畜牧兽医学报  2023, Vol. 54 Issue (8): 3173-3182. DOI: 10.11843/j.issn.0366-6964.2023.08.005    PDF    
过瘤胃葡萄糖对围产期奶畜营养调控的研究进展
吴志立, 姚军虎, 雷新建     
西北农林科技大学动物科技学院, 杨凌 712100
摘要:围产期奶畜的干物质采食量明显下降,同时还需要满足妊娠后期胎儿快速生长发育以及泌乳前期乳汁合成分泌的营养需求,导致奶畜能量代谢紊乱。能量代谢紊乱可引发多种代谢性疾病和炎性疾病,对奶畜生产性能和繁殖机能造成重大负面影响。过瘤胃葡萄糖(rumen-protected glucose,RPG)在消化过程中受瘤胃影响小,可在小肠部位高效吸收进入血液并提供安全高效的能量来源,以此缓解奶畜在围产期出现的不良反应。本文总结了围产期的生理代谢特点,综述了RPG对围产期能量代谢、产奶量、乳成分、氧化应激、免疫和炎症的影响及其机制,并对RPG生物利用率进行了简要评价,以期为RPG在围产期奶畜营养调控中的研究与应用提供参考。
关键词过瘤胃葡萄糖    围产期    奶畜    生理代谢    营养调控    
Research Progress of Rumen-protected Glucose on Nutritional Regulation in Perinatal Dairy Animals
WU Zhili, YAO Junhu, LEI Xinjian     
College of Animal Science and Technology, Northwest A&F University, Yangling 712100, China
Abstract: During the perinatal period, decreased dry matter intake, increased nutritional requirements of the rapid growth fetus in late gestation and the synthesis and secretion of milk in early lactation result in energy metabolism disorder of dairy animals. Energy metabolism disorders can cause a variety of metabolic and inflammatory diseases, which have a significant negative impact on the performance and reproduction of dairy animals. Rumen protected glucose (RPG) is less affected by the rumen microbes during digestion and can be efficiently absorbed within the small intestine, thereby providing a safe and efficient energy source to alleviate energy metabolism disorder of dairy animals during the perinatal period. This paper summarizes the physiological metabolism characteristics during perinatal period, reviews the effects and mechanisms of RPG on energy metabolism, milk yield, milk composition, oxidative stress, immunity and inflammation of perinatal dairy animals, and briefly summarizes the bioavailability of different type of RPG, in order to provide reference for the research and application of RPG in perinatal dairy animals.
Key words: rumen-protected glucose    perinatal period    dairy animals    physiological metabolism    nutritional regulation    

围产期是奶畜繁殖周期中的关键阶段,围产前期干物质采食量(dry matter intake,DMI)下降30%以上[1],同时由于流向胎儿和乳汁中的营养素迅速增加[2],易导致围产期奶畜处于多种营养素不平衡状态,特别是能量负平衡状态(negative energy balance,NEB)。如果不能有效应对NEB,易引起围产期奶畜的氧化应激以及免疫能力的下降,导致奶畜罹患酮病和脂肪肝等代谢性疾病以及乳腺炎和子宫内膜炎等炎性疾病,对动物福利、生产寿命以及经济效益产生负面影响[2-3]。代谢葡萄糖(metabolizable glucose,MG)是机体大部分细胞新陈代谢的主要供能物质,来源于机体糖异生和小肠主动吸收[4]。奶畜主要依靠肝糖异生途径维持MG稳态[5],但由于合成葡萄糖的前体数量有限,且机体对MG的需求量迅速增加[6],肝糖异生和小肠吸收葡萄糖将无法满足围产期奶畜的MG需要。日粮直接添加葡萄糖会在瘤胃内降解,且葡萄糖添加量过高可能导致瘤胃酸中毒[7]。过瘤胃保护技术处理葡萄糖,即瘤胃保护葡萄糖(rumen-protected glucose,RPG),受瘤胃微生物影响较小,可越过瘤胃在小肠吸收,弥补肝糖异生的不足,缓解围产期奶畜NEB[8]。日粮添加RPG已被视为一种提高奶畜生产性能的有效营养调控策略[9],本文主要综述了围产期奶畜的生理代谢特点以及日粮添加RPG对围产期奶畜的营养调控作用及机制,并对RPG生物利用率进行简要评价,为通过RPG改善围产期奶畜机体健康和生产性能的研究与应用提供借鉴。

1 围产期奶畜生理代谢特点

围产期亦称过渡期,一般是指奶畜产前21 d(围产前期)和产后21 d(围产后期)[10]。围产期在经历“妊娠-分娩-泌乳”剧烈生理转变的同时也伴随着激烈的代谢、激素以及免疫变化,给奶畜带来了巨大的生理挑战[11]。瘤胃受到胎儿增大的机械压迫及日粮和激素改变等造成的应激将引起围产期奶畜DMI降低,然而胎儿后期发育和乳汁合成对营养元素的需求却在增加,这种矛盾则可能导致机体NEB的发生。合成乳汁所需的MG远超其他生命活动的能量消耗,此时奶畜往往表现出胰岛素抵抗(insulin resistance,IR),造成组织MG分配失衡[12-13]。MG不仅是合成乳糖的前体,同时对乳汁合成的渗透压调节有重要意义。当血清中葡萄糖不能满足机体能量需求时,围产期奶畜体内的脂肪动员速度加快,导致大量非酯化脂肪酸(non-esterified fatty acid,NEFA)流入血液和肝,引发脂肪肝和酮病[14]。秦敏等[15]研究表明,奶牛血清葡萄糖含量在围产期较低,尤以围产后期最低,而血清NEFA及β-羟丁酸(β-hydroxybutyrate,BHBA)含量在围产期处于较高水平。脂肪组织分解代谢过程中,体内的活性氧将会增加,一旦超出机体抗氧化能力,将诱发奶畜的氧化应激,进一步危害机体免疫功能,并诱发其他疾病[3, 16]。有研究表明,在免疫系统急性激活的条件下,奶牛可在12 h内消耗1 kg葡萄糖[17],这将加剧机体的代谢紊乱,形成恶性循环。

奶畜所需MG主要来源于小肠吸收的外源葡萄糖和糖异生合成的内源葡萄糖[18]。糖异生尤其是肝糖异生是奶畜MG的主要来源[19]。产奶期间,乳腺将消耗机体MG总量的60%~85%[20],每产生1 kg乳汁大约需要消耗72 g葡萄糖[21],而乳腺对MG的过度利用正是围产期奶畜能量代谢紊乱的主要原因之一。为补偿乳汁合成导致的MG不足,肝糖异生作用将增强。但由于DMI降低导致生糖前体如丙酸等物质有限,围产期奶畜的血清葡萄糖浓度维持在较低水平。桑丹等[22]研究表明,奶山羊血清葡萄糖浓度除在分娩当天较高之外,产后30 d内均维持在2.70 μIU·mL-1以下。围产期奶畜往往表现出短暂的IR,由于不同组织细胞膜上的葡萄糖转运蛋白(glucose transporter,GLUT)对胰岛素依赖性的不同,骨骼肌和脂肪组织对MG的利用会被抑制,而乳腺和子宫可利用的MG增多,这有利于保障乳汁合成和胎儿生长发育。

为满足机体外周组织的能量需要,围产期奶畜的脂肪动员加快,表现为脂肪分解快于脂肪合成,导致血液中NEFA和BHBA浓度升高[23]。脂肪分解是由脂肪甘油三酯脂肪酶、激素敏感脂肪酶和单酰甘油脂肪酶连续介导[24]。脂肪动员相关酶的活性受到多种激素调控:生长激素、儿茶酚胺、肾上腺素、去甲肾上腺素、胰高血糖素会促进脂肪分解且作用机制不尽相同;胰岛素会抑制脂肪分解,但IR将会减弱这种作用[25]。甘油三酯(triglyceride,TG)水解后的产物是甘油和NEFA,甘油可进入糖异生途径并用于乳糖的合成[26],而NEFA有一部分被乳腺利用合成乳脂,另一部分则被肝代谢[27]。NEFA入肝后有三条代谢途径[28-29]:重新酯化成TG并以极低密度脂蛋白(very low-density lipoprotein,VLDL)的形式输出;经历β-氧化后进入三羧酸循环(tricarboxylic acid cycle,TCA)彻底氧化供能;部分氧化生成酮体流入血液。VLDL运输TG的能力有限,当TG合成速率大于VLDL的输出速率时将导致TG在肝内沉积并引发脂肪肝[30]。酮体释放到血液后可为肌肉和神经组织供能,但当生成酮体的速率超过酮体消耗速率时,将在剧烈的脂质动员过程中引发酮症。检测酮体含量已被广泛应用于评估脂肪动员程度和酮病。有研究表明,泌乳第3~16天是监测奶山羊高酮血症的最佳采样窗口,同时进行BHBA和葡萄糖测试可更好地反映羊群的脂肪动员和能量状态[31]。此外,Zamuner等[32]研究发现,脂肪动员水平与产奶量和胎次显著相关:与经产奶山羊相比,初产奶山羊围产期的脂肪动员水平均较低。脂肪动员是围产期奶畜MG不足的必然结果,但脂质动员可诱导内皮细胞和白细胞功能的改变,直接作用包括脂肪酶的底物及其产物对细胞的毒性作用即脂毒性,间接作用包括改变细胞内信号传导途径、诱导氧化应激,进而引发乳腺炎、子宫炎等炎性疾病[33-34]

组织蛋白质动员也是围产期奶畜的一大生理代谢特征,主要是因为胎儿生长发育以及乳蛋白合成对氨基酸需求的激增。Bell等[35]认为,高产奶牛在产后7~10 d可能需要每天动员多达1 kg组织蛋白,以满足乳腺对氨基酸和葡萄糖的需求。Reid等[36]研究显示,奶牛分娩后骨骼肌纤维直径下降了25%。同时有研究发现,与干奶期山羊相比,产后第3周山羊后肢骨骼肌蛋白合成的绝对值减少了29%[35]。因此,骨骼肌可能是围产期奶畜内源性氨基酸的重要来源[37]。虽然生糖氨基酸在围产后期的肝糖异生中起重要作用,但从定量的角度来看,对糖异生贡献最大的碳源是内源循环的乳酸,间接表明了围产期奶畜MG的极度缺乏[38]

综上所述,MG的匮乏是引起围产期奶畜代谢紊乱的重要因素,提高MG水平将对改善围产期奶畜机体健康具有重要意义。

2 RPG对围产期奶畜的营养调控作用

提高围产期奶畜MG水平主要有两种方法:改善肝糖异生进而获取更多内源葡萄糖,或者提高小肠吸收葡萄糖以获得更多外源葡萄糖。目前,改善肝糖异生的研究已经比较透彻,但受限于DMI下降引起的生糖前体不足,其效果有限。此外,糖异生的调控必须控制在一定限度内,以防引起生糖氨基酸和能量的浪费[18]。通过静脉注射[39-40]和真胃灌注[41-42]的方式提高外源葡萄糖摄入可改善奶畜NEB,但并不实用。另一方面,由于瘤胃的特殊性,直接饲喂普通葡萄糖亦非合理策略。因此,添加RPG是一种提高MG进而改善围产期奶畜机体健康的有效措施。

2.1 RPG对能量代谢的调控

围产期奶畜面临着巨大生理代谢压力,如果适应失败,将对奶畜造成重大危害[43]。葡萄糖代谢失衡是围产期奶畜代谢紊乱的根本原因[44]。由于围产期子宫和乳腺对MG的需求激增,机体将动员脂肪和蛋白质以补偿MG的不足。改善围产期葡萄糖代谢状况对奶畜的繁殖机能和生产性能影响深远[45]。奶畜日粮中添加适量不同类型的RPG可显著提高血清葡萄糖含量,降低NEFA和BHBA含量[46-47]。添加200 g·d-1 RPG(纯度≥45%)也可对奶畜隐性酮病起到良好的防治效果[48-49],但是添加400 g·d-1 RPG(纯度>45%,微胶囊技术)可能引起血清葡萄糖含量下降以及BHBA含量上升[50-52]。高剂量RPG导致奶畜血清葡萄糖含量下降以及BHBA含量上升的具体原因尚不明晰,有待进一步探究。一般而言,血清胰岛素浓度会随葡萄糖含量的增加而提升,胰岛素可通过降低肝细胞色素P450酶类的丰度提高类固醇激素如孕酮的浓度,保障母畜顺利妊娠[40]。但Sauls-Hiesterman等[8]研究发现,添加RPG并未导致血浆葡萄糖、血浆胰岛素或血浆孕酮浓度的增加,否定了饲喂RPG增加循环孕酮浓度这一假设。血尿素氮(blood urea nitrogen,BUN)是反映机体蛋白质代谢的重要指标,奶畜处于NEB时,氨基酸进入糖异生后BUN含量增多。李妍等[53]研究表明,添加300和400 g·d-1的RPG(纯度≥98%,乙基纤维素包被技术,过瘤胃率57.42%)可使BUN含量在产后维持在较低水平,并在产后第7天与对照组差异极显著,表明RPG可能改善了围产期的蛋白质动员,减少了生糖氨基酸的转化。而刘骞等[51]研究发现,在泌乳早期奶牛日粮中添加不同含量RPG(纯度≥90%,微胶囊技术)对BUN无显著影响,但BUN水平有随着RPG含量升高而降低的趋势。此外,研究表明RPG也可能通过提高粗蛋白摄入量增加奶牛乳汁中尿素氮的含量[8]

代谢组学分析技术目前广泛应用于动物健康评估、疾病诊断、生物产品表征等各方面研究[54]。血清代谢组学分析[55]表明,奶牛日粮添加350 g·d-1 RPG(纯度45%,脂肪包被技术,过瘤胃率54.03%)后,差异最大的前30个代谢产物包括溶血磷脂酰乙醇胺和脂肪醇等都与脂质代谢有关,表明RPG有降低脂肪分解的潜力。值得注意的是,RPG降低脂肪分解效果与使用剂量密切相关[56]。肝是机体糖代谢、脂代谢、氨基酸代谢的枢纽,因此对肝进行代谢组学分析获得的指标数据具有重要参考价值。对肝组织进行代谢组学分析发现,添加200 g·d-1的RPG(纯度45%,脂肪包被技术)后,β-D-葡萄糖-6-磷酸水平升高,核黄素、黄素单核苷酸、L-色氨酸和L-丝氨酸水平降低,对差异代谢物进行富集分析共得到18种显著富集的代谢通路,其中包括核黄素代谢通路、葡萄糖代谢通路、氨基酸代谢通路,这表明添加RPG可能提高肝的代谢强度,但RPG影响这些差异代谢物及相关通路的机制尚不清楚[57]。同时也有研究表明,添加200 g·d-1 RPG(纯度45%,脂肪包被技术)会导致奶牛IR更加严重,引起更高的循环NEFA含量并提高肝TG浓度,且肝中与脂肪分解功能相关的差异表达蛋白涉及到TCA循环等代谢通路[58-59]。这提示添加RPG可能增加围产期奶畜肝代谢紊乱的风险。造成RPG作用效果差异的因素是多方面的,不仅与RPG的添加剂量有关,也与奶畜的机体状况和试验条件有很大关系。

2.2 RPG对氧化应激、免疫和炎症的调控

围产期奶畜氧化应激水平升高、免疫能力下降、炎性疾病多发,这三者存在密切且复杂的联系。简单来说,NEB引起机体脂肪组织分解加速,导致机体氧化应激水平升高并伴随着围产期特征性的低血钙症和酮症,造成了机体的免疫抑制,并进一步诱发乳腺炎、子宫内膜炎等疾病[60-61]。免疫抑制以及炎症反应并非绝对病理性的,同时也是围产期奶畜正常生理反应的一部分。在围产期,脂肪组织也可能对环境因素做出反应,高温易发的热应激便是其表现形式之一,这可能加剧奶畜的不良反应[62]。奶畜能否成功渡过围产期与环境因素和遗传因素都有关系[2],对围产期奶畜进行营养调控时需要综合考量。

添加RPG被认为可通过改善围产期奶畜的能量代谢进而对应激、免疫和炎症反应产生影响。PI3K/AKT/mTOR通路是免疫细胞激活和炎症平衡的关键调控通路,对于机体炎症的评估具有重要参考价值[63]。胰岛素样生长因子(insulin-like growth factor,IGF)系统包括IGF和IGF结合蛋白(insulin growth factor binding proteins,IGFBPs),其中IGF1[64]和IGF2[65]对细胞分裂、分化和组织修复具有重要作用。Wang等[66]研究发现,200 g·d-1RPG(纯度45%,脂肪包被技术)可能促进IGF1和IGF2与受体结合并激活AKT/mTOR通路,促进子宫内膜修复。患有子宫内膜炎的奶畜血清白蛋白含量更低,且血清白蛋白和球蛋白的比值与奶畜的子宫内膜炎的发生存在负相关关系[67]。研究发现,添加200 g·d-1 RPG可提高围产期奶牛血清白蛋白和球蛋白的比值,其中白蛋白的含量增高,球蛋白含量降低[59]。以上研究都表明了RPG对预防围产后期子宫内膜炎具有重要意义。炎症生物标志物是衡量机体炎性反应水平的重要指标,RPG可降低围产期奶牛产后血清白细胞介素-8、白细胞介素-17A、干扰素γ、脂多糖结合蛋白和触珠蛋白等炎症生物标志物的含量[9, 59, 68]。基质金属蛋白酶(matrix metalloproteinases,MMP)和Toll样受体(Toll-like receptors,TLR)与机体的炎症反应和免疫机能有关[69]。Zhang等[9, 70]研究表明,200 g·d-1 RPG可下调盲肠黏膜MMP1、MMP3, MMP9、MMP13、TLR4、TLR6和TLR7的基因表达以及回肠黏膜TLR4、TLR9等的基因表达,同时提高肠上皮细胞紧密连接蛋白Occludin的基因表达。由此可见,RPG可能通过抑制促炎信号通路和改善屏障功能增强肠道黏膜的免疫活性,抑制炎症的发生。在严啊妮等[71]的研究中,添加10 g·d-1 RPG(纯度≥50%,过瘤胃率≥80%)可显著提高绵羊血清免疫球蛋白G和免疫球蛋白M的含量,显著降低血清中肿瘤坏死因子-α和白细胞介素-2的含量及动物呼吸频率,表明低剂量RPG可提高反刍动物的免疫能力并缓解热应激的不良反应,为预防夏季热应激加剧围产期奶畜的不良反应提供了借鉴。值得关注的是,添加200 g·d-1 RPG(纯度45%,脂肪包被技术)可使肝中丙二醛浓度和谷胱甘肽过氧化物酶活性升高,过氧化氢酶和超氧化物歧化酶活性也有升高趋势,表明RPG可能增加肝的氧化应激水平[58]

2.3 RPG对产奶量和乳成分的影响

MG是乳糖合成的前体,而乳糖是乳汁合成的渗透压调节剂,泌乳所需的MG远超其他生命功能的能量消耗,干奶期向泌乳期转变时MG不足正是导致围产期奶畜NEB的重要原因之一[72]。NEB造成的肝机能损害如脂肪肝对奶畜的产奶量影响重大。如果添加RPG可有效增加围产期奶畜MG水平,对提高奶畜产奶量以及改善乳成分可能有积极作用。但是对于奶畜而言,当添加RPG提高了其产奶量时,意味着肝将代谢更多的脂肪酸和氨基酸以满足机体对乳脂和乳蛋白的需求,从而有可能导致肝的脂质蓄积,增加肝组织负担[58-59]。RPG可提高围产后期奶牛产奶量,但对于乳糖、乳蛋白、乳脂的比例增加效果轻微,且目前对于RPG的最佳添加量还存在争议,但综合考虑200~400 g·d-1为宜[47, 52-53, 59, 73-74]。此外,Benak等[56]、McCarthy等[68]和Sauls-Hiesterman等[8]的研究表明,RPG并没有对奶牛的产奶量和乳成分产生显著影响,这可能与试验条件、饲喂时期和泌乳潜力有关。对于泌乳中后期、日粮淀粉含量较高或奶畜生产性能低下等情况,因MG需要量低且自身MG供应充足,补充RPG的作用可能不明显。

2.4 RPG的其他影响

以静脉输液或真胃灌注等非自然方式增加外源葡萄糖可能会对奶畜造成一定的机体损伤并降低其DMI,但RPG没有类似的效应[59, 68]。RPG还能在一定程度上降低奶畜在围产期后期的体重损失[53]。瘤胃微生物区系对于机体健康具有重要意义并很大程度上受到日粮的影响[75]。过瘤胃保护技术虽然能减轻瘤胃微生物对葡萄糖的影响,但一般而言RPG的瘤胃保护率在55%左右,所以添加RPG可能对瘤胃微生物区系产生影响。Wang等[76]报道,日粮添加350 g·d-1 RPG(纯度45%,脂肪包被技术,过瘤胃率54.03%)可显著提高瘤胃细菌的多样性和有益菌的丰度,促进了瘤胃内脂肪代谢和有机酸的生成。但这同样提示着如果RPG添加过量有可能造成瘤胃pH急剧下降,引起瘤胃酸中毒[77]

2.5 RPG生物利用率评价

目前对各类营养物质进行过瘤胃保护常采取的物理加工方式有加压加热处理、颗粒技术和包被技术,化学方法包括试剂处理、氨基酸类似物和衍生物以及氨基酸金属离子螯合法[78]。在实际生产中常用物理加压法和包被技术对葡萄糖进行处理制成RPG。过瘤胃率是评价RPG生物利用率的关键指标。薛倩等[79]研究表明,与聚丙烯酸树脂和壳聚糖相比采用乙基纤维素包被葡萄糖可有效降低葡萄糖在瘤胃内的降解率并保障葡萄糖能够在小肠内释放,过瘤胃率可达57.42%,而聚丙烯酸树脂和壳聚糖分别为45.5%和44.14%。目前仍缺乏对RPG过瘤胃率的系统性研究,但随着过瘤胃技术的不断发展,RPG的生物利用率有望得到进一步提高。

3 小结与展望

围产期奶畜承受着巨大的生理代谢压力,极易发生能量代谢紊乱并引发一系列代谢性和炎性疾病。围产期健康调控的成功与否直接影响着奶畜繁殖机能和生产性能的发挥,对奶业高质量发展影响深远。RPG能够高效地向围产期奶畜提供外源葡萄糖,缓解由于妊娠和泌乳对MG的过度消耗而引发的代谢紊乱,对脂肪动员和蛋白质代谢具有一定的改善作用。越来越多的研究表明,RPG对改善围产期奶畜的能量代谢、减轻应激和炎性反应、提高产奶量等具有重要作用。适宜的剂量对RPG发挥作用具有重要影响。RPG添加量过低可能导致调控效果并不显著,剂量过高也可能造成瘤胃酸中毒以及酮体的升高。目前,主流的RPG产品有两种:一种是由45%的葡萄糖通过脂肪包被技术制成,另一种是由微胶囊技术制成,其含糖量变异较大。对于脂肪包被的RPG产品,250~350 g·d-1可能是比较适宜的添加量。此外,RPG与其他添加剂配合使用能否能取得更好的调控效果,是一个将来值得关注的问题。RPG对于围产期奶畜的影响是全面而又深刻的,相信随着生物技术的发展以及相关研究的深入,围产期奶畜的营养调控措施将会更加科学合理。

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