Amino acids are organic compounds that contain both an amino group (-NH2) and carboxylic acid group (-COOH). When linked via peptide bonds, they function as the basic blocks used to construct a variety of different proteins. Hundreds of amino acids can be found in nature, but only about 20 to 22 are necessary to make the proteins present in the human body.1 Of these, nine are considered essential, meaning that they must be sourced from the diet. The human body can then synthesize the remaining (“non-essential”) amino acids using these nine amino acids.
The nine essential amino acids (EAAs) are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Different foods vary in terms of whether they are good sources of the nine EAAs. Animal-based foods, such as meat and eggs, are considered complete sources of protein because they contain all nine EAAs, while vegetarian sources of protein often must be combined with one another in a meal (e.g., rice and beans) to provide all nine EAAs, although some soy products like edamame and tofu do contain all nine EAAs.
The recommended protein digestibility-corrected amino acid scoring pattern (which is a method of evaluating the quality of a protein based on the amino acid requirements of humans and their ability to digest them) for individuals one year of age or older is as follows (in mg/g of protein): histidine, 18 mg/g; isoleucine, 25 mg/g; leucine, 55 mg/g; lysine, 51 mg/g; methionine + cysteine, 25 mg/g; phenylalanine + tyrosine, 47 mg/g; threonine, 27 mg/g; tryptophan, 7 mg/g; and valine, 32 mg/g.2 Up to 35 percent of the total daily energy intake may be from protein, but no tolerable upper intake level for either total protein or any of the amino acids exists due to insufficient data.2
In the Body
Each of the nine EAAs plays unique roles in the human body. Histidine is biosynthesized into histamine, a chemical released by cells in response to an injury, allergen, or another inflammatory reaction that triggers the contraction of smooth muscle and vasodilation via a vitamin B–dependent decarboxylation reaction by histidine decarboxylase.3 Isoleucine, alongside the other two branched-chain amino acids (BCAAs) leucine and valine, participates in building and repairing muscle, with each having their own effects. One study suggests that isoleucine increases muscle mass by promoting myogenesis and intramyocellular fat deposition.4 Elsewhere, long-term supplementation with leucine improved acquired growth hormone resistance in rats with protein-energy malnutrition,5 and another rat study reported that leucine supplementation accelerated connective tissue repair of the injured tibialis anterior muscle.6 The metabolite of valine, β-aminoisobutyric acid, is also increased by exercise and acts on other tissues, such as white adipose tissue, to enhance energy expenditure.7 Isoleucine may additionally have a hypoglycemic effect by stimulating both glucose uptake in muscle and whole-body glucose oxidation, as well as reducing gluconeogenesis in the liver,8 potentially avoiding hyperglycemia; increased dietary leucine intake may improve glucose and cholesterol metabolism.9 Finally, isoleucine may induce the expression of peptides (i.e., β-defensins) that regulate host innate and adaptive immunity10 and, together with leucine and valine again, shifts the lymphocyte immune response toward a Th1 (pro-inflammatory) type.11
Most of the other EAAs are also involved in immune function, metabolism, and the production of hormones and/or energy. In addition, however, lysine significantly increases the intestinal absorption of calcium and may improve the renal conservation of absorbed calcium,12 while methionine assists with the absorption of zinc.13 Lysine is required in collagen biosynthesis,14 as are other amino acids, such as glycine, of which threonine is a precursor.15 Finally, tryptophan is essential for the synthesis of serotonin,16 and phenylalanine is important for the production of other neurotransmitters, such as dopamine.17
The first step in the process of amino acid absorption is the mechanical breakdown of foods containing protein by the teeth.18 In the stomach, gastric juices containing hydrochloric acid and the enzyme pepsin initiate the breakdown of protein, and the partially digested protein is mechanically churned by stomach contractions into chyme.18 In the small intestine, digestive juice from the pancreas containing enzymes, such as chymotrypsin and trypsin, further breaks down the protein fragments into individual amino acids.18 Further along in the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood, then to the liver, which acts as a checkpoint for amino acid distribution throughout the body.18
Effects of Intake
Inadequate intake of EAAs through the diet, which typically leads to a lack of protein synthesis in the body, can cause a variety of symptoms, including emotional disorders (e.g., depression, anxiety), fatigue, weakness, anemia, loss of libido, increased rates of infections due to immune system impairment, cardiovascular diseases, and osteoporosis.1,19 In children, inadequate EAA intake may disrupt the metabolic process, stunting their growth.20
Under certain circumstances, some amino acids that are not normally essential may become so. For example, glutamine becomes a conditionally EAA under conditions of severe stress to the body, such as critical illness, surgery, or trauma (i.e., when its endogenous use exceeds its endogenous production).21 Arginine may be a conditionally EAA in neonates and infants, where it helps to prevent necrotizing enterocolitis; cysteine is similarly believed to be a conditionally EAA in preterm neonates, who may be unable to convert methionine, its precursor, in the liver.21 By extension, taurine, which requires cysteine as a precursor, may also be conditionally essential in premature neonates.21
Excessive amino acid intake has emerged as a more significant problem in recent years with increased intake of dietary supplements to enhance physical performance or gain other health benefits. Some effects of high amino acid intake, such as nausea (various), are innocuous, but others like hepatic dysfunction (methionine) and alterations in visual acuity and mental state (histidine) can be more serious.22 Certain conditions, such as tyrosinemia II (tyrosine) and phenylketonuria (phenylalanine, may result in abnormally high amino acid levels due to genetic mutations that cause dysfunction of the enzymes required for amino acid processing.22 High levels of certain amino acids in the body may also correlate with disease states, such as the increased concentrations of BCAA found in insulin-deficient and -resistant states, such as diabetes and obesity.23
As Therapeutics
Various studies have suggested therapeutic uses of the amino acids. For example, in a randomized crossover trial of 26 patients with multiple sclerosis, threonine reduced signs of spasticity on clinical examination, without the side or toxic effects associated with other treatments.24 Lysine may reduce the occurrence, severity, and healing time of recurrent herpes simplex virus infections.25 Tryptophan acts in a manner reminiscent of serotonergic antidepressants to induce a positive bias in the processing of emotional material and could benefit people with mild depression.26 In some cases, the abundance of a specific amino acid may have antitumor effects, while, in others, interfering with amino acid availability can selectively kill tumor cells.27 Finally, supplemental dietary administration of BCAAs was demonstrated to extend the average lifespan of middle-aged mice,28 and in yeast the restriction of methionine similarly facilitated an extension of the lifespan.29
Editor’s note: Please consult with a qualified healthcare professional to determine what kind of diet and supplements, if any, are best for you.
Sources
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