Dietary requirements of “nutritionally non-essential amino acids” by animals and humans
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Amino acids are necessary for the survival, growth, development, reproduction and health of all organisms. They were traditionally classified as nutritionally essential or non-essential for mammals, birds and fish based on nitrogen balance or growth. It was assumed that all “non-essential amino acids (NEAA)” were synthesized sufficiently in the body to meet the needs for maximal growth and health. However, there has been no compelling experimental evidence to support this assumption over the past century. NEAA (e.g., glutamine, glutamate, proline, glycine and arginine) play important roles in regulating gene expression, cell signaling, antioxidative responses, neurotransmission, and immunity. Additionally, glutamate, glutamine and aspartate are major metabolic fuels for the small intestine to maintain its digestive function and protect its mucosal integrity. Therefore, based on new research findings, NEAA should be taken into consideration in revising the classical “ideal protein” concept and formulating balanced diets to improve protein accretion, food efficiency, and health in animals and humans.
KeywordsAmino acids Food efficiency Health Metabolism Nutrition
AMP-activated protein kinase
Nutritionally essential amino acids
Eukaryotic translation initiation factor 4E-binding protein-1
Mechanistic target of rapamycin
Nutritionally non-essential amino acids
National Research Council
Adequate provision of dietary amino acids (AA) is essential for the health, growth, development and survival of animals and humans (Ren et al. 2012; Wu 2009). Based on growth or nitrogen balance, AA have traditionally been classified as nutritionally essential (indispensable) or non-essential (dispensable) for mammals, birds and fish (Le Ple’nier et al. 2012); Liu et al. 2012; Obayashi et al. 2012). Nutritionally essential AA (EAA) are defined as either those AA whose carbon skeletons cannot be synthesized de novo in animal cells or those that normally are insufficiently synthesized de novo by the animal organism relative to its needs for maintenance, growth, development, and health and which must be provided in the diet to meet the requirements (Wu 2010). In contrast, nutritionally non-essential AA (NEAA) are those AA which can be synthesized de novo in adequate amounts by the animal organism to meet the requirements for maintenance, growth, development and health and, therefore, need not be provided in the diet. To date, there is no compelling evidence for sufficient synthesis of all AA that are currently not classified as EAA in animal nutrition. Clearly, the terms EAA and NEAA are only the matters of definitions. However, guided by the concept of “nutritional non-essentiality”, nutritionists have long ignored NEAA in dietary formulation. The major objective of this article is to bring, into readers’attention, the conceptual and practical limitations of the NEAA definition and to propose the needs for dietary NEAA by animals and humans.
Physiological roles of NEAA in nutrition
Second, NEAA regulate the synthesis of nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S), which participate in gaseous signaling in cells through cGMP and cAMP production to enhance blood flow, nutrient transport, protein deposition, and immunity (Li et al. 2009). In particular, glycine is the precursor of heme, oxidation of which yields CO, whereas cysteine is the substrate for H2S generation. NO is synthesized from arginine by one of the three isoforms of NO synthase (NOS): neuronal NOS (nNOS also known as NOS1), inducible NOS (iNOS also known as NOS2), and endothelial NOS (eNOS also known as NOS3) (Oess et al. 2006). Many NEAA have been reported to regulate the production of NO, CO, and H2S in a cell-dependent manner (Li et al. 2009; Wu and Meininger 2002). For example, arginine, citrulline, glutamate, glycine, taurine and γ-aminobutyrate increase NO synthesis by constitutive NOS in endothelial cells or brain, whereas glutamine inhibits NO generation by both constitutive and inducible NOSs (Flynn et al. 2005). Furthermore, arginine, glutamine, glutamate, alanine, taurine, and glycine promotes CO synthesis by heme oxygenase in endothelial cells and non-vascular tissues, but N-acetyl-cysteine attenuates CO formation in injured brain and vascular smooth muscle cells (Li et al. 2009). Thus, inadequate production of NO, CO and H2S will impair whole-body insulin sensitivity and the efficiency of nutrient utilization for protein accretion.
Third, NEAA (e.g., arginine, glutamate, glutamine, and proline) participate in cell signaling pathways involving MTOR, cAMP-dependent kinases, cGMP-dependent kinases, G-protein coupled receptors, AMPK, mitogen-activated protein kinases (MAPK) (Chiu et al. 2012; Ray et al. 2012; Rhoads et al. 2000, 2008; Wu et al. 2011a). These are complex networks of metabolic regulation, but they are all regulated by protein phosphorylation and protein dephosphorylation mechanisms (Wu 2010). These pathways also involve activation of various proteins in the cytoplasm and the nucleus to regulate cellular processes, including (1) gene expression; (2) nutrient metabolism; (3) cell proliferation, differentiation, and migration; (4) mitosis and cell survival; (5) cell cycle progression; (6) cell survival and apoptosis; and (7) inflammatory responses (Clemmensen et al. 2012; Go et al. 2012; Zhou et al. 2012).
Fourth, NEAA are substrates for the synthesis of many nitrogenous substances with important functions (Wu 2009). Some of these bioactive molecules include carnosine, creatine, glutathione, neurotransmitters, polyamines, taurine, and low-molecular-weight hormones (del Favero et al. 2012; Ito et al. 2012; Jung et al. 2012; Peters et al. 2012). They are all essential for the growth, lactation, reproduction, health, and survival of animals, including humans. Examples include: (1) nutrient absorption and metabolism (e.g., nutrient transport, protein turnover, fat synthesis and oxidation, glucose synthesis and oxidation, AA synthesis and oxidation, urea and uric synthesis for ammonia detoxification, and efficiency of food utilization); (2) regulation of endothelial cell function, blood flow, lymph circulation, as well as immune function and health (e.g., T cell proliferation and B cell maturation, antibody production by B-cells, killing of pathogens, obesity, diabetes, and metabolic syndrome); (3) spermatogenesis, male fertility, ovulation, ovarian steroidogenesis, embryonic implantation and survival, placental angiogenesis and growth, fetal growth and development, and lactogenesis); (4) acid–base balance, neurotransmission, extracellular and intracellular osmolarity, antioxidative defense, and whole-body homeostasis; (5) post-natal survival, growth and development, and (6) the development, regeneration and remodeling of tissues, including brown adipose tissue and the vasculature (Brosnan and Brosnan 2010; Kimura 2010; Satterfield et al. 2011, 2012; Wu et al. 2012).
Fifth, NEAA can regulate the utilization of dietary protein by bacteria in the lumen of small intestine, thereby affecting its nutritive value in animals (Dai et al. 2011). Analysis of AA composition and the incorporation of AA into polypeptides has shown that protein synthesis is a major pathway for AA metabolism in all the porcine intestinal lumen bacteria studied (Dai et al. 2010). Of particular interest, arginine and glutamine play roles in modulating AA metabolism by intestinal microbes. For example, arginine increases the net utilization of threonine, glycine, phenylalanine, and branched-chain AA by Streptococcus sp. and Klebsiella sp., while decreasing the net utilization of lysine, threonine, isoleucine, leucine, glycine and alanine by jejunal or ileal mixed bacteria (Dai et al. 2012a). Furthermore, glutamine reduces net utilization of asparagine, lysine, leucine, valine, ornithine and serine by jejunal or ileal mixed bacteria (Dai et al. 2012b, c). These results have important implications for developing new means to formulate diets for animals.
Dietary requirements of NEAA
Deficiency of NEAA limits tissue protein synthesis and growth of piglets (25–39 days) fed a low-protein diet
Dietary protein content
12.7 % + EAAa
Protein synthesis (%/day)
Feed intake (FI; g/day)
Body-weight gain (g/day)
Feed:Gain ratio (g/g)
The fact that some AA can be synthesized in the body at the expense of considerable amounts of energy speaks highly for their physiological importance (Reeds 2000). Therefore, pathways for their de novo syntheses have evolved or have been highly conserved in the body. Likewise, all NEAA undergo metabolic transformations and have crucial physiological functions. For example, the unusually high concentration (~1 mM) of glycine in the plasma of post-natal pigs has an important role in stimulating rapid growth (Flynn et al. 2000), and the abundance of arginine (up to 6 mM) in porcine allantoic fluid during early gestation promotes placental growth (including placental angiogenesis) and fetal development (Wu et al. 2006). Thus, there is compelling evidence that an inadequate supply of NEAA in the diet impairs growth and production performance of swine (Wu et al. 2010, 2011b). For example, results of recent studies indicate that (1) diets must contain sufficient amounts of arginine and glutamine to support optimal fetal, neonatal and post-weaning growth in pigs (Kim and Wu 2004, 2009; Wu et al. 2004, 2010, 2011b); and (2) dietary supplementation with proline (Kirchgessner et al. 1995; Wu et al. 2011a) or glutamate (Rezaei et al. 2012) enhances growth performance and feed efficiency of early-weaned pigs. Based on the whole-body oxidation of phenylalanine fed a milk protein-based diet, Ball et al. (1986) have suggested that proline is an EAA for young pigs.
Recommendations of dietary NEAA requirements for swine
NEAA and criteria
% of diet (as-fed basis; typical corn- and soybean meal-based diet)
Composition of AA in the bodies of animals
mg AA/G protein
Asp + Asn
Glu + Gln
Pro + OH-Pro
Based on the composition of EAA in animals (primarily the carcass), the “ideal protein” concept (optimal proportions and amounts of EAA) has been developed for poultry and swine (Baker 2009; Kim et al. 2001). The common features shared by these “ideal protein” models included (1) all protein EAA that are not synthesized by the animals; (2) several AA (cystine, glutamate, glycine, proline, and tyrosine) that are synthesized by animals to various extents; and (3) no data on alanine, aspartate, asparagine, glutamine, or serine. Because the content of proline plus hydroxyproline in the body was not determined, the relatively small amount of proline in the recommended ideal protein was only arbitrarily set and may limit maximal growth of animals. In contrast, very large amounts of glutamate (e.g., 13 times the lysine value) were used to presumably provide the entire need for “non-specific AA N”. However, it was unknown whether glutamate fulfilled this role and whether excess glutamate might interfere with the transport, metabolism and utilization of other AA in the body. In light of the new notion on AA nutrition developed herein, we propose that the “ideal protein” include both EAA and NEAA in correct amounts and proportions (Fig. 2).
The traditional classification of AA as EAA or NEAA has major conceptual limitations in protein nutrition. It is unfortunate that dietary requirements of NEAA have not been recommended for animals and humans. However, emerging evidence shows that traditionally classified NEAA, particularly glutamine, glutamate, proline, and arginine, play important roles in regulating gene expression at both transcriptional and translational levels. There is also growing recognition that these NEAA participate in cell signaling via MTOR, AMPK, MAPK, and gases (NO, CO and H2S). Exquisite integration of these regulatory networks has profound effects on cell proliferation, differentiation, metabolism, homeostasis, survival, and function. Thus, the classic concept of “the ideal protein” should include both EAA and NEAA to improve food efficiency, growth, and health of mammals (including humans), birds, and fish.
Work in our laboratories was supported by National Research Initiative Competitive Grants from the Animal Reproduction Program (2008-35203-19120) and Animal Growth and Nutrient Utilization Program (2008-35206-18764) of the USDA National Institute of Food and Agriculture, AHA (10GRNT4480020), Texas A&M AgriLife Research (H-8200), the National Natural Science Foundation of China (no. u0731001, 30810103902, 30928018, 30972156, 31172217 and 31272450), China Postdoctoral Science Foundation (2012T50163), Chinese Universities Scientific Funds (No. 2012RC024), and the Thousand-People Talent program at China Agricultural University. Important contributions of our graduate students and colleagues to the recent development of the field are gratefully appreciated.
Conflict of interest
The authors declare that they have no conflict of interests.
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