The 20 Amino Acids – The Building Blocks Of Our Muscles – Complete Guide to Essential, Non-Essential, and Conditional

Protein is often called the most important macronutrient for anyone who trains — and amino acids are the reason why. When you eat protein, your digestive system breaks it down into amino acids and small fragments of two or three amino acids, which are absorbed and used to build the body’s own proteins. Twenty amino acids make up the proteins in food and in your body. They’re the building blocks your body uses to build and repair muscle, make enzymes and hormones, support immune function, and carry out thousands of other biological processes. (Two rarer ones are built into a small number of special proteins: selenocysteine, found in selenium-containing enzymes, and pyrrolysine, found only in some microbes.)

Understanding amino acids isn’t just academic. Knowing which ones your body can’t make, which one drives muscle protein synthesis, and which supplements are actually supported by evidence helps you make smarter decisions about your diet — and avoid spending money on products that don’t deliver.

For a broader overview of how protein fits into your overall nutrition, visit our Basic Nutrition page.

The Three Categories of Amino Acids

The 20 amino acids are commonly divided into three groups, based on whether your body can make them:

Essential amino acids — your body can’t make these, so they must come from food. There are 9 essential amino acids.

Non-essential amino acids — your body makes these from other compounds in normal metabolism. They’re just as important biologically; “non-essential” only means you don’t need them from food. This page lists 4 here.

Conditionally essential amino acids — your body normally makes enough, but in certain situations — such as premature birth, severe illness or injury, or some inherited disorders — it may not keep up. This page lists 7 here. Classifications vary slightly between sources; serine, for example, is often listed as non-essential.

For healthy people eating enough protein from varied sources, all 20 are covered by diet and the body’s own production — no individual amino acid needs special attention.

Essential Amino Acids

These 9 amino acids can’t be made by your body and must come from food. A diet that regularly falls short of one or more of them — usually because total protein is too low, or because it relies on a single plant source — impairs growth, muscle maintenance, recovery, and immune function.

1. Leucine

Leucine is one of the three branched-chain amino acids (BCAAs) and the most important amino acid for muscle building. It’s the main signal that switches on muscle protein synthesis, by activating the mTORC1 pathway — although the other essential amino acids are needed as building blocks for the new muscle protein.

Key functions: Triggering muscle protein synthesis, helping preserve muscle during calorie restriction and with age, and stimulating insulin release.

Best food sources: Dairy (especially whey protein), meat, poultry, fish, eggs, soy, and legumes.

Particularly important for: Anyone building or preserving muscle — a protein’s leucine content is one of the key factors in how well it stimulates muscle growth.

2. Isoleucine

Isoleucine is the second BCAA. It supports muscle protein synthesis much more weakly than leucine, can be broken down in muscle for energy, and — unlike leucine — can be used to make glucose as well as ketones. In animal and cell studies, it increased glucose uptake into muscle, but there’s no good evidence that isoleucine supplements improve blood sugar control or performance in people.

Key functions: Building muscle and other proteins, providing fuel during prolonged exercise, and — in laboratory studies — glucose uptake and gut immune defenses.

Best food sources: Cheese, chicken, pork, beef, fish, eggs, soy, and seeds.

Good to know: Isoleucine is usually the least abundant of the three BCAAs in food, but any diet with enough protein provides several times the requirement.

3. Valine

Valine is the third BCAA. It’s an essential building block for almost every protein, can be used for energy or to make glucose, and is broken down mainly in muscle. Valine and the other BCAAs are at the center of the “central fatigue” hypothesis — the idea that they could delay fatigue in long events by competing with tryptophan for entry into the brain — but BCAA supplements haven’t reliably improved endurance performance.

Key functions: Building muscle and other proteins, providing fuel during prolonged exercise, and supporting nitrogen balance.

Best food sources: Cheese, meat, poultry, fish, eggs, soy, seeds, and peanuts.

4. Histidine

Histidine was long thought to be essential only for children, because adults on histidine-free diets seemed fine at first — the body was drawing on histidine stored in hemoglobin and carnosine. Longer studies showed that hemoglobin levels fall without it, and it’s now recognized as essential at all ages. Histidine is the precursor of histamine — involved in immune responses, stomach acid production, and wakefulness — and, together with beta-alanine, forms carnosine, which buffers acid in muscle during high-intensity exercise.

Key functions: Making histamine, forming carnosine with beta-alanine, maintaining healthy hemoglobin levels, enzyme catalysis, and binding metals in enzymes.

Best food sources: Pork, hard cheeses, poultry, beef, fish, and seeds.

Good to know: In fish such as tuna and mackerel that isn’t kept cold, bacteria convert histidine into histamine, which can cause “scombroid” food poisoning — and cooking doesn’t destroy it.

5. Lysine

Lysine is needed for protein synthesis, forms the cross-links that give collagen its strength, and — together with methionine — is used to make carnitine. It’s the amino acid most often in short supply in diets built mainly on wheat, maize, and similar grains, which is why combining grains with legumes matters. Lysine is also the amino acid most damaged by heat processing, through the browning (Maillard) reaction.

Key functions: Protein synthesis, collagen cross-linking, carnitine production, and immune function. It’s also used for cold sores, though the evidence is mixed.

Best food sources: Meat, poultry, fish, dairy, eggs, and legumes — the key plant source. Nuts, seeds, and most grains are low in lysine.

Particularly important for: Vegans and vegetarians who rely heavily on grains — including legumes or soy every day solves this.

6. Tryptophan

Tryptophan is the body’s only raw material for serotonin, which influences mood, appetite, and sleep, and for melatonin, which helps time the sleep-wake cycle. It can also be converted into niacin (vitamin B3). It’s the least abundant essential amino acid in food, but a normal diet provides two to three times the requirement — and eating more tryptophan-rich food has little effect on brain serotonin, because other amino acids in protein compete with it for entry into the brain.

Key functions: Making serotonin and melatonin, making niacin, and gut serotonin production.

Best food sources: Seeds (especially pumpkin seeds), cheese, poultry, fish, oats, and tofu. Turkey has about the same tryptophan as chicken — the post-holiday-meal drowsiness has other causes — and bananas contain very little.

Good to know: If you take antidepressants or other medicines that raise serotonin, don’t take tryptophan or 5-HTP supplements without your doctor’s approval.

7. Methionine

Methionine is the only essential amino acid that contains sulfur. It starts every protein the body makes, and it’s the source of SAM, the body’s main methyl donor — used for DNA methylation, making creatine, and many other reactions. Methionine can also be converted into cysteine, which is needed for glutathione, the main antioxidant inside cells. Adequate folate, vitamin B12, and vitamin B6 keep its by-product, homocysteine, in check.

Key functions: Starting protein synthesis, supplying methyl groups, making creatine, and supplying cysteine for glutathione and taurine.

Best food sources: Brazil nuts (the richest), meat, poultry, fish, eggs, dairy, sesame seeds, and grains.

8. Phenylalanine

Phenylalanine is the precursor of tyrosine, and through tyrosine of dopamine, noradrenaline, adrenaline, thyroid hormones, and melanin. It comes in L- and D-forms: L-phenylalanine is the natural form in protein, while D-phenylalanine and DL-phenylalanine (DLPA) are sold for pain and mood, with weak evidence.

Key functions: Protein synthesis and making tyrosine, which is used to make catecholamines and thyroid hormones.

Best food sources: Cheese, meat, fish, eggs, soy, nuts, and seeds.

Note: People with phenylketonuria (PKU), a rare inherited condition, can’t process phenylalanine and must strictly limit it — including from the sweetener aspartame.

9. Threonine

Threonine is needed for protein synthesis, and its standout role is in the gut: mucins — the proteins that form the gut’s protective mucus layer — are unusually rich in threonine, and animal studies show the gut uses a large share of dietary threonine to make them. Threonine is also abundant in antibodies.

Key functions: Protein synthesis, gut mucus production, and immune function.

Best food sources: Meat, poultry, fish, cheese, eggs, soy, and legumes. Unlike lysine, threonine isn’t a limiting factor in grain-based diets for human adults.

Non-Essential Amino Acids

Your body makes these four amino acids from other compounds — but they’re still vital for health and performance. “Non-essential” refers to dietary need, not biological importance.

10. Asparagine

Asparagine was the first amino acid ever isolated — from asparagus juice, in 1806. It’s the attachment point for the sugar chains (N-linked glycans) carried by most secreted and cell-surface proteins, which help them fold and function. Some leukemia cells can’t make their own asparagine, so an enzyme that removes it from the blood is a cornerstone of childhood leukemia treatment. In food, free asparagine reacts with sugars at high temperatures to form acrylamide.

Key functions: Protein glycosylation, energy production and DNA building blocks (through aspartate), and brain development.

Best food sources: Protein-rich foods such as meat, poultry, fish, dairy, nuts, seeds, and legumes. Potatoes and asparagus store nitrogen as free asparagine, so their protein is unusually asparagine-rich, but they contain little protein overall.

11. Glutamic Acid

Glutamic acid — usually called glutamate in the body — is the brain’s main excitatory neurotransmitter, central to learning and memory. It’s also the hub of nitrogen metabolism, a fuel for the cells lining the gut, and the starting material for GABA, glutathione, proline, and glutamine. Together with glutamine, it’s the most abundant amino acid in almost all food proteins, and free glutamate is the source of umami (savory) taste.

Key functions: Excitatory neurotransmission, learning and memory, nitrogen metabolism, gut fuel, and umami taste.

Best food sources: All protein foods — especially dairy, nuts, and seeds. Aged cheese, tomatoes, mushrooms, soy sauce, and fermented foods are rich in free glutamate. MSG is its sodium salt; well-controlled studies haven’t shown consistent reactions at normal food doses.

12. Alanine

Alanine plays a central role in the glucose–alanine cycle: during fasting and prolonged exercise, muscles release alanine, which carries nitrogen to the liver, where it’s used to make glucose. Note that beta-alanine, the popular supplement that raises muscle carnosine, is a different molecule — L-alanine supplements don’t raise carnosine.

Key functions: The glucose–alanine cycle, carrying nitrogen from muscle to the liver, glucose production, and protein structure.

Best food sources: Meat, poultry, fish, eggs, seeds, nuts, and legumes; gelatin is especially rich.

13. Aspartic Acid

Aspartic acid (aspartate) converts directly into oxaloacetate, a Krebs cycle intermediate, which makes it central to energy production — including through the malate–aspartate shuttle, which helps the heart and muscles get the most energy from glucose. It also supplies nitrogen to the urea cycle, which removes ammonia, and provides atoms for building DNA and RNA. Its mirror-image form, D-aspartic acid, is sold as a testosterone booster, but trials in men who train found no benefit.

Key functions: Energy production, the urea cycle, and building DNA and RNA.

Best food sources: Meat, poultry, fish, dairy, eggs, nuts, seeds, and legumes.

Conditionally Essential Amino Acids

Your body normally makes enough of these seven amino acids. In certain situations — such as premature birth, severe illness or burns, or specific inherited disorders — production may not keep up. For healthy people, including athletes eating enough protein, supplements of these amino acids are generally unnecessary, with a few specific exceptions noted below.

14. Glutamine

Glutamine is the most abundant free amino acid in the blood and muscle, a major fuel for gut and immune cells, and the body’s main safe carrier of nitrogen between organs. Blood and muscle levels fall during severe illness, which made glutamine an appealing supplement — but large trials in critically ill patients found no benefit, and higher mortality in some groups. In athletes, evidence for benefits on recovery, soreness, or immunity is weak.

Key functions: Fueling gut and immune cells, carrying nitrogen, kidney acid-base balance, and supplying glutamate for neurotransmitters and glutathione.

Best food sources: Meat, poultry, fish, dairy, eggs, legumes, and wheat (gluten is rich in it). Food databases report glutamine combined with glutamic acid.

Supplementation: Not routinely recommended for athletes or for people who are ill. Small studies suggest high doses before exercise may reduce gut permeability in the heat; prescription glutamine is used in sickle cell disease.

15. Arginine

Arginine is the substrate the body uses to make nitric oxide, which widens blood vessels — the reason arginine and citrulline appear in pre-workout products. It’s also part of the urea cycle, the first building block in creatine production, and a source of proline and polyamines. High doses stimulate growth hormone release, but oral doses have only modest, short-lived effects.

Key functions: Nitric oxide production, the urea cycle, creatine production, wound healing, and immune function.

Best food sources: Pumpkin seeds, peanuts, and other nuts and seeds (the richest), meat, poultry, fish, and legumes.

Supplementation: Much of an oral arginine dose is broken down before it reaches the bloodstream, so citrulline raises blood arginine more effectively — though performance effects of both are small. For endurance, dietary nitrate (such as beetroot juice) has stronger evidence. People who’ve had a heart attack shouldn’t take arginine supplements.

16. Tyrosine

Tyrosine is made from phenylalanine and is the direct precursor of dopamine, noradrenaline, adrenaline, thyroid hormones, and melanin. It becomes essential in phenylketonuria (PKU). In people eating enough protein, tyrosine doesn’t limit thyroid hormone production — iodine does.

Key functions: Making dopamine, noradrenaline, and adrenaline, making thyroid hormones and melanin, and supporting mental performance under stress.

Best food sources: Cheese, poultry, pork, fish, seeds, peanuts, eggs, and soy. Bananas contain very little.

Supplementation: L-tyrosine has one of the clearer evidence bases among amino acid supplements — for preserving mental performance during acute stress, such as sleep deprivation, cold, or high altitude. It has little effect on physical performance. Don’t combine it with MAO inhibitor medicines.

17. Serine

Serine is made from glucose and is the main source of the single-carbon units that feed the folate cycle, which the body needs to make DNA and methyl groups. It’s also the starting point for sphingolipids (important fats in nerve membranes and myelin), the body’s main source of glycine, and a building block for cysteine and phosphatidylserine. Its mirror-image form, D-serine, helps switch on NMDA receptors involved in learning and memory. Rare inherited disorders of serine production cause severe brain problems, which high-dose serine can help treat.

Key functions: Single-carbon metabolism, making sphingolipids, glycine, cysteine, and phosphatidylserine, D-serine signalling in the brain, and cell signalling as the most common phosphorylation site in proteins.

Best food sources: Eggs (the highest share of protein), dairy, soy, legumes, seeds, nuts, meat, and fish.

18. Proline

Proline is, together with glycine, one of the two main building blocks of collagen, the body’s most abundant protein. About 12–13% of collagen is proline, and a similar share is hydroxyproline — proline modified with the help of vitamin C, which stabilizes collagen’s triple helix. That’s why vitamin C deficiency causes scurvy. Proline’s unique ring structure helps collagen chains coil into their rope-like shape.

Key functions: Collagen production for tendons, ligaments, cartilage, bone, skin, and blood vessels, and wound healing.

Best food sources: Gelatin and collagen (by far the richest), hard cheese (casein is proline-rich), meat, poultry, and wheat. Bone broth contains some, but amounts are low and variable.

Particularly relevant for: Athletes interested in tendon and ligament health — see the gelatin and vitamin C protocol on our proline page.

19. Glycine

Glycine is the simplest amino acid, and the most abundant one in collagen — one in every three of collagen’s amino acids is glycine, sitting at the center of the triple helix where only the smallest amino acid fits. It’s also needed to make creatine, glutathione, bile salts, and heme (the iron-containing part of hemoglobin), and it acts as a calming neurotransmitter in the spinal cord. Some researchers argue the body may not make enough for all its needs, though this hasn’t been established.

Key functions: Collagen production, making creatine, glutathione, heme, and bile salts, and calming nerve signalling.

Best food sources: Gelatin and collagen, pork skin, meat (especially cuts with more connective tissue), poultry, fish, nuts, seeds, and legumes. Dairy is notably low.

Supplementation: Several small studies found that 3g of glycine before bed improved sleep quality. The evidence is promising but limited.

20. Cysteine

Cysteine is a sulfur-containing amino acid that the body makes from methionine and serine. It’s usually the limiting building block of glutathione, the main antioxidant inside cells. Its sulfur atoms form disulfide bonds that hold many proteins in shape — including keratin in hair and nails, insulin, and antibodies. Its supplement form, N-acetylcysteine (NAC), is the standard hospital antidote for paracetamol (acetaminophen) overdose.

Key functions: Glutathione production, protein structure through disulfide bonds, keratin, and taurine production.

Best food sources: Oats and other whole grains, eggs, sunflower seeds, Brazil nuts, poultry, meat, fish, and legumes. As a share of protein, oats and eggs beat meat.

Supplementation: NAC has established medical uses, but routine use around training isn’t recommended — in some studies, high-dose NAC interfered with recovery signalling.

Complete vs. Incomplete Proteins

An important practical point when applying this to your diet:

Complete proteins contain all 9 essential amino acids in adequate amounts. Meat, fish, poultry, eggs, and dairy are complete proteins, and so is soy. The big exception among animal proteins is collagen and gelatin: they contain no tryptophan and are low in several other essential amino acids, so they don’t count toward your protein needs for muscle building in the same way.

Incomplete proteins are low in one or more essential amino acids. Grains are low in lysine, and legumes are relatively low in the sulfur amino acids (methionine and cysteine) — but their strengths complement each other. Eating a variety of plant proteins over the day, such as grains with beans or lentils, provides a complete amino acid profile without any animal products. They don’t need to be eaten at the same meal.

Amino Acids and Supplementation

For most people, enough protein from varied foods covers every amino acid. Here’s what the evidence says about the most common amino acid-related supplements:

Protein powders (whey, casein, soy, pea) — a convenient way to reach your protein target. Whey is especially rich in leucine and quickly digested.

BCAAs (leucine, isoleucine, valine) — popular, but they add little for people already eating enough protein. On their own, BCAAs produce a smaller muscle-building response than whey or an essential amino acid (EAA) blend, which provides all the building blocks.

Creatine — not an amino acid itself, but made in the body from arginine, glycine, and methionine. It’s one of the best-researched and most effective supplements for strength and power.

Beta-alanine — not one of the 20 protein amino acids, but the limiting building block for carnosine. It’s well supported for high-intensity efforts lasting roughly one to four minutes.

Collagen and gelatin — rich in glycine, proline, and hydroxyproline. Taken with vitamin C before loading exercise, gelatin increased a marker of collagen production, which is promising for tendons and ligaments; evidence for joint pain and skin is mixed. Remember that collagen isn’t a complete protein.

L-tyrosine and glycine — tyrosine for mental performance under acute stress, and glycine for sleep, both with modest supporting evidence.

Glutamine — widely used, but the evidence for recovery and immune benefits in healthy athletes is weak.

As with any supplement, choose products that are third-party tested — especially if you compete in drug-tested sport.

For a broader understanding of how amino acids and protein fit into your overall nutritional approach, visit our Basic Nutrition page.