阿拉伯木聚糖(arabinoxylan，AX)具有高黏度、高持水性等特征。从20世纪80年代起，小麦AX逐步引起世界范围内的广泛关注。动物营养研究者发现小麦中的非淀粉多糖(non-starch polysaccharides，NSP)的主要成分是AX，其含量与小麦的代谢能值呈负相关，是小麦饲粮中主要的抗营养因子，从而开展了广泛地降低其抗营养作用的研究。而食品研究者发现小麦AX可作为食品的增稠剂、稳定剂，对面团和面包品质有重要的影响。人类营养学家更是发现小麦AX有许多重要的生理活性，如降低血清胆固醇水平、调节血糖水平[1]、益生作用[2]、抗氧化[3, 4]、免疫调节[5]和抗肿瘤[6]等。在小麦育种界，AX已成为除蛋白质和淀粉之外最受关注的一种小麦组分。AX在小麦中的含量并不高，但作为一种次要组分能够在农业育种、谷物化学、食品加工、膳食营养中备受重视，与其独特的结构、性质有直接关系。全国每年有2 000万t小麦麸皮资源可供利用，其中蕴藏着400多万t的AX，小麦活性多糖的开发将为小麦麸皮的增值转化提供科学依据。本文重点介绍国内外近年来有关AX及其水解产物，特别是阿拉伯木聚寡糖(arabinoxylan oligosaccharide，AXOS)益生功能的研究进展。
AX是构成植物细胞壁的主要成分，广泛存在于小麦等谷物的种皮中，AX的基本结构中包括由β-D-吡喃木糖残基经β-1，4-糖苷键连接而成的木聚糖主链和α-L-呋喃阿拉伯糖基(arabinose，Ara)侧链，β-D-吡喃木糖残基可在C2或/和C3位被α-L-呋喃阿拉伯糖基单取代或者双取代。
AX主要由阿拉伯糖(arabinose，Ara)和木糖(xylose，Xyl)等五碳糖组成，因此过去习惯称其为戊聚糖。小麦中含有50～80 g/kg DM的戊聚糖[7]，是植物中继纤维素之后含量较多的第二大生物多聚体[8]。
在畜牧业生产中，能量饲料占全价饲料组成的60%～70%。在玉米价格较高或供应困难时，在动物生产中往往用小麦替代玉米。与玉米相比，小麦中含有较高的AX，可增加胃肠道食糜黏性，增加消化液的分泌，降低消化酶的活性，增加肠道内微生物数量[9, 10]，影响胃肠道形态结构和蛋白质的内源性分泌，干扰营养物质的消化和重吸收，并最终影响畜禽的生长速度和健康水平[11, 12, 13, 14]。AX对幼龄畜禽的影响更为严重，其是影响小麦应用于畜牧业生产的主要限制因素。在饲粮中添加木聚糖酶则可消除这种抗营养作用，小麦饲粮中添加木聚糖酶能显著提高动物的生长性能、免疫功能和体内激素水平[15, 16]。
研究发现，AX及其水解产物AXOS均具有益生活性。近年来，小麦AX对肠道微生态的调节作用受到世界范围内的广泛关注，爱尔兰、比利时、美国、德国、澳大利亚等国均已大量开展提纯小麦AX/AXOS对人类益生功能的研究，一些公司已开发出小麦AX的商业产品。
小麦AX可以促进动物和人肠道双歧杆菌、乳酸杆菌、真细菌等益生菌的生长[17, 18]，抑制类杆菌(Bacteroides spp.)和梭菌类(Clostridium spp.)的生长[18]，提高益生指数，刺激肠道微生物分泌木聚糖酶和阿魏酸酯酶[19]，促进肠道丁酸[17]和乳酸[20]等短链脂肪酸(short-chain fatty acid，SCFA)的生成。此外，以商品猪作为人类营养的研究模型，发现饲粮中添加高水平(12.9%)的小麦AX可提高盲肠SCFA含量，降低肠道中有害蛋白质分解产物(p-甲酚与苯酚)的含量，缓解结肠细胞DNA损伤，提高结肠中段普雷沃菌属(Prevotella spp.)和梭菌Ⅳ(Clostridial cluster Ⅳ)的含量[21]。
小麦AXOS的益生功能与AX类似，并且效果更强，研究报道更多。体外发酵法证实，人源结肠微生物仅可利用一少部分的AX，但可利用绝大部分的AXOS[22]。小麦AXOS可被一些双歧杆菌、乳酸杆菌和类杆菌利用，不能被大肠杆菌、梭状芽孢杆菌利用[23, 24]。小麦AXOS可提高人肠道双歧杆菌[25, 26]、乳酸杆菌、类杆菌属-普氏菌属(Bacteroides-Prevotella)、梭菌属球菌-直肠真杆菌(Clostridium coccoides-Eubacterium rectale)[27]等益生菌的生长，降低盲肠大肠杆菌、金黄色葡萄球菌和链球菌的数量[28]，改变人体的碳水化合物和蛋白质代谢，降低生物标记氨的经尿排出量，增加经粪的排出量[29]，减少肠道中有害蛋白质分解产物(p-甲酚与苯酚)的含量[25, 26, 27]，提高胃肠健康指数。AXOS可降低攻毒肉鸡盲肠肠炎沙门氏菌群数量及其造成的脾脏转位[30]。
AXOS能刺激微生物，尤其是双歧杆菌的增殖[31, 32]，促进木聚糖酶、木糖苷酶等降解酶的分泌[33]，暗示AXOS作为益生元可能更具优势。
AX和AXOS的功能活性与其分子质量大小、分子结构，尤其是与分子聚合度(avDP)以及侧链取代程度(avDAS)密切相关[21]。随着分子质量的降低，小麦AX促进双歧杆菌和乳酸杆菌增殖的活性也随之增强[17]；对5种不同avDP以及avDAS的AXOS比较发现，较高聚合度(avDP为29)的AXOS可以抵达结肠末端，而较低聚合度(avDP为8)的AXOS则主要在结肠近端发酵[27]。分子聚合度较低(avDP为3)的AXOS可促进肠道双歧杆菌的生长和乙酸、丁酸的产生，但对肠道SCFA的总含量没有影响；分子聚合度较高(avDP为61)的AXOS可降低肠道SCFA含量，但不具有促进双歧杆菌生长的作用[20]。
AX结构中的阿魏酰基团对于AX的许多功能性质有着重要影响。AX和AXOS的抗氧化功能与其分子组成中共价联结的阿魏酸有关[3]。而在碱性的提取条件下，阿魏酰基团很容易被水解下来而影响小麦AX功能的发挥。
近年来，国际上对小麦AX的益生功能进行了大量的研究，这为动物营养学家提供了一些新的视角。虽然动物营养学家针对小麦AX对动物生长性能、胃肠道结构与功能方面已经开展了一定程度的研究，但大多研究是在小麦饲粮水平上进行的，利用分离提纯的小麦AX开展的系统研究较少，且总体仍属于黑箱研究，许多深层次的重要问题仍未能解决，更无法解释小麦饲粮中添加木聚糖酶改善动物的生长性能、免疫功能的机理。因此，借鉴近年来小麦AX，特别是AXOS益生功能的研究，有必要针对性地开展提纯小麦AX/AXOS对畜禽生物学功能的分子构效研究，弥补国内外在小麦AX/AXOS益生功能研究上的空白，为多糖益生功能的研究提供试验证据，对开发新的生物活性多糖具有重要意义。
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