Amino Acids

What are amino acids? Definition and Structure

Amino acids are organic nutrients that appear in foods and in the human body either as building blocks of proteins or as free amino acids.

Amino acids are made of the amino group (NH2), carboxyl group (COOH) and a side chain containing carbon, hydrogen or oxygen; two amino acids (cysteine and methionine) also contain sulfur and one (selenocysteine) contains selenium.

Amino Acid Formula Picture

Picture 1. General amino acid structure:
All amino acids contain the amino and carboxyl group;
it is the side chain that makes amino acids different from each other.

Essential, Conditionally-Essential and Nonessential Amino Acids

21 amino acids can form proteins in the human body; they are called proteinogenic, standard, typical, canonical or natural amino acids.

Chart 1. List of 21 Proteinogenic Amino Acids

AminoAcidNamewith (Abbreviation)


1. Histidine (His)
2. Isoleucine (Ile)
3. Leucine (Leu)
4. Lysine (Lys)
5. Methionine (Met)
6. Phenylalanine (Phe)
7. Threonine (Thr)
8. Tryptophan (Trp)
9. Valine (Val)


The 9 amino acids on the right are essential (vital), which means they are necessary for the human life and health but cannot be produced in your body so you need to get them from foods [1].

10. Arginine (Arg)
11. Cysteine (Cys)
12. Glutamine (Gln)
13. Glycine (Gly)
14. Proline (Pro)
15. Serine (Ser)
16. Tyrosine (Tyr)


These amino acids can be synthesized in your body, but in certain circumstances, like young age, illness or hard exercise, you need to get them in additional amounts from foods to meet the body requirements for them. Ornithine is also considered conditionally essential amino acid, but it does not form proteins [2].

17. Alanine (Ala)
18. Asparagine (Asn)
19. Aspartic acid (Asp)
20. Glutamic acid (Glu)
21. Selenocysteine (Sec)


These amino acids can be synthesized in your body from other amino acids, glucose and fatty acids, so you do not need to get them from foods [44].

Types of Amino Acids

Branched-Chain Amino Acids (BCAAs)

Branched-chain amino acids (BCAAs), which include leucine, isoleucine and valine, are essential amino acids that stimulate protein synthesis in the muscles.

Acidic and Basic Amino Acids

ACIDIC amino acids are aspartic and glutamic acid, and BASIC amino acids are arginine, histidine and lysine [25].

Sulfur-Containing Amino Acids

Sulfur-containing amino acids include cysteine, homocysteine, methionine and taurine [23].

  • Animal foods high in cysteine and methionine: chicken, turkey, fish (bluefish, yellowtail, tuna, salmon), pork (ham) beef, veal, lamb, bison, crabs, mollusks, cheese [20].
  • Plant foods high in cysteine and methionine: nuts (butternuts, peanuts), seeds (pumpkin, sunflower), legumes (beans, soybeans, lentils) [20].
  • Foods high in taurine include red meat and fish [21] and certain energy drinks.
  • Homocysteine is produced in the body during protein breakdown.

In individuals with celiac or Crohn’s disease or other disorders with impaired amino acid absorption, foods high in sulfur-containing amino acids can cause sulfur-smelling gas [24].

Glucogenic and Ketogenic Amino Acids

In the human body, glucogenic amino acids can be converted to glucose in the process called gluconeogenesis; they include all amino acids except lysine and leucine [3].

Ketogenic amino acids, which can be converted to ketones: isoleucine, leucine, lysine, phenylalanine, threonine, thryptophan and tyrosine [3]. Ketones can be used by the brain as a source of energy during fasting or in a low-carbohydrate diet.

Functions of Amino Acids

  • Amino acids are a source of energy; like proteins, they can provide about 4 Calories per gram [41].
  • In the human body, certain amino acids can be converted to other amino acids, proteins, glucose, fatty acids or ketones [42,43].
  • Other functions of amino acids:
    • Chemical messengers (neurotransmitters) in the nervous system: aspartate, GABA, glutamate, glycine, serine
    • Precursors of other neurotransmitters or amino acid-based hormones:
      • Tyrosine is a precursor of dopamine, epinephrine, norepinephrine [44] and thyroxine [48].
      • Tryptophan is a precursor of melatonin and serotonin [44] and nicotinic acid (vitamin B3)
      • Histidine is a precursor of histamine [46].
    • Glycine is a precursor of heme, a part of hemoglobin [45].
    • Aspartate, glutamate and glycine are precursors of nucleic acids, which are parts of DNA [47].

Foods that Contain All 9 Essential Amino Acids

Food protein containing all 9 amino acids in adequate amounts is called complete or high-quality protein.

ANIMAL FOODS with complete protein include liver (chicken, pork, beef), goose, duck, turkey, chicken, lamb, pork, most fish, rabbit, eggs, milk, cheese (cottage, gjetost, cream, swiss, ricotta, limburger, gruyere, gouda, fontina, edam) and certain beef cuts [18]. Animal foods with incomplete protein include certain yogurts and beef cuts.

PLANT FOODS with complete protein include spinach, beans (black, cranberry, french, pink, white, winged, yellow), soy, split peas, chickpeas, chestnuts, pistachios, pumpkin seeds, avocado, potatoes, quinoa, a seaweed spirulina, tofu [18] and hummus [52]. Common plant foods with incomplete protein: rice (white and brown), white bread (including whole-wheat), pasta, beans (adzuki, baked, kidney, lima, pinto, snap), peas, lentils, nuts (walnuts, peanuts, hazelnuts, almonds, coconut), sunflower seeds, kamut.

Foods made of mycoprotein also contain complete protein [50-pp.249,250].

Limiting Amino Acids and Complementary Proteins

A limiting amino acid is an essential amino acid that is present in a certain food in the lowest amount, which prevents protein synthesis in the body beyond the rate at which that amino acid is available. Here’s a short video that describes the principle of the limiting amino acids. For example, the limiting amino acid in cereal grains and nuts is usually lysine, and the ones in legumes are usually methionine and cysteine. You can prevent the limiting effect of amino acids on protein synthesis by eating foods that, in combination, contain all essential amino acids in sufficient amounts.

Limiting amino acid picture

Picture 2. Limiting amino acid
The barrel represents a protein and the staves essential amino acids. On the left, lysine (Lys) is the limiting amino acid that limits the protein synthesis as suggested by the water level. On the right, when lysine is added, protein synthesis can increase to the point allowed by the next limiting amino acid ─ methionine (Met).

Complementary proteins are those that, when taken together, provide all essential amino acids. For example, cereals, which tend to be low in lysine but high in methionine, can be combined with beans, which are high in lysine but low in methionine [17]. According to The American Journal of Clinical Nutrition, complementary proteins do not need to be eaten at the same meal but “over number of days” to provide the adequate synthesis of the body proteins [16].

They are usually only vegans who need to complement food proteins from various plant foods to get all essential amino acids in adequate amounts. Practically all animal foods contain all essential amino acids in adequate amounts, so omnivores usually do not need to think about protein complementation.

Foods LOW in LYSINE [49]:

  • Most cereal grains (barley, bulgur, cornmeal, couscous, kamut, millet, oats, semolina, sorghum, spelt, pasta, pita, rice, teff, triticale, wheat, seitan–plant-based protein derived from wheat gluten [54]), except  amaranth, buckwheat and quinoa.
  • Tapioca
  • Fruits and vegetables

Foods HIGH in LYSINE [30]:

  • Most animal foods: meat, fish, eggs, milk
  • Legumes: most beans (especially soybeans), peas and lentils
  • Nuts: cashews, peanuts, pistachios
  • Seeds: chia, cottonseed, pumpkin, squash

Amino Acid Supplements (Free-Form AA)

Amino acid supplements contain free-form amino acids, which are not bound to each other so they do not need to be digested, so they are absorbed easier than intact protein from food.

Amino acid supplements as “amino acid complex,” “amino acid combination,” “amino blend,” “amino acid chelates” with chromium, magnesium or zinc, amino acids as part of vitamin supplements or as single amino acids are available.

Possible Benefits of Amino Acid Spplements

The use of AA in malabsorption. Individuals who have problem digesting natural proteins from food, for example those with lack of protein-digesting enzymes due to chronic pancreatitis, or those with malabsorption syndromes (short bowel syndrome after partial removal of the small intestine, severe celiac disease [52]), can use oral supplements with mixtures of all 9 essential amino acids as a source of protein [26].

Total parenteral diet (no food by mouth). Patients in hospitals who cannot or should not take food by mouth can get amino acid supplements as the source of proteins by injection into a vein [27].

There is SOME EVIDENCE that branched-chain amino acids (BCAAs) supplements can stimulate muscle recovery and decrease muscle soreness after physical exercise [13].

There is SOME EVIDENCE that in individuals with sleep disorders amino acids hydroxytryptophan (5-HT) and gamma aminobutyric acid (GABA) taken before sleep can reduce time to fall asleep, sleep duration and quality [37].

There is INSUFFICIENT EVIDENCE about the effect of orally taken individual amino acids beta-alanine, arginine, aspartate, branched-chain amino acids, (leucine, isoleucine, valine) glutamine, glycine, lysine, ornithine, tryptophan, tyrosine on muscle power or endurance that would be greater than the effect of natural protein from food [10,11,13,14,15,28].

There is INSUFFICIENT EVIDENCE about the effectiveness of amino acid supplements in the prevention or treatment of mental disorders, such as schizophrenia, bipolar disorders, obsessive compulsive disorders (OCD), attention deficit hyperactivity syndrome (ADHD), autism and major depression [29,36].

Amino acid supplements are NOT LIKELY EFFECTIVE in promoting weight loss.

“Amino Acid Therapy”

The so called “amino acid therapy” refers to the use of supplemental amino acids to help balance chemical messengers (neurotransmitters) in the brain: epinephrine, norepinephrine, dopamine and serotonin. It is used to treat anxiety, depression, memory loss, insomnia, headaches, migraine, chronic pain, attention deficit disorder (ADD), addictive disorders and to promote weight loss. None of these effects have been proven effective, so far.

“Liquid Amino Diet”

Amino acid drops for weight loss are on the market. There is no proof that they actually promote weight loss.

Amino Acids, Working Out and Bodybuilding

Specific amino acids DO NOT LIKELY increase muscle mass or athletic performance more than intact protein [28,32]. Intravenous and oral arginine, lysine and ornithine supplements can stimulate the release of the human growth hormone (HGH), but there is no proof that any of them would have the actual anabolic effect, that is an increase of the muscle mass [32].

The Effect of Amino Acids on Insulin

Some amino acids trigger the insulin release. Amino acids that trigger the highest insulin release include leucine, isoleucine, valine (branched-chain amino acids), lysine and threonine, followed by phenylalanine, tyrosine and arginine [33,35]. Glucose and other substances in foods also trigger insulin release, so it is hard to predict the level of insulin increase solely from the amino acid content of  specific food proteins. Milk, whey and cottage cheese appear to stimulate insulin more than fish and egg white [33,34]. Free amino acids stimulate insulin more than intact proteins from foods [33,35].

Safety: Side Effects and Dangers of Amino Acid Supplements

Oral amino acid supplements in recommended doses are POSSIBLY SAFE for most healthy adults [11].

No major side effects are known, but overdose of certain amino acids, such as BCAAs, arginine, citrulline, ornithine, cystine and N-acetylcysteine (>9 g/dose), when taken orally, can cause nausea, vomiting, bloating, diarrhea, fatigue and muscle incoordination [14,26,38]. Allergic reactions to amino acids with swollen lips and face, itchy skin and rash (hives), vomiting or diarrhea are possible [26].

Taking too much of certain supplements, such as arginine, can lower blood pressure [39].

High intake of amino acid supplements may have harmful effects on the kidneys in individuals with kidney disease or kidney stones [11] but not likely in healthy individuals [40].

Pregnant women who intend to take oral amino acid supplements should speak with their doctors. Injectable amino acids are pregnancy category C, which means not enough studies have been done to establish their safety during pregnancy, so pregnant women should avoid them [27].

Legality of Amino Acid Supplements

The use of amino acid supplements is not prohibited by World Anti-Doping Agency (WADA) [12].

Amino Acid Pool or Nitrogen Pool

Amino acid pool is the total amount of free amino acids (300-600 grams) that are at a given moment available in the human body for the synthesis of new proteins [51]. These free amino acids mainly appear in the muscles, liver and blood [51]. The main source of amino acid pool are proteins from the muscles and liver, and the everyday food protein contributes only a little to the pool, so the pool can be maintained even during prolonged fasting.

Amino Acid Metabolism

Amino acids in the human body can be converted into other amino acids, glucose and fatty acids [44]. Every day, some amino acids are broken down and excreted as urea with the urine, so, to maintain the protein balance in the body, you need to regularly consume amino acids by eating foods that contain protein [51].

Amino Acid Profile

Amino acid profile is a summary of blood amino acid levels.

Blood amino acid levels can be INCREASED, for example, in eclampsia (high blood pressure during pregnancy), hereditary fructose intolerance, ketoacidosis (from diabetes), kidney failure, Reye syndrome or genetic disorders of amino acid metabolism in infants [55].

Blood amino acid levels can be DECREASED, for example, in hyperactivity of the adrenal gland (Cushing’s syndrome), fever, Hartnup disease, Huntington’s chorea, malnutrition, nephrotic syndrome (a kidney disorder with loss of proteins in the urine), phlebotomus fever (caused by Phlebovirus transmitted by a sandfly) and rheumatoid arthritis [55].

Blood amino acid levels may be also affected by diet and medications [55].

Amino acid profile should be interpreted along with other tests and by a specialist, such as endocrinologist.

Nonproteinogenic, Nonstandard or Unnatural Amino Acids

Amino acids that are not coded by DNA are called nonnproteinogenic, nonstandard or unnatural (which does not mean synthetic) amino acids. They include:

Amino Acid Derivatives

Absorption of Amino Acids

When proteins are digested, the resulting amino acids are absorbed in the small intestine. In order for amino acids to be properly absorbed, enough vitamins B12 and C, chromium and sleep is required.

Amino Acid Deficiency Syndromes

Amino acid deficiency syndromes are described in the articles about the specific amino acids.

Frequently Asked Questions

1. How many amino acids form proteins: 20 or 21?

21 amino acids form proteins. Selenocysteine as the 21st proteinogenic amino acid has been discovered only recently and this is why many health sites still mention only 20 amino acids.

2. What amino acids vegans tend to lack?

Vegans that eat only cereals may lack the amino acid lysine and those who eat only fruits and vegetables may lack methionine. Examples of rich vegan sources of lysine are legumes, peanuts and pumpkin seeds [30]. Examples of plant foods high in methionine are cereal grains, butternuts, sunflower seeds and soybeans [31]. So, vegans can obtain sufficient amounts of various amino acids by combining different protein-containing foods.

3. Why are amino acids added to certain beverages?

The amino acid taurine has a calming effect on the brain, so it may be added to certain energy drinks to oppose some of the stimulant actions of caffeine.

4. Can taking amino acid supplements cause cold sore?

Arginine supplements are sometimes mentioned as a possible trigger of cold sores, but this is uncertain.

5. Amino acid is to proteins as…?

Amino acid is to proteins as glucose is to starch.

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