This review explores the potential of including glutamine, a so-called non-essential amino acid, in the formulation of reduced-crude protein (CP) diets for broiler chickens. There is a precedent for benefits when including glycine and serine in reduced-CP diets. Fundamentally this is due to decreases in non-essential amino acid concentrations in reduced-CP diets — an unavoidable consequence of reducing CP without amino acid supplementation. The situation for glutamine is complicated because analysed dietary concentrations are very rarely provided as standard assays do not differentiate between glutamine and glutamate and are reported on a combined basis as glutamic acid. The dietary requirement for glutamic acid is approximately 36.3 g/kg but it is increasingly unlikely that this requirement will be met as dietary CP levels are progressively reduced. Glutamine is an abundant and versatile amino acid and constitutes 50.5 mg/g of whole-body chicken protein and is the dominant free amino acid in systemic plasma where it has been shown to provide 22.6% (139.9 of 620.3 μg/mL) of the total in birds offered 215 g/kg CP, wheat-based diets. In addition to dietary intakes, glutamine biosynthesis is derived mainly from the condensation of glutamate and ammonia (NH3) catalysed by glutamine synthetase, a reaction that is pivotal to NH3 detoxification. Glutamate and NH3 are converted to glutamine by phosphate-dependent glutaminase in the reciprocal reaction; thus, glutamine and glutamate are interchangeable amino acids. However, the rate of glutamine biosynthesis may not be adequate in rapidly growing broiler chickens and exogenous and endogenous glutamine levels are probably insufficient in birds offered reduced-CP diets. The many functional roles of glutamine, including NH3 detoxification and maintenance of acid-base homeostasis, then become relevant. Twenty feeding studies were identified where dietary glutamine supplementation, usually 10 g/kg, was evaluated in birds kept under thermoneutral conditions. On balance, the outcomes were positive, but the average dietary CP was 213 g/kg across the twenty feeding studies, which indicates that CP and, in turn, glutamine concentrations would have been adequate. This suggests that glutamine inclusions in reduced-CP diets hold potential and consideration is given to how this may be best confirmed.

中文翻译：

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在低粗蛋白饮食中添加谷氨酰胺对鸡肉生产的潜力


本综述探讨了在肉鸡低粗蛋白 （CP） 日粮配方中加入谷氨酰胺（一种所谓的非必需氨基酸）的潜力。在低 CP 饮食中加入甘氨酸和丝氨酸是有好处的先例。从根本上说，这是由于低 CP 饮食中非必需氨基酸浓度的降低——这是在没有补充氨基酸的情况下降低 CP 的不可避免的结果。谷氨酰胺的情况很复杂，因为很少提供分析的膳食浓度，因为标准分析不区分谷氨酰胺和谷氨酸，而是以谷氨酸的组合形式报告。谷氨酸的膳食需求量约为 36.3 g/kg，但随着膳食 CP 水平逐渐降低，满足这一需求的可能性越来越小。谷氨酰胺是一种丰富且用途广泛的氨基酸，占 50.5 mg/g 的全身鸡蛋白，是全身血浆中的主要游离氨基酸，已被证明可提供 22.6%（620.3 μg/mL 中的 139.9%）在提供 215 g/kg CP 的鸟类中以小麦为基础的饮食。除了饮食摄入外，谷氨酰胺的生物合成主要来自谷氨酰胺合成酶催化的谷氨酸和氨 （NH3） 的缩合，这种反应对 NH3 解毒至关重要。谷氨酸和 NH3 在互惠反应中被磷酸盐依赖性谷氨酰胺酶转化为谷氨酰胺;因此，谷氨酰胺和谷氨酸是可互换的氨基酸。然而，在快速生长的肉鸡中，谷氨酰胺的生物合成速率可能不够，而在提供低 CP 饮食的鸡中，外源性和内源性谷氨酰胺水平可能不足。 谷氨酰胺的许多功能作用，包括 NH3 解毒和维持酸碱稳态，就变得相关了。确定了 20 项饲喂研究，其中在热中性条件下饲养的鸡中评估了日粮谷氨酰胺补充剂（通常为 10 g/kg）。总的来说，结果是积极的，但在 20 项喂养研究中，平均膳食 CP 为 213 g/kg，这表明 CP 和谷氨酰胺浓度本来是足够的。这表明低 CP 饮食中的谷氨酰胺含量具有潜力，并考虑如何最好地证实这一点。