Huntley Nichole F, Patience John F
Department of Animal Science, 213 Kildee Hall, Iowa State University, Ames, 50011 IA USA.
Department of Animal Science, 201B Kildee Hall, Iowa State University, Ames, 50011 IA USA.
J Anim Sci Biotechnol. 2018 Jan 7;9:4. doi: 10.1186/s40104-017-0226-9. eCollection 2018.
Xylose, as β-1,4-linked xylan, makes up much of the hemicellulose in cell walls of cereal carbohydrates fed to pigs. As inclusion of fibrous ingredients in swine diets continues to increase, supplementation of carbohydrases, such as xylanase, is of interest. However, much progress is warranted to achieve consistent enzyme efficacy, including an improved understanding of the utilization and energetic contribution of xylanase hydrolysis product (i.e. xylooligosaccharides or monomeric xylose). This review examines reports on xylose absorption and metabolism in the pig and identifies gaps in this knowledge that are essential to understanding the value of carbohydrase hydrolysis products in the nutrition of the pig. Xylose research in pigs was first reported in 1954, with only sporadic contributions since. Therefore, this review also discusses relevant xylose research in other monogastric species, including humans. In both pigs and poultry, increasing purified D-xylose inclusion generally results in linear decreases in performance, efficiency, and diet digestibility. However, supplementation levels studied thus far have ranged from 5% to 40%, while theoretical xylose release due to xylanase supplementation would be less than 4%. More than 95% of ingested D-xylose disappears before the terminal ileum but mechanisms of absorption have yet to be fully elucidated. Some data support the hypothesis that mechanisms exist to handle low xylose concentrations but become overwhelmed as luminal concentrations increase. Very little is known about xylose metabolic utilization in vertebrates but it is well recognized that a large proportion of dietary xylose appears in the urine and significantly decreases the metabolizable energy available from the diet. Nevertheless, evidence of labeled D-xylose-1-C appearing as expired CO in both humans and guinea pigs suggests that there is potential, although small, for xylose oxidation. It is yet to be determined if pigs develop increased xylose metabolic capacity with increased adaptation time to diets supplemented with xylose or xylanase. Overall, xylose appears to be poorly utilized by the pig, but it is important to consider that only one study has been reported which supplemented D-xylose dietary concentrations lower than 5%. Thus, more comprehensive studies testing xylose metabolic effects at dietary concentrations more relevant to swine nutrition are warranted.
木糖作为β-1,4-连接的木聚糖，构成了猪所采食的谷物碳水化合物细胞壁中大部分的半纤维素。随着猪日粮中纤维性成分添加量的持续增加，补充木聚糖酶等碳水化合物酶变得很有必要。然而，要实现酶的稳定功效仍有很大进展空间，这包括对木聚糖酶水解产物（即低聚木糖或木糖单体）的利用和能量贡献有更深入的了解。本综述考察了关于猪体内木糖吸收和代谢的报告，并找出了这方面知识的空白，这些空白对于理解碳水化合物酶水解产物在猪营养中的价值至关重要。猪的木糖研究最早于1954年报道，此后仅有零星的研究成果。因此，本综述还讨论了其他单胃动物（包括人类）的相关木糖研究。在猪和家禽中，增加纯化D-木糖的添加量通常会导致生产性能、效率和日粮消化率呈线性下降。然而，目前所研究的添加水平在5%至40%之间，而木聚糖酶添加理论上释放的木糖量应低于4%。超过95%摄入的D-木糖在回肠末端之前就消失了，但吸收机制尚未完全阐明。一些数据支持这样的假设：存在处理低木糖浓度的机制，但随着肠腔浓度增加会不堪重负。关于脊椎动物体内木糖的代谢利用知之甚少，但人们普遍认识到，日粮中很大一部分木糖会出现在尿液中，并显著降低日粮的可代谢能量。然而，在人类和豚鼠中，标记的D-木糖-1-C以呼出的二氧化碳形式出现的证据表明，木糖氧化虽可能性小但仍有潜力。尚未确定猪是否会随着对添加木糖或木聚糖酶日粮的适应时间增加而提高木糖代谢能力。总体而言，猪对木糖的利用率似乎很低，但需要考虑的是，仅有一项研究报道了日粮中D-木糖添加浓度低于5%的情况。因此，有必要开展更全面的研究，测试与猪营养更相关的日粮浓度下木糖的代谢效应。
