Glutamate (Glu) and glutamine (Gln) comprise a large proportion of total amino acids (AAs) in fish in the free and protein-bound forms. Both Glu and Gln are synthesized de novo from other α-amino acids and ammonia. Although these two AAs had long been considered as nutritionally non-essential AAs for an aquatic animal, they must be included adequately in its diet to support optimal health (particularly intestinal health) and maximal growth. In research on fish nutrition, Glu has been used frequently as an isonitrogenous control on the basis of the assumption that this AA has no nutritional or physiological function. In addition, purified diets used for feeding fish generally lack glutamine. As functional AAs, Glu and Gln are major metabolic fuels for tissues of fish (including the intestine, liver, kidneys, and skeletal muscle), and play important roles not only in protein synthesis but also in glutathione synthesis and anti-oxidative reactions. The universality of Glu and Gln as abundant intracellular AAs depends on their enormous versatility in metabolism. Dietary supplementation with Glu and Gln to farmed fish can improve their growth performance, intestinal development, innate and adaptive immune responses, skeletal muscle development and fillet quality, ammonia removal, and the endocrine status. Glu (mainly as monosodium glutamate), glutamine, or AminoGut (a mixture of Glu and Gln) is a promising feed additive to reduce the use of fishmeal, while gaining the profitability of global aquaculture production. Thus, the concept of dietary requirements of fish for Glu and Gln is a paradigm shift in the nutrition of aquatic animals (including fish).

Nutrition and metabolism of glutamate and glutamine in fish

As a functional amino acid (AA), L-arginine (Arg) serves not only as a building block of protein but also as an essential substrate for the synthesis of nitric oxide (NO), creatine, polyamines, homoarginine, and agmatine in mammals (including humans). NO (a major vasodilator) increases blood flow to tissues. Arg and its metabolites play important roles in metabolism and physiology. Arg is required to maintain the urea cycle in the active state to detoxify ammonia. This AA also activates cellular mechanistic target of rapamycin (MTOR) and focal adhesion kinase cell signaling pathways in mammals, thereby stimulating protein synthesis, inhibiting autophagy and proteolysis, enhancing cell migration and wound healing, promoting spermatogenesis and sperm quality, improving conceptus survival and growth, and augmenting the production of milk proteins. Although Arg is formed de novo from glutamine/glutamate and proline in humans, these synthetic pathways do not provide sufficient Arg in infants or adults. Thus, humans and other animals do have dietary needs of Arg for optimal growth, development, lactation, and fertility. Much evidence shows that oral administration of Arg within the physiological range can confer health benefits to both men and women by increasing NO synthesis and thus blood flow in tissues (e.g., skeletal muscle and the corpora cavernosa of the penis). NO is a vasodilator, a neurotransmitter, a regulator of nutrient metabolism, and a killer of bacteria, fungi, parasites, and viruses [including coronaviruses, such as SARS-CoV and SARS-CoV-2 (the virus causing COVID-19). Thus, Arg supplementation can enhance immunity, anti-infectious, and anti-oxidative responses, fertility, wound healing, ammonia detoxification, nutrient digestion and absorption, lean tissue mass, and brown adipose tissue development; ameliorate metabolic syndromes (including dyslipidemia, obesity, diabetes, and hypertension); and treat individuals with erectile dysfunction, sickle cell disease, muscular dystrophy, and pre-eclampsia.

Role of L-Arginine in Nitric Oxide Synthesis and Health in Humans.

The liver plays a central role in amino acid (AA) metabolism in humans and other animals. In all mammals, this organ synthesizes many AAs (including glutamate, glutamine, alanine, aspartate, asparagine, glycine, serine, and homoarginine), glucose, and glutathione (a major antioxidant). Similar biochemical reactions occur in the liver of birds except for those for arginine and glutamine hydrolysis, proline oxidation, and gluconeogenesis from AAs. In contrast to mammals and birds, the liver of fish has high rates of glutamate and glutamine oxidation for ATP production. In most animals (except for cats and possibly some of the other carnivores), the liver produces taurine from methionine or cysteine. However, the activity of this pathway is limited in human infants (particularly preterm infants) and is also low in adult humans as compared with rats, birds and livestock species (e.g., pigs, cattle and sheep). The liver exhibits metabolic zonation and intracellular compartmentation for ureagenesis, uric acid synthesis, and gluconeogenesis, as well as AA degradation and syntheses. Capitalizing on these extensive bases of knowledge, dietary supplementation with functional AAs (e.g., methionine, N-acetylcysteine, and glycine) to humans and other animals can alleviate or prevent oxidative stress and damage in the liver. Because liver diseases are common problems in humans and farm animals (including fish), much research is warranted to further both basic and applied research on hepatic AA metabolism and functions.

Amino Acid Metabolism in the Liver: Nutritional and Physiological Significance.

Protein accretion in some fish species is affected by dietary lipids, starch and their interactions, but this aspect of nutrition is largely unknown in largemouth bass (LMB). Therefore, we designed six experimental diets with three starch levels (5%, 10%, and 15%; dry matter  basis) and two lipid levels (10% and 12.5%; dry matter basis) to evaluate the effects of dietary starch and lipid levels on the protein retention, growth, feed utilization, and liver histology of LMB. There were three tanks (18 fish per tank, ~ 4.85 g per fish) per dietary treatment group and the trial lasted for 8 weeks. Fish were fed to apparent satiation twice daily. Results indicated that increasing the dietary starch level from 5 to 15% reduced (P < 0.05) absolute feed intake (AFI; − 9.0%, − 15% and − 14% on days 14–28, 28–42, and 42–56, respectively) and weight gains (− 4.4% and − 6.5% on days 42 and 56, respectively) of LMB. Increasing the dietary lipid level from 10 to 12.5% reduced (P < 0.05) AFI (− 9.7%, − 11.7% and − 11.9% on days 14–28, 28–42; and 42–56, respectively), weight gains (− 4.2%, − 5.9% and − 6.9% on days 28, 42 and 56, respectively), and survival rate (by a 5.6% unit) of LMB. The retention of dietary protein and some amino acids in the body was affected by dietary starch or lipid levels and their interactions. The viscerosomatic index (VSI), hepatosomatic index (HSI), and intraperitoneal fat ratio (IPFR) increased with increasing the dietary starch level from 5 to 15%. Compared with 10% lipids, 12.5% lipids in diets increased IPFR but had no effect on VSI or HSI. The concentrations of glucose in serum increased with increasing the dietary starch level from 5 to 15% at 4 to 24 h after feeding, with the effect of dietary lipids being time-dependent. Compared with a 5%-starch diet, fish fed a diet with 10%- or 15%-starch exhibited an enlarged and pale liver with excessive glycogen. Based on these findings, we recommend dietary lipid and starch levels to be 10% and < 10%, respectively, for juvenile LMB to maximize the retention of dietary protein in their bodies.

Effects of dietary starch and lipid levels on the protein retention and growth of largemouth bass ( Micropterus salmoides )

Aquaculture is increasingly important for providing humans with high-quality animal protein to improve growth, development and health. Farm-raised fish and shellfish now exceed captured fisheries for foods. More than 70% of the production cost is dependent on the supply of compound feeds. A public debate or concern over aquaculture is its environmental sustainability as many fish species have high requirements for dietary protein and fishmeal. Protein or amino acids (AAs), which are the major component of tissue growth, are generally the most expensive nutrients in animal production and, therefore, are crucial for aquatic feed development. There is compelling evidence that an adequate supply of both traditionally classified nutritionally essential amino acids (EAAs) and non-essential amino acids (NEAAs) in diets improve the growth, development and production performance of aquatic animals (e.g., larval metamorphosis). The processes for the utilization of dietary AAs or protein utilization by animals include digestion, absorption and metabolism. The digestibility and bioavailability of AAs should be carefully evaluated because feed production processes and AA degradation in the gut affect the amounts of dietary AAs that enter the blood circulation. Absorbed AAs are utilized for the syntheses of protein, peptides, AAs, and other metabolites (including nucleotides); biological oxidation and ATP production; gluconeogenesis and lipogenesis; and the regulation of acid-base balance, anti-oxidative reactions, and immune responses. Fish producers usually focus on the content or digestibility of dietary crude protein without considering the supply of AAs in the diet. In experiments involving dietary supplementation with AAs, inappropriate AAs (e.g., glycine and glutamate) are often used as the isonitrogenous control. At present, limited knowledge is available about either the cell- and tissue-specific metabolism of AAs or the effects of feed processing methods on the digestion and utilization of AAs in different fish species. These issues should be addressed to develop environment-friendly aquafeeds and reduce feed costs to sustain the global aquaculture.

Nutrition and Functions of Amino Acids in Fish.

We analyzed the composition of amino acids (AAs) in oligopeptides, proteins, and the free pool, as well as creatine, agmatine, polyamines, carnosine, anserine, and glutathione, in animal- and plant-derived feedstuffs. Ingredients of animal origins were black soldier fly larvae meal (BSFM), chicken by-product meal, chicken visceral digest, feather meal, Menhaden fishmeal, Peruvian anchovy fishmeal, Southeast Asian fishmeal, spray-dried peptone from enzymes-treated porcine mucosal tissues, poultry by-product meal (pet-food grade), spray-dried poultry plasma, and spray-dried egg product. Ingredients of plant origins were algae spirulina meal, soybean meal, and soy protein concentrate. All animal-derived feedstuffs contained large amounts of all proteinogenic AAs (particularly glycine, proline, glutamate, leucine, lysine, and arginine) and key nonproteinogenic AAs (taurine and 4-hydroxyproline), as well as significant amounts of agmatine, polyamines, creatine, creatinine, creatine phosphate, and glutathione. These nitrogenous substances are essential to either DNA and protein syntheses in cells or energy metabolism in tissues (particularly the brain and skeletal muscle). Of note, chicken by-product meal, poultry by-product meal, and spray-dried poultry plasma contained large amounts of carnosine and anserine (potent antioxidants). Compared with most of the animal-derived feedstuffs, plant-derived feedstuffs contained much lower contents of glycine and proline, little 4-hydroxyproline, and no creatine, creatinine, creatine phosphate, carnosine or anserine. These results indicate the unique importance of animal-source feedstuffs in improving the feed efficiency, growth and health of animals (including fish and companion animals). Because soy protein concentrate is consumed by infants, children and adults, as are BSFM and algae for children and adults, our findings also have important implications for human nutrition.

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Peng Li

Papers

Composition of amino acids and related nitrogenous nutrients in feedstuffs for animal diets.