硝酸盐后运动能力的提升并不显著[62,95]。有两项研究表明,自行车、越野滑雪运动员在补充硝酸盐后,肌肉耗氧量及其运动表现没有显著变化[62,95]。而另一项研究认为,甜菜根汁的补充显著提高了中长跑运动员在1 500 m项目中的成绩,但其10 000 m的成绩却没有显著提升[97]。究其原因,可能是研究方法不同,研究方案并不完善,优秀运动员营养摄入量比普通人大,营养比例更均衡,而试验中对这些优秀运动员膳食硝酸盐的补充可能不足,运动能力提升效果不明显。而且优秀运动员多年从事高强度运动训练,心血管功能、骨骼肌利用率甚至线粒体产能效率具备更高的能力[98],从而使膳食硝酸盐的对优秀运动员的补充效果并不明显,关于这点还需要进一步研究。虽然补充硝酸盐可能对优秀运动员的运动表现提升并不明显,但是对于速度、力量或耐力方面的能力极小的提升都会使优秀运动员在重要比赛中起到非常重要的作用,不能忽视膳食硝酸盐补充可能带来的影响。
4 结论
膳食硝酸盐作为一种运动补剂已经被体育界关注,如今许多国外运动员在训练及比赛前已经开始使用含膳食硝酸盐较高的甜菜根汁作为营养补剂,以期提升其在训练或比赛期间的运动表现。因此应进一步研究膳食硝酸盐作为运动补剂的可能性。
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Influence of Dietary Nitrate Supplementation on Human Exercise Health
CHEN Qi,ZHU Yong-bo
(Gansu Research Institute of Sports Science,Lanzhou 730000,China)
Abstract:Objective and Method The paper reviewed research advancements on dietary nitrate how improve the ability of human exercise.Result and Conclusion Most research showed that dietary supplementation with nitrate resulted in significant enhance exercise performance,and dietary nitrate would become a popular sports supplement in the future.
Keywords:dietary nitrate;NO;exercise health
(責任编辑 李婷婷)