Glycine is a major amino acid in mammals and other animals. It is synthesized from serine, threonine, choline, and hydroxyproline via inter-organ metabolism involving primarily the liver and kidneys. Under normal feeding conditions, glycine is not adequately synthesized in birds or in other animals, particularly in a diseased state. Glycine degradation occurs through three pathways: the glycine cleavage system (GCS), serine hydroxymethyltransferase, and conversion to glyoxylate by peroxisomal D-amino acid oxidase. Among these pathways, GCS is the major enzyme to initiate glycine degradation to form ammonia and CO2 in animals. In addition, glycine is utilized for the biosynthesis of glutathione, heme, creatine, nucleic acids, and uric acid. Furthermore, glycine is a significant component of bile acids secreted into the lumen of the small intestine that is necessary for the digestion of dietary fat and the absorption of long-chain fatty acids. Glycine plays an important role in metabolic regulation, anti-oxidative reactions, and neurological function. Thus, this nutrient has been used to: (1) prevent tissue injury; (2) enhance anti-oxidative capacity; (3) promote protein synthesis and wound healing; (4) improve immunity; and (5) treat metabolic disorders in obesity, diabetes, cardiovascular disease, ischemia-reperfusion injuries, cancers, and various inflammatory diseases. These multiple beneficial effects of glycine, coupled with its insufficient de novo synthesis, support the notion that it is a conditionally essential and also a functional amino acid for mammals (including pigs and humans).

Amino acids (AA) have enormous physiological importance, serving as building blocks for proteins and substrates for synthesis of low-molecular-weight substances. Based on growth or nitrogen balance, AA were traditionally classified as nutritionally essential or nonessential for animals. Although those AA that are not synthesized in eukaryotes (nutritionally essential AA, EAA) must be present in animal diets, nutritionally nonessential AA (NEAA) have long been ignored for all species. Emerging evidence shows that nonruminants cannot adequately synthesize NEAA or conditionally essential AA (CEAA) to realize their growth or anti-infection potential. Likewise, all preformed AA are needed for high-producing cows and rapidly growing ruminants. Many NEAA and CEAA (e.g., arginine, glutamine, glutamate, glycine, and proline) and certain EAA (e.g., leucine and tryptophan) participate in cell signaling, gene expression, and metabolic regulation. Thus, functions of AA beyond protein synthesis must be considered in dietary formulations to improve efficiency of nutrient use, growth, development, reproduction, lactation, and well-being in animals.

Bacteria in the gastrointestinal (GI) tract play an important role in the metabolism of dietary substances in the gut and extraintestinal tissues. Amino acids (AA) should be taken into consideration in the development of new strategies to enhance efficiency of nutrient utilization because they are not only major components in the diet and building blocks for protein but also regulate energy and protein homeostasis in organisms. The diversity of the AA-fermenting bacteria and their metabolic redundancy make them easier to survive and interact with their neighboring species or eukaryotic host during transition along GI tract. The outcomes of the interactions have important impacts on gut health and whole-body homeostasis. The AA-derived molecules produced by intestinal bacteria affect host health by regulating either host immunity and cell function or microbial composition and metabolism. Emerging evidence shows that dietary factors, such as protein, non-digestible carbohydrates, probiotics, synbiotics and phytochemicals, modulate AA utilization by gut microorganisms. Interdisciplinary research involving nutritionists and microbiologists is expected to rapidly expand knowledge about crucial roles for AA in gut ecology and host health.

Embryonic loss and intrauterine growth restriction (IUGR) are significant problems in humans and other animals. Results from studies involving pigs and sheep have indicated that limited uterine capacity and placental insufficiency are major factors contributing to suboptimal reproduction in mammals. Our discovery of the unusual abundance of the arginine family of amino acids in porcine and ovine allantoic fluids during early gestation led to the novel hypothesis that arginine plays an important role in conceptus (embryo and extra-embryonic membranes) development. Arginine is metabolized to ornithine, proline, and nitric oxide, with each having important physiological functions. Nitric oxide is a vasodilator and angiogenic factor, whereas ornithine and proline are substrates for uterine and placental synthesis of polyamines that are key regulators of gene expression, protein synthesis, and angiogenesis. Additionally, arginine activates the mechanistic (mammalian) target of rapamycin cell signaling pathway to stimulate protein synthesis in the placenta, uterus, and fetus. Thus, dietary supplementation with 0.83 % L-arginine to gilts consuming 2 kg of a typical gestation diet between either days 14 and 28 or between days 30 and 114 of pregnancy increases the number of live-born piglets and litter birth weight. Similar results have been reported for gestating rats and ewes. In sheep, arginine also stimulates development of fetal brown adipose tissue. Furthermore, oral administration of arginine to women with IUGR has been reported to enhance fetal growth. Collectively, enhancement of uterine as well as placental growth and function through dietary arginine supplementation provides an effective solution to improving embryonic and fetal survival and growth.

Recent years have witnessed growing interest in the role of peptides in animal nutrition. Chemical, enzymatic, or microbial hydrolysis of proteins in animal by-products or plant-source feedstuffs before feeding is an attractive means of generating high-quality small or large peptides that have both nutritional and physiological or regulatory functions in livestock, poultry and fish. These peptides may also be formed from ingested proteins in the gastrointestinal tract, but the types of resultant peptides can vary greatly with the physiological conditions of the animals and the composition of the diets. In the small intestine, large peptides are hydrolyzed to small peptides, which are absorbed into enterocytes faster than free amino acids (AAs) to provide a more balanced pattern of AAs in the blood circulation. Some peptides of plant or animal sources also have antimicrobial, antioxidant, antihypertensive, and immunomodulatory activities. Those peptides which confer biological functions beyond their nutritional value are called bioactive peptides. They are usually 2–20 AA residues in length but may consist of >20 AA residues. Inclusion of some (e.g. 2–8%) animal-protein hydrolysates (e.g., porcine intestine, porcine mucosa, salmon viscera, or poultry tissue hydrolysates) or soybean protein hydrolysates in practical corn- and soybean meal-based diets can ensure desirable rates of growth performance and feed efficiency in weanling pigs, young calves, post-hatching poultry, and fish. Thus, protein hydrolysates hold promise in optimizing the nutrition of domestic and companion animals, as well as their health (particularly gut health) and well-being.