Department of Animal Sciences, Auburn University, Auburn, AL, USA.
Adv Exp Med Biol. 2021;1285:29-42. doi: 10.1007/978-3-030-54462-1_3.
Proteins have been recognized for a long time as an important dietary nutritional component for all animals. Most amino acids were isolated and characterized in the late nineteenth and early twentieth century. Initially dietary proteins were ranked high to low quality by growth and N balance studies. By the 1950s interest had shifted to studying the roles of individual amino acids in amino acid requirements by feeding studies with non-ruminants as rodents, poultry and pigs. The direct protein feeding approaches followed by measurements of nutritional outcomes were not possible however in ruminants (cattle and sheep). The development of measuring free amino acids by ion exchange chromatography enabled plasma amino acid analysis. It was thought that plasma amino acid profiles were useful in nutritional studies on proteins and amino acids. With non-ruminants, nutritional interpretations of plasma amino acid studies were possible. Unfortunately with beef cattle, protein/amino acid nutritional adequacy or requirements could not be routinely determined with plasma amino acid studies. In dairy cows, however, much valuable understanding was gained from amino acid studies. Concurrently, others studied amino acid transport in ruminant small intestines, the role of peptides in ruminant N metabolism, amino acid catabolism (in the animal) with emphasis on branched-chain amino acid catabolism. In addition, workable methodologies for studying protein turnover in ruminants were developed. By the 1990s, nutritionists could still not determine amino acid requirements with empirical experimental studies in beef cattle. Instead, computer software (expert systems) based on the accumulated knowledge in animal and ruminal amino acids, energy metabolism and protein production were realized and revised frequently. With these tools, the amino acid requirements, daily energy needs, ruminal and total gastrointestinal tract digestion and performance of growing beef cattle could be predicted.
长期以来，蛋白质一直被认为是所有动物重要的膳食营养成分。大多数氨基酸是在 19 世纪末和 20 世纪初被分离和鉴定的。最初，根据生长和氮平衡研究，膳食蛋白质的质量被分为高到低。到 20 世纪 50 年代，人们的兴趣转向通过非反刍动物（如啮齿动物、家禽和猪）的喂养研究来研究单个氨基酸在氨基酸需求中的作用。然而，在反刍动物（牛和羊）中，直接用蛋白质喂养并测量营养结果的方法是不可行的。通过离子交换色谱法测量游离氨基酸的方法的发展，使得可以进行血浆氨基酸分析。人们认为，血浆氨基酸谱在蛋白质和氨基酸的营养研究中是有用的。对于非反刍动物，可以对血浆氨基酸研究进行营养解释。不幸的是，对于肉牛来说，不能通过血浆氨基酸研究常规确定蛋白质/氨基酸的营养充足性或需求。然而，在奶牛中，从氨基酸研究中获得了很多有价值的认识。与此同时，其他人研究了反刍动物小肠中的氨基酸转运、肽在反刍动物氮代谢中的作用、动物体内的氨基酸分解代谢（重点是支链氨基酸分解代谢）。此外，还开发了用于研究反刍动物蛋白质周转的可行方法。到 20 世纪 90 年代，营养学家仍然不能通过在肉牛中的经验性实验研究来确定氨基酸的需求。相反，基于动物和瘤胃氨基酸、能量代谢和蛋白质生产方面的积累知识的计算机软件（专家系统）被实现，并经常进行修订。有了这些工具，可以预测生长肉牛的氨基酸需求、每日能量需求、瘤胃和整个胃肠道消化以及性能。
