Histidine is an essential amino acid (EAA) in mammals, fish, and poultry. We aim to give an overview of the metabolism and physiological effects of histidine in humans and different animal species through a systematic review following the guidelines of PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses). In humans, dietary histidine may be associated with factors that improve metabolic syndrome and has an effect on ion absorption. In rats, histidine supplementation increases food intake. It also provides neuroprotection at an early stage and could protect against epileptic seizures. In chickens, histidine is particularly important as a limiting factor for carnosine synthesis, which has strong anti-oxidant effects. In fish, dietary histidine may be one of the most important factors in preventing cataracts. In ruminants, histidine is a limiting factor for milk protein synthesis and could be the first limiting AA for growth. In excess, histidine supplementation can be responsible for eating and memory disorders in humans and can induce growth retardation and metabolic dysfunction in most species. To conclude, the requirements for histidine, like for other EAA, have been derived from growth and AA composition in tissues and also have specific metabolic roles depending on species and dietary levels.

Lysine cannot be synthesized by mammals and, as a consequence, is an indispensable amino acid. The main role of lysine is to participate in protein synthesis. The catabolism of lysine is principally located in the liver. Lysine released from digested protein undergoes a significant first-pass metabolism of ∼30 to 42% in humans and piglets. An important question regarding the biological basis of the requirement of lysine is the possible participation of microbial de novo synthesized amino acids in the whole-body fluxes. Recent intake recommendations to meet the lysine requirement range from 64 to 30 mg/(kg · d) for 0.5-y infants and adults (>18 y), respectively. Lysine intake in the Western human diet is in the range 40–180 mg/(kg · d). An upper limit of 300–400 mg/(kg · d) can be considered in humans.

Dietary protein and amino acids, including glutamate, generate signals involved in the control of gastric and intestinal motility, pancreatic secreti

Glutamate (Glu), either as one of the amino acids of protein or in free form, constitutes up to 8–10% of amino acid content in the human diet, with an intake of about 10–20 g/day in adults. In the intestine, postprandial luminal Glu concentrations can be of the order of mM and result in a high intra-mucosal Glu concentration. Glu absorbed from the intestinal lumen is for a large part metabolized by enterocytes in various pathways, including the production of energy to support intestinal motility and functions. Glu is the most important fuel for intestinal tissue, it is involved in gut protein metabolism and is the precursor of different important molecules produced within the intestinal mucosa (2-oxoglutarate, L-alanine, ornithine, arginine, proline, glutathione, γ-aminobutyric acid [GABA]). Studies in adult humans, pigs, piglets or preterm infants indicate that a large proportion of Glu is metabolized in the intestine, and that for the usual range of Glu dietary intake (bound Glu and free Glu including added Glu as a food additive in normal amounts up to 1 g/day), circulating Glu is tightly maintained at rather low concentrations. Systemic blood levels of Glu transiently rise when high doses monosodium glutamate (> 10–12 g), higher than normal human dietary consumption, are ingested and normalize within 2 h after the offset of consumption. Glu is also involved in oral and post oral nutrient chemosensing that involves gustatory nerves and both humoral and neural (vagal) gut-brain pathways with an impact on gut function and feeding behavior. Glu functions as a signaling molecule in the enteric nervous system and modulates neuroendocrine reflexes in the gastrointestinal tract. The oral taste sensation of Glu involves its binding to the oral umami taste receptors that triggers the cephalic phase response of digestion to prepare for food digestion. Glu is sensed again in the gut, inducing a visceral sensation that enhances additional gut digestive processes through the visceral sense (vago-vagal reflex).

Dietary proteins are found in animal products, plant products and single-cell organisms. Proteins are present in variable proportions in these different food sources and the different proteins also differ in their amino acid composition, dietary indispensable amino acid content and physico-chemical properties. Different criteria can be used to define dietary protein requirements and different markers can be used to assess nutritional protein quality according to the criteria used for protein requirement estimation. The current approach to determining protein requirements is related to nitrogen balance and the dietary indispensable amino acid score approach relates protein quality to the capacity of protein to allow reaching nitrogen balance by providing nitrogen and indispensable amino acids. A second approach considers more directly protein nitrogen utilization by the body and includes measurement of protein digestibility and of the efficiency of dietary nitrogen retention at maintenance or for protein deposition at the whole body level or in more specific body areas. Another approach is related to protein turnover and protein synthesis in relation to maintenance and/or efficiency for deposition or development (growth) at the whole body level or for different and more specific target tissues such as muscle or bone. Lastly, protein quality can also be evaluated from different markers used as risk factors for metabolic dysfunction and disorders related for instance to insulin resistance, diabetes and obesity or cardio-vascular disease. The accuracy and relevance of these different approaches is discussed regarding the capacity of the different protein sources (i.e. animal as meat, milk or eggs, legume as soya or pea, or cereal as wheat or rice) to satisfy protein requirements according to these different criteria and markers.