畜牧兽医学报  2024, Vol. 55 Issue (1): 31-38. DOI: 10.11843/j.issn.0366-6964.2024.01.004    PDF    
家禽支链氨基酸营养需要研究进展
牟湘钰, 徐云若, 胡静怡, 周欣妍, 朱勇文     
华南农业大学动物科学学院, 广州 510642
摘要:支链氨基酸(branched-chain amino acids,BCAA)在家禽生长、生产性能、免疫功能和肠道健康等方面发挥着重要作用,涉及器官发育、免疫反应、肌肉蛋白周转和基因调控等关键过程。然而,单一BCAA的过量、缺乏或比例失衡均会影响家禽蛋白质合成和肠道健康,因此保持BCAA比例的平衡对于家禽的生产性能至关重要。尽管NRC(1994)提供了肉鸡、蛋鸡和种鸡的BCAA推荐量,但不同日粮蛋白质水平以及不同品种和生长阶段的家禽对BCAA的需求存在差异。同时,鉴于抗生素生长促进剂的禁用及养殖端所面临的多种疾病的挑战,也需要重新评估家禽对BCAA的推荐摄取水平。另外,BCAA在维持肠道完整性、调控肠道微生物组成以及提高机体蛋白周转效率的作用机理研究仍有待进一步探究。本综述在阐述BCAA在家禽中的营养生理作用以及其对生产和健康的基础上,推荐了不同饲养条件下不同肉鸡品种(肉鸡、蛋鸡和种鸡)BCAA需要量,以期为家禽低蛋白日粮BCAA推荐量提供理论参考。
关键词家禽    支链氨基酸    生产性能    免疫功能    肠道健康    
Advances in Research on the Nutritional Requirements of Branched-Chain Amino Acids in Poultry
MU Xiangyu, XU Yunruo, HU Jingyi, ZHOU Xinyan, ZHU Yongwen     
College of Animal Science, South China Agricultural University, Guangzhou 510642, China
Abstract: Branched-chain amino acids (BCAA) play important roles in poultry growth, production performance, immune function and intestinal health. They are involved in key processes such as organ development, immune response, muscle protein turnover and gene regulation. However, an excess, deficiency or imbalance of a single BCAA can affect poultry protein synthesis and intestinal health. Therefore, maintaining a balanced ratio of BCAA is critical for poultry performance. Although the NRC (1994) provides recommended amounts of BCAA for broilers, laying hens, and breeders, there are variations in BCAA requirements among poultry breeds, dietary protein levels, and growth stages. Meanwhile, the ban on antibiotic growth promoters and the challenges of multiple diseases in poultry farming necessitate a re-evaluation of the recommended levels of BCAA intake. Additionally, more research is needed to explore the mechanisms of action of BCAA in maintaining intestinal integrity and regulating gut microbial composition and enhancing protein turnover efficiency in the body. This review aims to describe the nutritional and physiological roles of BCAA in poultry, as well as their effects on production and health. And also provides BCAA requirements for different broiler breeds (broilers, laying hens and breeders) under various feeding conditions. These recommendations aim to provide theoretical references for the recommended levels of BCAA in low-protein diets for poultry.
Key words: poultry    branched-chain amino acids    productive performance    immune function    intestinal health    

蛋白质是促进家禽生长发育、提高生产性能的关键营养物质。近年来,蛋白质资源的短缺以及家禽摄入过量蛋白质对环境造成的负面影响已成为制约家禽养殖业发展的重要因素[1]。因此,开发低蛋白(crude protein,CP)饲粮至关重要。目前低CP饲粮的开发主要针对赖氨酸(lysine,Lys)、蛋氨酸(methionine,Met)和苏氨酸(threonine,Thr)等限制性必需氨基酸,而关于支链氨基酸(branched-chain amino acids,BCAA)的研究相对较少。一方面,Lys和Met等合成氨基酸的添加容易导致BCAA的缺乏和比例失衡;另一方面,由于三种BCAA结构相似,容易引起代谢竞争和对酶降解的干扰。在亮氨酸含量通常较高的低CP饲粮中,这种情况进一步加剧了BCAA比例失衡,从而对家禽的生长产生负面影响[2]。因此,在研究低CP饲粮时,应特别关注BCAA在家禽生长中发挥的作用及适宜的添加比例。

基于此,本文综述了BCAA的营养生理作用及其对家禽生产性能、产蛋性能、免疫功能及肠道健康的影响,并根据NRC(1994)提供的家禽BCAA推荐量探究其在肉鸡、蛋鸡和种鸡中的不同需求。基于现有BCAA需要量的相关数据,本文还指出了目前存在的研究空白,为重新评估疾病挑战下和不同饲养条件下的BCAA需要量提供了研究方向。

1 BCAA的营养生理作用

家禽的生长、生产、免疫、酶反应和组织周转等多种活动都需要蛋白质的参与。BCAA是指分子结构中侧链具有分支结构的三种必需氨基酸,分别为亮氨酸(leucine, Leu)、异亮氨酸(isoleucine, Ile)和缬氨酸(valine, Val);其中,Leu为2-氨-4-甲基戊酸,Ile为2-氨基-3-甲基戊酸,Val为2-氨基-3-甲基丁酸。三种BCAA结构相似,与支链脂肪酸一样携带疏水侧链。BCAA在支链氨基酸转氨酶(branched-chain aminotransferase, BCAT)的作用下初步降解,后经支链ɑ-酮酸脱氢酶(branched-chain ɑ-keto acid dehydrogenase complex, BCKD)作用完全降解,BCKD同时可导致辅酶A复合物的不可逆分解。该酶促反应往往由其中一种BCAA触发(通常是Leu),最终导致其他BCAA被降解[3]

尽管家禽高蛋白饲粮含有更多限制性氨基酸可满足其生长早期的营养需求,但BCAA的生物学利用率尚不清楚。此外,家禽饲粮中任一BCAA水平过高或过低均会影响其他两种的吸收与利用。例如,饲粮中Leu过量和Ile不足均会阻碍雏鸡的生长,同时导致BCAA比例失衡,不利于其他BCAA的利用;特别是在低CP饲粮中添加Leu会增加其与Ile和Val的拮抗作用,并加剧Ile和Val的降解,阻碍家禽生长[4]。然而,在满足饲粮中Ile和Val最低需求时,过量Leu不会表现出不利影响[5]。在玉米-豆粕型饲粮中,除Lys、Met和Thr三种限制性氨基酸外,BCAA也可能成为限制性因素,但目前仍无法确定将哪一种氨基酸作为家禽饲粮的第四限制性氨基酸[6]。相较于Ile,Val在Leu参与的反应中更容易出现拮抗反应和酶降解,目前可确定Val是玉米-豆粕型饲粮的限制性氨基酸,因此低CP饲粮通常会额外添加Val以满足家禽所需;而Val水平过高(0.82%)并不会对Leu与Ile的利用产生较大影响[7]

尽管BCAA在调节免疫功能和肠道健康上的重要性已得到普遍认可,但目前有关家禽靶向免疫、微生物群和肠道健康方面的研究应用十分有限。Konashi等[8]研究表明,低BCAA饲粮显著降低了10~24日龄雄性科宝肉鸡胸腺和法氏囊的重量,说明BCAA在淋巴器官发育和免疫机能中起着关键作用。有研究发现BCAA可以通过调节靶组织基因表达和蛋白酶信号转导影响家禽的免疫功能及肠道健康[9]。而相较于其他BCAA,Leu能够激活mTOR通路、上调蛋白酶以增强家禽的免疫功能[10]。此外,禽肠道菌群也会对BCAA的消化吸收产生影响。因此,在确定饲粮BCAA的最佳比例之前,有必要评估家禽肠道菌群多样性对BCAA需要量的影响。

2 BCAA对肉禽生产性能的影响

NRC(1994)提供的BCAA推荐需要量仅适用于2~6周龄的肉鸡,且该推荐量是在最低CP水平饲粮下得出的。而随着新品系的培育和饲养模式的变化,家禽的营养需求也在不断发生变化[11-12]。BCAA比例失衡会导致多余的部分被用于能量而非蛋白质的合成,而目前家禽BCAA的最佳比例尚未明确。

任何一种BCAA的过量与缺乏均会导致BCAA比例失衡,进而对肉鸡的生产性能产生不利影响。Leu促进蛋白质合成的能力最强,但Leu过量会降低血浆中Ile和Val的浓度。Chen等[13]研究表明,将饲粮Leu水平由1.88%提高到2.73%(控制Ile和Val比例分别为Leu的59%和69%),可显著提高1~21日龄肉仔鸡末重、采食量、饲料利用率和胸肌率。Erwan等[14]研究表明,饲粮中添加0.5%Leu可显著提高3~6周龄科宝肉鸡胴体重并减少脂肪沉积,同时发现口服包被型Leu(每kg体重6 mmol·10 mL-1)能够显著降低7日龄肉仔鸡血浆Ile和Val水平。一些研究者认为,饲粮配方中Leu的最佳水平应根据饲料原料类型来确定[15-16]。在Ile水平方面,蒋守群等[17]研究发现,随着饲粮Ile从0.55%增加到0.66%,22~42日龄黄羽肉鸡平均日增重和饲料利用率显著提高。Kidd等[18]发现,低Ile(饲粮Ile水平为4.2 g·kg-1)会显著降低雄性罗斯肉鸡胸肌率;而在0.51%Ile的玉米-豆粕型饲粮中继续添加额外的50%Ile可提高3~8周龄雄性肉鸡生产性能。Park和Austic[19]通过分段回归分析确定Ile在肉仔鸡饲粮中的添加量应为0.63%~0.65%(占CP水平的3.28%~3.38%),且添加其他氨基酸(Lys、Arg除外)会加剧Ile的失衡。因此,在满足主要限制性氨基酸需要量的基础上,也要适当考虑Ile的需要量及BCAA的比例。在Val水平方面,Maynard等[20]研究发现,在高Leu和Ile饲粮中添加Val可显著降低29~42日龄科宝肉鸡羽毛的异常率。因此补充Leu和Ile之前,应先满足Val的需要。从这一角度可认为Val是家禽玉米-豆粕型饲粮的第四限制性氨基酸。Maynard等[21]发现,缺乏Val会导致8~21日龄雌性科宝肉鸡的生产性能下降、羽毛生长异常和腿部畸形。饲粮中Ile和Val比例的改变,同样会使肉鸡的生产性能下降。Corrent和Bartelt[22]研究发现,7~21日龄罗斯肉鸡饲粮Ile∶Lys应为0.7~0.8,Val∶Lys应为0.8左右,Leu∶Lys是可变的,但需保持在1.30左右。Maynard等[23]发现,在花生粕-动物蛋白混合饲粮中添加Ile/Lys(67%~69%)和Val/Lys(75%~78%)显著降低了0~48日龄雄性罗斯肉鸡氮排放并增加胸肉质量。同时有研究发现,在饲粮中添加1 g·kg-1的Ile可显著提高28~42日龄雄性罗斯肉鸡胸肌率;添加1 g·kg-1的Val可显著提高28~42日龄肉鸡平均日增重;而Ile和Val联合补充则会降低0~21日龄肉鸡料重比及腹脂率,并提高42日龄肉鸡末重[24-25]。Miranda等[26]研究发现,在不限制饲粮CP水平的情况下,Val∶Lys和Ile∶Lys分别为0.77和0.67时1~6周龄雄性科宝肉鸡的生产性能显著提升,脂肪沉积减少。Lima等[27]采用折线模型估算30周龄科宝肉鸡的Val和Ile需要量分别为803和708 mg·d-1。在其他BCAA比例组合方面,Ospina-Rojas等[28]研究发现,饲粮1.19%Leu+0.86%Val水平可显著提高21~42日龄雄性科宝肉鸡的采食量和平均日增重;此外,饲粮Leu水平为1.13%时胸肌率最高,Val水平为0.71%时腿肌率最高,因此添加Val的同时应考虑Leu水平,以提高肉鸡的生产性能。Maynard等[23]研究发现,在饲粮低CP(19%)水平下,低Ile(0.78%)和Leu(1.57%)组15~35日龄雌性科宝肉鸡的翅重和采食量明显高于低Val(0.87%)组;而低Ile组15~35日龄雄性科宝肉鸡的料重比则明显低于低Val和Leu组。

关于3种BCAA的组合效果,Pastor等[29]采用非线性回归模型确定了雄性科宝肉鸡在10~20日龄和25~35日龄Lys、Leu、Ile和Val的理想比例分别为100∶94∶55∶65和100∶106∶56∶72。Sakomura等[30]采用线性回归模型估算出在体重代谢基础上23周龄科宝肉鸡维持所需的Ile、Leu及Val分别为每日94、52和155 mg·kg-0.75,在蛋白代谢基础上分别为每日329、172和546 mg·kg-0.75。现有的文献和推荐标准指出,家禽生长所需的BCAA较少,但随着日龄增长,生长后期BCAA与Lys的比值应有所提升[11-12]。但Kop-Bozbay等[31]研究发现,BCAA水平并不会对生长后期科宝肉鸡和16~42日龄罗斯肉鸡的生产性能造成显著影响,这可能是饲粮中BCAA的来源不同造成的。其他特殊家禽方面,Martinez等[32]以及Hanafy和Attia[33]在对日本鹌鹑的研究中发现,饲粮Val水平为6.66 g·kg-1或0.2%可显著提升其采食量和体重。Kop-Bozbay和Ocak[34]研究1~7日龄雏火鸡发现,饲粮Leu∶Ile∶Val为3∶1∶2时,平均日增重和胸肌率显著提高。

3 BCAA对蛋禽产蛋性能的影响

饲粮BCAA能够在家禽肝组织中通过脂肪酸代谢产生卵黄和蛋白[35],在蛋鸡产蛋过程中发挥重要作用。饲粮Ile在不同条件下对蛋鸡日产蛋量和蛋品质产生不同影响。Miller等[36]发现,饲粮0.53%Ile水平就可满足以血粉为主要蛋白来源的单冠白莱航鸡的维持和产蛋需要。Ullah等[37]研究发现,在饲粮Ile含量从0.66%提升至0.72%时可显著提高20~46周龄海兰蛋鸡的日产蛋量和产蛋重。Peganova和Eder[38]研究发现,饲粮Ile水平高于1.0%时产蛋期罗曼蛋鸡的蛋品质降低;在低Val+Leu(0.63%+0.72%)组合中,Ile水平从0.57%提升至1.15%时蛋鸡的日产蛋量降低。也有研究显示,提高Ile水平并未使蛋鸡产蛋性能表现出显著差异[39]。上述研究结果不一致的原因推测可能是不同饲粮CP水平或不同家禽品种对Ile的消化率不同引起的。在Val水平方面,Liaqat等[40]研究表明,在饲粮Val含量从0.81%提升至0.87%可显著提高罗曼蛋鸡的产蛋率和蛋品质。对于其他特殊家禽,Hanafy和Attia[33]研究发现,在饲粮18%CP水平下,0.2%Val可显著提高36~96日龄日本鹌鹑的产蛋率。而Jian等[41]研究发现33~41周龄凤达蛋鸡的产蛋率随着饲粮Val水平升高而升高,而哈氏单位、蛋壳厚度、蛋黄颜色出现下降。相对肉禽,蛋禽以及主要饲料成分中不同的BCAA含量及其可利用率仍有待完善。

4 BCAA对种禽胚胎发育的影响

充分的BCAA营养供给对初生幼禽至关重要。为改善家禽胚胎的生长发育,除了在孵化后提供特定营养外,还可以通过卵内注射为发育中后期家禽胚胎提供足够的营养和生物活性物质。Kita等[42]研究发现,卵内注射1%的Leu和Ile(453 mg Leu和271 mg Ile)可以加速鸡胚的生长,缩短雏鸡的孵化时间。Chowdhury等[43]研究发现,卵内注射500 μL含34.5 μmol Leu的无菌水显著提高了出雏后肉鸡初生重及热应激期间的热耐受性。Han等[44]研究发现,与注射Ile或Val相比,在第7天向胚胎卵黄囊注射500 μL含35 μmol Leu的无菌水显著降低了出雏时雏鸡的体温并提高其5日龄的体重;而注射500 μL含70 μmol Leu的无菌水显著提高了鸡胚血浆甲状腺素水平并显著降低出雏10日龄时发生热应激的雄性雏鸡直肠温度。崔洋洋等[45]在卵内注射9.06 mg Leu,显著改善了慢性热应激雌性罗斯肉鸡的十二指肠绒毛高度和回肠隐窝深度。上述研究均表明,卵内注射Leu可以提高雏鸡的耐热性。Bhanja和Mandal[46]研究发现,卵内注射4~7 mg BCAA可显著提高鸡胚第1周的胚重。Kop-Bozbay和Ocak[47]研究发现,将0.2%的Leu、Ile和Val以3∶2∶1的比例混合注入鸡胚羊膜腔,鸡胚孵化率显著降低,但鸡胚的健雏率与初生重显著提高。

5 BCAA对家禽免疫功能的影响

家禽肝组织中的BCAA主要由Val刺激粒状淋巴细胞和无粒白细胞产生,参与自然杀伤细胞的增殖[48]。若BCAA添加充足,家禽黏膜会加速分泌免疫球蛋白A,减少固有层的致病性增殖。而作为白细胞、IL-12等促炎细胞因子和树突状细胞功能的动力来源,BCAA还在促进先天性和适应性免疫反应中发挥重要作用[49]。Sartori等[50]研究发现,添加BCAA可促进小鼠间充质细胞增殖,显著下调p-NF-κB/NF-κB并上调p-STAT-3/STAT-3 mRNA表达,同时减少IL-6和TNF-α并增加抗炎介质水平,证明BCAA具有合成代谢和免疫调节的作用。董小英等[51]研究表明,在饲粮低CP水平(17%)下,添加BCAA会显著降低仔猪血浆尿素和上皮内淋巴细胞数量并增加十二指肠绒毛长度和免疫球蛋白A数量。Prates等[52]研究发现,在25 mg·kg-1低脂多糖(lipopolysaccharide, LPS)水平下,额外补充0.3%的BCAA、Arg和Cys,可部分逆转断奶仔猪因LPS应激而增加的炎症反应及激素变化产生的应激生物标志物。在肉鸡方面,Liu等[53]研究发现,在19胚龄鸡胚肠细胞中添加Leu可观察到NF-κB磷酸化并下调炎症反应。Hale等[54]研究发现,饲粮低Ile水平(0.42%)显著降低了30~42日龄雌性罗斯肉鸡的胸腺重量和CD8+T细胞数量。Konashi等[8]研究发现,饲粮低BCAA水平(13.5 g·kg-1 Leu、8 g·kg-1 Ile和8.2 g·kg-1 Val)显著降低了10~24日龄雄性科宝肉鸡胸腺和法氏囊的重量,说明BCAA在淋巴器官发育和免疫功能中起着关键作用。

6 BCAA对家禽肠道发育和菌群组成的影响

为了更好地吸收养分,家禽需要增加肠绒毛高度以产生更大的吸收表面积。有研究表明,饲粮1.37%~2.17%Leu水平可显著提高AA肉鸡空肠和回肠中的绒毛高度及十二指肠、空肠和回肠中的绒毛高度与隐窝深度的比[55]。Liu等[56]研究发现,在饲粮中添加400 mg·kg-1 Ile可显著增加初生AA肉鸡空肠和回肠绒毛高度。Allameh和Toghyani[57]在低CP饲粮中补充可消化Val(9.9 g·kg-1),罗斯肉鸡空肠和回肠的绒毛高度及杯状细胞的数量显著增加。BCAA可在转氨过程中为Glu和Asp等氨基酸的合成提供氨基,而上述两种氨基酸被认为是小肠黏膜细胞内蛋白质周转和营养物质运输的主要原料[58]。有研究表明,若BCAA比例不平衡,补充Ile不会对仔猪肠细胞的生长和增殖起促进作用[59]。常银莲[60]发现,上调饲粮BCAA水平可激活mTOR信号通路以促进初生AA肉鸡肠道蛋白质的合成,从而促进肠道上皮细胞的增殖,增加肠绒毛高度。

目前,BCAA对肠道菌群的具体影响仍不明确,有待进一步研究。van der Wielen等[61]研究发现,从31日龄科宝肉鸡盲肠中分离出的乳酸发酵菌(菌株G17T)、丙酸梭菌(93<5%)及新丙酸梭菌(93<5%)相似,且当底物中存在BCAA时生长平缓。Jian等[41]发现随着饲粮Val水平的提高,33周龄罗曼蛋鸡盲肠FusobacteriumAeriscardoviaAnaerobiospirillumAerococcusCorynebacteriumCampylobacter的相对丰度显著降低,OribacteriumFrisingicoccus的相对丰度显著增加。Liu等[56]发现,饲喂800 mg·kg-1 Ile饲粮时,初生AA肉鸡盲肠在门水平上的Firmicutes和Bacteroidota的相对丰度显著降低,Proteobacteria和Cyanobacteria的相对丰度显著增加。上述结果显示,添加BCAA可能对家禽肠道菌群普遍存在有益影响,从而促进家禽的生长。

7 小结

BCAA作为蛋白质合成的关键调控因子之一,对于促进肌肉生长发育和提高免疫机能起着至关重要的作用。近年来,更多研究表明BCAA在维持家禽肠道完整性、调控营养转运蛋白和肠道微生物上具有重要作用,但对其具体作用机制仍有待明确,特别在家禽低蛋白饲粮配方中,应根据基础饲粮CP含量、饲料原料种类及其BCAA含量和利用率,精准预测BCAA适宜需要量及其比例。在现有禁抗条件下,未来的研究应根据家禽采食量变化、肠道健康、应激水平和免疫状态来精准调整适宜的可消化BCAA的需要量,以实现家禽饲料节本增效。

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