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Definition: amino acid from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Amino acids are natural organic compounds that make up proteins and can thus be considered the basic molecules of life. There are 20 different common amino acids. They consist mainly of carbon, oxygen, hydrogen, and nitrogen. Each amino acid has a common core structure (consisting of two carbon atoms, two oxygen atoms, a nitrogen atom, and four hydrogen atoms) to which is attached a variable group, known as the R group. In glycine, the R group is a single hydrogen atom; in alanine, the R group consists of a carbon and three hydrogen atoms (methyl group).

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organic chemistry

amino acid

Summary Article: Amino Acids
From The Gale Encyclopedia of Nutrition and Food Labels

Amino acids are small molecules that have a variety of functions in the human body, including forming the linear chains that make up proteins and participating in many different metabolic pathways. Some of the 20 common amino acids can be made by the body, but others must be obtained from foods.


L-amino acids are the building blocks of proteins— the major structural and functional components of every cell in the body. Amino acids are also required for the synthesis of nucleic acids (DNA and RNA), hormones, vitamins, and other essential molecules. They have additional roles in the following:

  • energy supply

  • gene regulation

  • health and immune-system function maintenance

  • growth and development

  • metabolism regulation

  • endocrine regulation

  • acid-base balance maintenance

  • cell signaling

  • blood flow

  • reproduction

  • lactation

  • anti-oxidation

  • stress responses

  • appetite control

  • skin pigmentation

  • behavior

  • protein breakdown

  • nitrogen (ammonia) movement in and removal from the body

Free amino acids—individual amino acids in cells and body fluids that are not incorporated into protein—account for only a very small proportion of the body's total amino-acid content, but they have essential roles in nutrition and metabolism. Amino acids can be oxidized (broken down) to smaller molecules to supply energy to cells. When dietary carbohydrates (sugars) are limited, certain amino acids are converted into glucose for fuel in a process called gluconeogenesis. During prolonged exertion, amino acids provide 3–6% of the body's energy requirements; when carbohydrates are limited, they may provide as much as 10%. Because extended food deprivation leads to extensive breakdown of muscle protein for gluconeogenesis, dietary amino acids and protein are essential for minimizing degradation of muscle protein.

Some amino acids regulate the activity (expression) of specific genes. Several amino acids regulate gene activity and physiologic functions by modifying DNA and proteins—a process called epigenetic regulation. Because some epigenetic modifications are passed on to future generations, amino acids can affect genetic inheritance. Amino acids also have roles in the regulation of the transcription (copying) of DNA into RNA and in the translation of RNA into protein.

The amino acids glutamine and arginine are sometimes called immunonutrients because of their important roles in immune-system function. Glutamine—the most abundant free amino acid in the body—helps regulate the immune system, is a major fuel source for immune-system cells, and is essential for the production of glutathione that protects cells from oxidative damage. Glutamine is a nonessential amino acid (NEAA)— meaning that it can be produced by the body—and most healthy people obtain all the extra glutamine they need from their diets. However, disease or injury, as well as cancer treatments such as chemotherapy and radiation therapy, can cause physiologic stress characterized by glutamine deficiency. Thus, glutamine supplementation may be necessary during recovery from disease or injury. It also may help protect normal tissues during cancer treatment and increase the susceptibility of cancer cells to the treatment.


Amino acids—free and in peptides (short amino-acid chains) and proteins—along with carbohydrates and fats are macronutrients required by the body in large amounts. The human body can make the 11 NEAA via carbohydrate metabolism. These are also called dispensable amino acids. Some NEAA, such as glutamine, are considered conditional amino acids because under certain conditions such as illness or other forms of stress or in premature infants, the body cannot synthesize adequate amounts, and these amino acids must be supplied by the diet or supplements. The other nine common amino acids—essential amino acids (EAA) or indispensable amino acids—cannot be made by the body and must be obtained from food. During digestion, proteins in food are broken down into their constituent amino acids, which are absorbed by the bloodstream and delivered to cells for the synthesis of new proteins and other functions. Amino acids are also obtained from the normal breakdown and turnover of the body's own proteins. Amino-acid and protein deficiencies affect all of the organs and many body systems, especially the immune system, gastrointestinal functions (including absorption), kidney function, and brain function in infants and young children.

EAA include:

  • histidine

  • isoleucine

  • leucine

  • lysine

  • methionine

  • phenylalanine

  • threonine

  • tryptophan

  • valine

NEAA include:

  • alanine

  • asparagine

  • aspartic acid (aspartate)

  • glutamic acid (glutamate)

  • serine

Conditional EAA or conditionally indispensable amino acids include:

  • arginine

  • cysteine

  • glutamine

  • glycine

  • proline

  • tyrosine

Some amino acids, including arginine, glutamate, and glutamine, are considered functional amino acids (FAA) because, in addition to their incorporation into proteins, they play essential roles in the regulation of important metabolic pathways that are necessary for growth and development, reproduction, lactation, and health and survival. FAA can be either EAA or NEAA, but their dietary inclusion or supplementation can help prevent conditions such as obesity, diabetes, and fetal growth retardation.


When diets are lacking in the correct amounts of various amino acids, the body's ability to use protein is adversely affected. Protein quality is determined by the protein digestibility-corrected amino acid score (PDCAAS), which is based both on human amino-acid requirements and the body's ability to digest the protein. In addition to good-quality protein, nonprotein energy from carbohydrates and fats are required to prevent protein-energy malnutrition (PEM). Worldwide, PEM is associated with approximately six million childhood deaths every year. PEM is also fairly common among adults in many parts of the world. In industrialized countries, PEM occurs primarily among the elderly and ill or hospitalized patients.

Recommended intakes
Protein requirements

Dietary protein requirements are based on the nitrogen balance provided by amino acids. The Food and Nutrition Board of the U.S. Institute of Medicine (IOM), Health Canada, the World Health Organization (WHO), and the European Union (EU) all recommend about 0.4 g of good-quality protein per pound of body weight (0.8 g/kg) daily for healthy adults, including vegetarians. Vegans who eat no animal products and older adults may benefit from about 0.5 g/lb. (1.0 g/kg). WHO also recommends additional protein for pregnant and lactating women.

Protein turnover is the continuous breaking down and resynthesizing of proteins that occurs in most cells of the body. In a steady state, protein breakdown equals protein synthesis, but daily protein turnover is higher in infants and lower in the elderly compared with young adults. Inadequate intake of protein or of an EAA results in decreased protein synthesis, but protein degradation continues in order to supply the required amino acids. An EAA dietary deficiency limits the utilization of other amino acids and prevents normal rates of protein synthesis, even with adequate total nitrogen intake; thus, the limiting amino acid determines the nutritional value of a protein.

The EAA composition of a protein source is compared with a reference amino-acid composition to determine its PDCAAS. Protein quality is defined as a protein's capacity for meeting the body's nitrogen and amino-acid requirements for growth, maintenance, and repair and its digestibility—the type and amount of amino acids that the protein makes available for the body to use.

EAA requirements

EAA requirements are based on additional studies and corrected for the digestibility of the protein. Adult daily amino-acid requirements according to the IOM and WHO, respectively, are:

  • histidine, 24 mg/lb. of body weight, 10 mg/kg

  • isoleucine, 9 mg/lb., 20 mg/kg

  • leucine, 19 mg/lb., 39 mg/kg

  • lysine, 17 mg/lb., 30 mg/kg

  • methionine plus cysteine (sulfur-containing amino acids), 9 mg/lb., 15 mg/kg

  • phenylalanine plus tyrosine, 15 mg/lb., 25 mg/kg

  • threonine, 9 mg/lb., 15 mg/kg

  • tryptophan, 2 mg/lb., 4 mg/kg

  • valine, 11 mg/lb., 26 mg/kg

EAA requirements per gram of protein consumed according to the IOM and WHO, respectively, are:

  • histidine, 18 mg/g of protein, 15 mg/g

  • isoleucine, 25 mg/g, 20 mg/g

  • leucine, 55 mg/g, 59 mg/g

  • lysine, 55 mg/g, 45 mg/g

  • methionine plus cysteine, 25 mg/g, 22 mg/g

  • phenylalanine plus tyrosine, 47 mg/g, 38 mg/g

  • threonine, 27 mg/g, 23 mg/g

  • tryptophan, 7 mg/g, 6 mg/g

  • valine, 32 mg/g, 39 mg/g

There is no evidence that high amino-acid intake from protein in food poses any risk, so no tolerable upper intake level (UL) has been established for amino acids. However, little is known about possible adverse effects from high levels of amino acids in dietary supplements. Therefore, caution is advised when consuming any individual amino acid at a significantly higher level than is normally present in food. Although excess amino acids are excreted in the urine without being absorbed by the body, adverse effects could include:

  • kidney damage

  • worsening liver disease from excess glutamine

  • allergic reactions to supplemental amino acids such as glutamine

  • increased sensitivity to amino acids in elderly people

  • unknown effects of supplementation during pregnancy or lactation

Special circumstances

Because some plant proteins are lower in the EAA cysteine, lysine, methionine, and threonine, vegetarians and vegans who restrict their diets to plant-based foods may be deficient in these amino acids. Diets that contain complementary mixtures of plant proteins provide the same protein quality as diets that include animal proteins.

Certain amino acids are the primary concern with the inherited metabolic disorders maple syrup urine disease (MSUD) and phenylketonuria (PKU). MSUD is the most common disorder affecting the metabolism of the branched-chain amino acids (BCAA)—isoleucine, leucine, and valine. In MSUD, BCAA, especially leucine, are not properly metabolized, leading to high levels in the blood. Failure to severely restrict intake of BCAA in people with MSUD can lead to intellectual disabilities and death. PKU causes phenylalanine or its breakdown byproducts to build up in the blood, leading to brain damage, growth retardation, and skin abnormalities beginning in infancy or childhood. Dietary phenylalanine must be restricted from the first month of life and continued at least through childhood and adolescence.

Intake sources

The amino-acid composition of a protein is its most important nutritional characteristic. Most animal proteins from meat, poultry, fish, eggs, and dairy products are considered complete, because they provide all nine EAA in the required ratios. For example, egg protein has a high PDCAAS because it contains a perfect balance of all nine EAA and is easily digestible. Plant proteins from legumes, whole grains, nuts, seeds, vegetables, and fruits are incomplete, because they lack or contain insufficient quantities of one or more EAA. However, soybeans, quinoa, and spinach are all considered high-quality proteins with large amounts of EAA. Soy protein most closely resembles milk protein and contains all nine EAA; in fact, soy can function as a sole protein source. All of the EAA need not be included in a single meal, since they can be stored in the liver; it is preferable if EAA intake is balanced over the course of the day.

An adult male can obtain the recommended amounts of EAA from any of the following:

  • eight large potatoes

  • 2.5 cups (0.6 L) tofu

  • 12.75 cups (3 L) cooked corn

  • 15.5 cups (3.7 L) cooked brown rice

Combining different plant foods creates complementary proteins that provide all of the EAA. For example:

  • Beans and other legumes and vegetables that are low in methionine can be complimented with grains, nuts, or seeds.

  • Grains, which are low in lysine and threonine, are complimented by legumes.

  • Nuts and seeds that are low in lysine are complimented by legumes.

  • Corn, which has limited lysine and tryptophan, can be complemented with legumes.

Role in population health

Many foods are fortified with amino acids, and amino-acid supplements are marketed aggressively to physically active people. Although most of these supplements are considered safe at the recommended dosages, there is little or no evidence that they improve muscle mass or strength. Most athletes and bodybuilders obtain the small amount of additional amino acids they require through protein in foods. Furthermore, some high-protein energy bars contain incomplete proteins such as gelatin or collagen.

There have been numerous claims that BCAA supplementation improves mental state and promotes muscle growth and recovery in athletes. BCAA do not appear to reduce muscle fatigue, and the European Food Safety Authority has rejected exercise-related health claims for BCAA. Furthermore, although there is some evidence that BCAA promote immune function, high plasma levels are associated with obesity, cardiometabolic risk, and markers for insulin resistance.

Importance to population health

Adequate intake of amino acids is essential for growth, development, and health. Most people in developed countries meet or exceed the Dietary Reference Intake (DRIs) for EAA and protein. This includes people on plant-based diets with more than 50% of all protein from plant sources, lacto-ovo vegetarians who consume only milk and egg products from animal sources, and vegans who consume no animal products. In contrast, adequate amino-acid intake is a concern for many populations in developing countries.

Legislation and regulation

In the United States, amino acids are regulated as dietary supplements rather than foods. The European Scientific Committee for Food was unable to conclude whether the amino-acid levels in energy drinks were safe or unsafe; however, health claims for amino acids in energy drinks are not permitted by the EU.

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  • Margaret Alic, PhD
    Copyright © 2017 Gale Cengage Learning

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