What Is an Amino Acid?
Proteins are the main structural and functional components of the body, from muscle, skin, and hair to enzymes, hormones, and antibodies. Every protein is built from a chain of smaller units called amino acids. You can think of amino acids as the letters of an alphabet: by combining 20 standard amino acids, the body builds thousands of different proteins.
When we eat protein-containing food, the digestive system breaks it down into amino acids. The body then uses these amino acids to build new proteins, repair tissues, and produce other important compounds, such as certain hormones and neurotransmitters.
Types of Amino Acids
From a nutritional standpoint, amino acids fall into three groups.
Essential amino acids (9): The body cannot make these, so they must be obtained from food.
Non-essential amino acids (11): The body can make these on its own. It is important to note that “non-essential” does not mean “unimportant.” It simply means they do not necessarily have to come from the diet.
Conditionally essential amino acids: Some non-essential amino acids, such as arginine, glutamine, cysteine, and tyrosine, may not be produced in sufficient amounts under certain conditions, such as severe illness, burns, metabolic stress, or periods of growth, and may need to be supplied through food.
The 9 essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Complete and Incomplete Proteins
A protein that contains all 9 essential amino acids in adequate amounts is called a “complete protein.” Most animal sources, such as meat, fish, eggs, and dairy, as well as soy, fall into this category. Many plant sources are low in one or more essential amino acids; for example, grains are typically limited in lysine, and legumes in methionine.
The good news is that these gaps can be filled through dietary variety. Combining grains and legumes is a simple example, and it doesn’t even have to happen in a single meal; getting them across the day is enough.
Final Note
Most people who eat a varied diet with adequate protein easily meet their needs for essential amino acids from food. Next, we will look at each of the 9 essential amino acids individually.
Histidine
1. Histidine
Overview: Histidine is one of the 9 essential amino acids. The body cannot make enough of it on its own, so it must come from food. Its distinctive chemical structure (the imidazole ring) allows it to act as a buffer in mildly acidic and alkaline environments and to bind metals such as zinc, iron, and copper.
Roles in the body:
- Histamine production: Histidine is the precursor of histamine, a compound involved in immune and inflammatory responses, stomach acid secretion, and signaling in the nervous system.
- Carnosine production: Together with beta-alanine, histidine forms carnosine, a compound found in muscle and brain that helps regulate tissue acidity.
- Protein structure: In many enzymes and proteins, including hemoglobin, histidine helps bind metals and maintain protein function.
- Growth and tissue repair: Like other essential amino acids, it is needed to build new proteins.
Daily requirement: According to international nutrition authorities, the adult requirement is estimated at about 10 mg per kilogram of body weight per day. A varied diet with adequate protein easily meets this amount.
Food sources: Red meat, poultry, fish, eggs, dairy, soy, nuts, seeds, and whole grains.
Deficiency: In people who eat enough protein, histidine deficiency is rare. Long-term deficiency may affect hemoglobin production and skin health, and in certain patients, such as some people with chronic kidney disease, it may be monitored by a doctor.
Important notes:
- Eating histidine-rich foods does not by itself cause allergies or a dangerous rise in histamine. The concern is histamine that builds up in spoiled, aged, or fermented foods.
- Fish such as tuna and mackerel, if stored improperly, can have their histidine converted into histamine by bacteria, which may cause food poisoning (scombroid poisoning). Always keep fish refrigerated or frozen.
- For most people, histidine supplements are unnecessary. If you have a medical condition or take medication, consult your doctor before using any supplement.
Isoleucine
2. Isoleucine
Overview: Isoleucine is one of the 9 essential amino acids and one of the three branched-chain amino acids (BCAAs), along with leucine and valine. The body cannot make it, so it must come from food. Unlike most amino acids, which are broken down mainly in the liver, BCAAs are largely metabolized in muscle tissue.
Roles in the body:
- Muscle metabolism and energy: Isoleucine can be used by muscle as an energy source, especially during prolonged exercise or fasting.
- Blood sugar regulation: It helps muscle cells take up glucose from the blood and contributes to glucose metabolism.
- Hemoglobin formation: It is involved in the production of hemoglobin, the oxygen-carrying protein in red blood cells.
- Immune function and tissue repair: It supports the building of new proteins needed for immune cells and wound healing.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 20 mg per kilogram of body weight per day. A varied diet with adequate protein easily meets this amount.
Food sources: Poultry, fish, red meat, eggs, dairy, soy, lentils and other legumes, nuts, seeds, and whole grains.
Deficiency: In people who eat enough protein, isoleucine deficiency is rare. It is more likely in situations of overall protein malnutrition, where several essential amino acids are low at the same time.
Important notes:
- Isoleucine, leucine, and valine compete for the same transport systems in the body. Taking a high dose of one BCAA on its own may lower the levels of the others, which is one reason supplements are not automatically beneficial.
- People with maple syrup urine disease (MSUD), a rare genetic disorder, cannot properly break down BCAAs, including isoleucine. They must follow a specialized diet under medical supervision.
- Higher blood BCAA levels have been linked to insulin resistance in some observational studies, but a cause-and-effect relationship has not been established.
- For most people who eat enough protein, BCAA supplements are unnecessary. If you have a medical condition, especially kidney or liver disease, or take medication, consult your doctor before using any supplement.
Leucine
3. Leucine
Overview: Leucine is one of the 9 essential amino acids and the most abundant of the three branched-chain amino acids (BCAAs), along with isoleucine and valine. The body cannot make it, so it must come from food. Leucine is one of the most common amino acids in muscle protein, and it has received particular attention in sports nutrition and in research on muscle loss with aging.
Roles in the body:
- Muscle protein synthesis: Leucine acts as a signal that activates mTORC1, a cellular pathway that switches on protein building. This is why leucine is considered the key trigger for muscle growth and repair after protein intake.
- Maintaining muscle mass: Adequate leucine, as part of enough total protein, helps preserve muscle, especially in older adults.
- Energy production: Muscle can use leucine as an energy source, particularly during prolonged exercise or fasting.
- Blood sugar regulation: It stimulates insulin release and helps muscle cells take up glucose.
- Wound healing and growth hormone regulation: It contributes to tissue repair and to hormonal processes involved in growth.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 39 mg per kilogram of body weight per day, the highest among the essential amino acids after lysine's close figure. A varied diet with adequate protein easily meets this amount.
Food sources: Dairy products (especially whey protein), red meat, poultry, fish, eggs, soy, lentils and other legumes, nuts, seeds, and whole grains.
Deficiency: In people who eat enough protein, leucine deficiency is rare. It is more likely in overall protein malnutrition, where several essential amino acids are low at the same time. In such cases, muscle loss, weakness, and slow wound healing may occur.
Important notes:
- Leucine works best as part of a complete protein meal. Research suggests that total daily protein intake and overall diet quality matter more for muscle health than taking leucine alone.
- Leucine, isoleucine, and valine compete for the same transport systems. A high dose of one BCAA on its own may lower the levels of the others.
- People with maple syrup urine disease (MSUD), a rare genetic disorder, cannot properly break down BCAAs, including leucine. They must follow a specialized diet under medical supervision.
- Higher blood BCAA levels have been linked to insulin resistance in some observational studies, but a cause-and-effect relationship has not been established.
- For most people who eat enough protein, leucine or BCAA supplements are unnecessary. If you have a medical condition, especially kidney or liver disease, or take medication, consult your doctor before using any supplement.
Lysine
4. Lysine
Overview: Lysine is one of the 9 essential amino acids. The body cannot make it, so it must come from food. It is especially important to know about because it is often the "limiting" amino acid in diets based mainly on cereals such as wheat, rice, and corn, which contain relatively little of it.
Roles in the body:
- Collagen formation: Lysine is needed to build and cross-link collagen, the protein that gives strength to skin, tendons, bones, cartilage, and blood vessels.
- Calcium absorption and retention: It supports calcium uptake from the gut and helps the body retain calcium, which is important for bone health.
- Carnitine synthesis: Lysine (with methionine) is a building block for carnitine, a compound that transports fatty acids into cells for energy production.
- Protein building and tissue repair: It is needed to make enzymes, hormones, and antibodies, and for growth and wound healing.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 30 mg per kilogram of body weight per day. A varied diet with adequate protein easily meets this amount.
Food sources: Red meat, poultry, fish, eggs, dairy products, soy and soy products, lentils, chickpeas, beans, quinoa, and pumpkin seeds. Grains are relatively low in lysine.
Deficiency: In people who eat enough varied protein, lysine deficiency is uncommon. It is more likely when the diet depends heavily on cereals and includes little animal protein or legumes. Possible effects of inadequate intake include fatigue, poor appetite, slowed growth in children, and impaired wound healing.
Important notes:
- Plant-based eaters can meet their lysine needs by regularly including legumes, soy products, and other lysine-rich plant foods alongside grains. Grains and legumes complement each other well, and they do not need to be eaten in the same meal.
- Lysine is sensitive to heat in the presence of sugars. Heavy cooking, such as prolonged high-temperature baking or frying, can make part of the lysine in food unavailable to the body.
- Lysine supplements are often promoted for cold sores (herpes simplex). The evidence is mixed and inconclusive, so they should not replace medical treatment.
- High supplemental doses may cause stomach discomfort and diarrhea. People with kidney disease should not take lysine supplements without medical advice.
- Lysinuric protein intolerance is a rare genetic disorder that affects the transport of lysine and a few other amino acids and requires specialized medical care.
- For most people who eat enough protein, lysine supplements are unnecessary. If you have a medical condition or take medication, consult your doctor before using any supplement.
Methionine
5. Methionine
Overview: Methionine is one of the 9 essential amino acids. The body cannot make it, so it must come from food. It is a sulfur-containing amino acid and the starting point for building every new protein in the cell. Legumes tend to be relatively low in methionine, so it is often the limiting amino acid in plant-based diets that rely heavily on beans, lentils, and peas.
Roles in the body:
- Methyl group donation: Methionine is converted into S-adenosylmethionine (SAMe), a key compound that supplies methyl groups for many reactions, including DNA regulation and the synthesis of creatine and phospholipids.
- Cysteine and antioxidant defense: The body can convert methionine into cysteine, which is needed to make glutathione, one of the main antioxidants in cells, and taurine.
- Protein synthesis: Methionine is the amino acid that starts the building of new protein chains.
- Carnitine and fat metabolism: Together with lysine, it contributes to the production of carnitine, which helps move fatty acids into cells for energy.
- Healthy hair, skin, and nails: As a source of sulfur, it supports the structure of keratin and connective tissue.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement for methionine plus cysteine combined is about 15 mg per kilogram of body weight per day, because cysteine can partly replace methionine. A varied diet with adequate protein easily meets this amount.
Food sources: Eggs, fish, poultry, red meat, dairy products, sesame seeds, Brazil nuts, sunflower seeds, oats, and soy. Most legumes contain less of it.
Deficiency: In people who eat enough protein, methionine deficiency is rare. It is more likely in overall protein malnutrition or in very restricted diets. Possible effects include poor growth in children, fatigue, and impaired liver function.
Important notes:
- Methionine metabolism depends on vitamin B6, vitamin B12, and folate. If these vitamins are low, a compound called homocysteine can build up in the blood. High homocysteine is associated with a greater risk of cardiovascular disease, though the benefit of lowering it with supplements is still debated.
- People with homocystinuria, a rare genetic disorder of methionine metabolism, must restrict methionine and follow a specialized diet under medical supervision.
- Animal studies have shown that restricting methionine may extend lifespan, but this has not been proven in humans, and people should not cut methionine on their own.
- Taking large doses of methionine as a supplement can raise homocysteine and cause side effects. Methionine supplements and SAMe supplements are different products.
- For most people who eat enough protein, methionine supplements are unnecessary. If you have a medical condition, especially liver or kidney disease, or take medication, consult your doctor before using any supplement.
Phenylalanine
6. Phenylalanine
Overview: Phenylalanine is one of the 9 essential amino acids. The body cannot make it, so it must come from food. It is an aromatic amino acid, and its most important job is to serve as the raw material for another amino acid, tyrosine, which in turn is used to make several hormones and neurotransmitters.
Roles in the body:
- Conversion to tyrosine: An enzyme in the liver (phenylalanine hydroxylase) converts phenylalanine into tyrosine. This is why tyrosine is considered non-essential as long as enough phenylalanine is available.
- Neurotransmitter production: Through tyrosine, phenylalanine contributes to the making of dopamine, norepinephrine, and epinephrine, which affect mood, alertness, and the body's stress response.
- Thyroid hormones: Tyrosine derived from phenylalanine is also needed to make thyroid hormones, which regulate metabolism.
- Melanin: It supports the production of melanin, the pigment of skin, hair, and eyes.
- Protein synthesis: Like other essential amino acids, it is needed to build proteins, including enzymes and structural proteins.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement for phenylalanine plus tyrosine combined is about 25 mg per kilogram of body weight per day, because tyrosine can partly replace phenylalanine. A varied diet with adequate protein easily meets this amount.
Food sources: Red meat, poultry, fish, eggs, dairy products, soy and soy products, legumes, nuts, and seeds. Phenylalanine is also a component of the artificial sweetener aspartame.
Deficiency: In people who eat enough protein, phenylalanine deficiency is rare. It is more likely in overall protein malnutrition, where several essential amino acids are low at the same time.
Important notes:
- Phenylketonuria (PKU): People with this rare genetic disorder lack the enzyme that converts phenylalanine to tyrosine, so phenylalanine builds up in the blood and can damage the brain, especially in early life. They must follow a strict, low-phenylalanine diet under medical supervision. In many countries, newborns are screened for PKU.
- Aspartame labels: Foods and drinks containing aspartame often carry a warning for people with PKU. For everyone else, aspartame is considered safe within the accepted daily intake limits set by food safety authorities.
- Phenylalanine supplements (including the "DL-phenylalanine" form) are sometimes marketed for mood or pain, but the evidence is limited. They can interact with certain medications, such as MAO inhibitors and levodopa, and are not suitable for people with PKU.
- For most people who eat enough protein, phenylalanine supplements are unnecessary. If you have a medical condition or take medication, consult your doctor before using any supplement.
Threonine
7. Threonine
Overview: Threonine is one of the 9 essential amino acids. The body cannot make it, so it must come from food. It is a structural building block of several important proteins, and it is especially concentrated in the proteins that form connective tissue and the protective mucus layer of the gut.
Roles in the body:
- Collagen and elastin: Threonine is a component of these proteins, which give strength and elasticity to skin, tendons, blood vessels, and other connective tissues.
- Gut barrier: It is a major component of mucins, the proteins that form the protective mucus lining of the digestive tract. A large share of the threonine we eat is used by the gut for this purpose.
- Immune function: It is needed to build antibodies and other immune proteins.
- Cell signaling: Threonine residues in proteins can be chemically tagged (phosphorylated), which is one of the ways cells switch enzymes and signaling pathways on and off.
- Protein synthesis and fat metabolism: Like other essential amino acids, it is needed for growth and tissue repair, and it takes part in normal fat metabolism in the liver.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 15 mg per kilogram of body weight per day. A varied diet with adequate protein easily meets this amount.
Food sources: Poultry, red meat, fish, eggs, dairy products (especially cottage cheese and other cheeses), soy, lentils, beans, pumpkin seeds, and other seeds and nuts.
Deficiency: In people who eat enough protein, threonine deficiency is rare. It is more likely in overall protein malnutrition or in diets that depend heavily on a single cereal with little variety, since cereals are low in lysine and, to a lesser degree, in threonine. Possible effects of inadequate intake include poor growth in children, fatigue, and impaired gut and immune health.
Important notes:
- Plant-based eaters can easily meet their threonine needs by combining grains with legumes, soy products, nuts, and seeds across the day.
- Because the gut uses so much threonine, conditions that damage the intestinal lining or increase protein loss may raise the body's needs. This is a matter for medical assessment, not self-supplementation.
- Threonine supplements have been studied for muscle spasticity in some neurological conditions, but the evidence is limited and inconsistent. They should not replace medical treatment.
- Very high doses of a single amino acid can disturb the balance of others and cause digestive upset.
- For most people who eat enough protein, threonine supplements are unnecessary. If you have a medical condition or take medication, consult your doctor before using any supplement.
Tryptophan
8. Tryptophan
Overview: Tryptophan is one of the 9 essential amino acids. The body cannot make it, so it must come from food. It is the least abundant amino acid in most foods and in the body, yet it is the starting material for several compounds with major roles in mood, sleep, and metabolism.
Roles in the body:
- Serotonin production: Tryptophan is the precursor of serotonin, a neurotransmitter involved in mood, appetite, and gut movement. Most of the body's serotonin is actually found in the gut.
- Melatonin production: Serotonin can be converted into melatonin, the hormone that helps regulate the sleep-wake cycle.
- Niacin (vitamin B3) synthesis: The body can convert tryptophan into niacin, although the process is inefficient (roughly 60 mg of tryptophan yields about 1 mg of niacin). This is why diets adequate in protein protect against niacin deficiency.
- Kynurenine pathway: Most dietary tryptophan is broken down through this pathway, which produces compounds involved in energy metabolism (NAD) and immune regulation.
- Protein synthesis: Like other essential amino acids, it is needed to build proteins.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 4 mg per kilogram of body weight per day, the lowest among the essential amino acids. A varied diet with adequate protein easily meets this amount.
Food sources: Poultry (including turkey and chicken), eggs, cheese and other dairy products, fish, soy and soy products, pumpkin seeds, sesame seeds, oats, legumes, and nuts.
Deficiency: In people who eat enough protein, tryptophan deficiency is rare. It is more likely in diets that rely almost entirely on corn with little other protein, since corn is low in both tryptophan and available niacin. This combination historically led to pellagra, a niacin-deficiency disease characterized by dermatitis, diarrhea, and cognitive problems. Medical conditions that impair absorption can also contribute.
Important notes:
- Turkey and sleepiness: The popular belief that turkey makes you sleepy because of tryptophan is largely a myth. Turkey does not contain unusually high amounts compared with other poultry or cheese. Drowsiness after a big meal is more likely related to meal size, carbohydrates, and fatigue.
- Brain access: Tryptophan competes with other large amino acids to cross into the brain. Eating tryptophan-rich foods does not automatically raise brain serotonin, and the effect of diet on mood is modest and varies between people.
- Drug interactions: Tryptophan supplements combined with antidepressants (such as SSRIs or MAO inhibitors), or other drugs that raise serotonin, can contribute to serotonin syndrome, a potentially dangerous condition. Do not combine them without medical supervision.
- Supplement safety: In 1989, contaminated tryptophan supplements from one manufacturer caused an outbreak of eosinophilia-myalgia syndrome in the United States. Supplements are now better regulated, but this history shows the importance of product quality.
- Possible side effects of supplements include nausea, dizziness, and drowsiness. Supplements are not recommended during pregnancy or breastfeeding unless a doctor advises them.
- For most people who eat enough protein, tryptophan supplements are unnecessary. If you have a medical condition, especially depression, or take any medication, consult your doctor before using any supplement.
Valine
9. Valine
Overview: Valine is one of the 9 essential amino acids and the third of the branched-chain amino acids (BCAAs), along with leucine and isoleucine. The body cannot make it, so it must come from food. Like the other BCAAs, valine is metabolized largely in muscle tissue rather than in the liver.
Roles in the body:
- Muscle growth and repair: Valine is a building block of muscle protein and contributes to the repair of tissues after exercise or injury.
- Energy production: Muscle can break valine down and use it as a fuel source, especially during prolonged exercise or fasting.
- Nitrogen balance: It helps the body maintain a healthy balance of nitrogen, which reflects whether the body is building or breaking down protein.
- Protein structure: It is a component of many proteins, where its water-repelling (hydrophobic) side chain helps proteins fold into their correct shape.
Daily requirement: According to WHO/FAO/UNU estimates, the adult requirement is about 26 mg per kilogram of body weight per day. A varied diet with adequate protein easily meets this amount.
Food sources: Dairy products, red meat, poultry, fish, eggs, soy and soy products, lentils, beans, peanuts, whole grains, and mushrooms.
Deficiency: In people who eat enough protein, valine deficiency is rare. It is more likely in overall protein malnutrition, where several essential amino acids are low at the same time. Possible effects include muscle wasting, fatigue, and impaired growth in children.
Important notes:
- Valine, leucine, and isoleucine compete for the same transport systems. A high dose of one BCAA on its own may lower the levels of the others, so single-amino-acid supplements are not automatically beneficial.
- People with maple syrup urine disease (MSUD), a rare genetic disorder, cannot properly break down BCAAs, including valine. They must follow a specialized diet under medical supervision.
- Valine has a well-known role in genetics: in sickle cell disease, a single change in the hemoglobin gene replaces glutamic acid with valine at one position in the protein, which alters its shape. This is a classic example of how one amino acid can change a protein's function.
- Higher blood BCAA levels have been linked to insulin resistance in some observational studies, but a cause-and-effect relationship has not been established.
- For most people who eat enough protein, valine or BCAA supplements are unnecessary. If you have a medical condition, especially kidney or liver disease, or take medication, consult your doctor before using any supplement.
Conclusion
Amino acids are the building blocks of every protein in the body, and proteins in turn carry out nearly every task that keeps us alive: building and repairing tissues, speeding up chemical reactions, carrying oxygen, defending against infection, and sending signals between cells. Of the 20 standard amino acids, nine are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Because the body cannot make them, they must come from food.
As we have seen, each essential amino acid has its own distinct role. Leucine, isoleucine, and valine support muscle metabolism. Lysine is needed for collagen and calcium handling. Methionine supplies sulfur and methyl groups for many reactions. Phenylalanine and tryptophan lead to neurotransmitters and hormones that influence mood, alertness, and sleep. Histidine and threonine contribute to immune function, connective tissue, and the protective lining of the gut. None of them works alone, and the body needs all nine in adequate amounts to build proteins properly.
The good news is that meeting these needs is usually straightforward. A varied diet that contains enough protein supplies all nine essential amino acids for most healthy people. Animal foods and soy provide complete proteins, while plant-based eaters can reach the same goal by combining grains, legumes, nuts, and seeds over the course of the day. Deficiencies are rare in people who eat adequately, and they are more often a sign of overall protein malnutrition than of a shortage of a single amino acid.
It is also worth remembering the limits of what amino acids can do. Supplements are marketed with many promises, but for most people who already eat enough protein, they offer little or no added benefit. Taking large doses of a single amino acid can upset the balance of others, and certain groups need particular caution, including people with phenylketonuria, maple syrup urine disease, kidney or liver disease, and those taking medications that affect serotonin or other pathways.
In short, the most reliable way to support your body’s amino acid needs is not a powder or a capsule, but a balanced, varied diet. If you have a medical condition, take medication, or are considering a supplement, speak with a doctor or a registered dietitian first.
Note: This article is for educational purposes and is not a substitute for medical advice.
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