Amino acids are organic compounds with a shared chemical structure: an amino group (–NH₂), a carboxyl group (–COOH), and a side chain (R group) that distinguishes each amino acid from the others. When linked together by peptide bonds in chains, they form proteins. When dietary protein is digested, it's broken back down into amino acids, which are absorbed and used by the body.
The 20 Amino Acids: Essential vs Non-Essential
The human body uses 20 different amino acids. These are divided into:
Essential Amino Acids (EAAs) — 9
The body cannot synthesise these, or cannot synthesise them in adequate amounts. They must come from diet.
| Essential Amino Acid | Key Functions |
|---|---|
| Histidine | Immune function, neurotransmitter production |
| Isoleucine | Muscle metabolism, haemoglobin production |
| Leucine | Protein synthesis, muscle repair, mTOR activation |
| Lysine | Collagen formation, calcium absorption, immune function |
| Methionine | Methylation reactions, glutathione synthesis |
| Phenylalanine | Neurotransmitter (dopamine, epinephrine) precursor |
| Threonine | Collagen, tooth enamel, fat metabolism |
| Tryptophan | Serotonin and melatonin precursor |
| Valine | Muscle growth, energy production |
Isoleucine, leucine, and valine are also known as branched-chain amino acids (BCAAs) — named for their branched molecular structure. Leucine, in particular, is the primary amino acid that triggers muscle protein synthesis via the mTOR pathway.
Non-Essential Amino Acids — 11
These can be synthesised by the body from other compounds. They are: alanine, asparagine, aspartate, glutamate, and serine, plus six conditionally essential ones.
Conditionally Essential Amino Acids — 6
Normally synthesisable by the body, but certain circumstances (illness, severe stress, rapid growth, premature birth) can make synthesis insufficient:
- Arginine
- Cysteine
- Glutamine
- Glycine
- Proline
- Tyrosine
Glutamine is notable — it's the most abundant amino acid in the body and becomes conditionally essential under critical illness, major surgery, and extreme exercise, when demand outstrips synthesis.
Complete vs Incomplete Protein
A complete protein contains all 9 essential amino acids in adequate amounts. Animal-derived proteins — meat, poultry, fish, eggs, dairy — are all complete. So are a few plant sources: quinoa, soy, buckwheat, and hemp seeds.
An incomplete protein is low in one or more essential amino acids. Most plant proteins are incomplete. The limiting amino acid — the one in shortest supply — restricts how much of that protein can be utilised.
For example: rice is low in lysine; beans are low in methionine. Combined, they provide all essential amino acids in balanced proportions — hence the traditional complementary protein pairings found in virtually every food culture (rice and beans, dal and roti, hummus and pitta).
Importantly, you don't need to combine proteins in the same meal — the body pools amino acids throughout the day. Eating varied plant protein sources across the day achieves adequate EAA intake for most people.
For a detailed comparison of protein quality between plant and animal sources, see our guide to plant vs animal protein.
How Amino Acids Are Used Beyond Building Muscle
Proteins (and therefore amino acids) do far more than build muscle:
- Enzymes: virtually all metabolic reactions are catalysed by protein enzymes.
- Hormones: insulin, growth hormone, and glucagon are proteins.
- Immune function: antibodies are proteins; individual amino acids like glutamine are fuel for rapidly dividing immune cells.
- Transport: haemoglobin (oxygen transport), albumin (drug and fatty acid transport) are proteins.
- Neurotransmitters: tryptophan → serotonin and melatonin; phenylalanine → dopamine and adrenaline; glutamate and GABA are themselves amino acid-derived neurotransmitters.
- Structural: collagen (the most abundant protein in the body), keratin, actin, and myosin are structural proteins.
- Energy: amino acids can be converted to glucose (via gluconeogenesis) or used directly for energy when carbohydrate or fat intake is insufficient.
Protein Quality Scores
Scientists use several metrics to evaluate how well dietary proteins meet human amino acid needs:
PDCAAS (Protein Digestibility Corrected Amino Acid Score): compares EAA content against human requirements, adjusted for digestibility. Max score = 1.0. Egg white, milk protein, and soy all score 1.0.
DIAAS (Digestible Indispensable Amino Acid Score): a newer, more precise version that uses ileal digestibility rather than faecal digestibility. Animal proteins and soy score above 1.0 (can meet more than 100% of requirements per gram); most other plant proteins score 0.4–0.8.
DIAAS is considered more accurate, but PDCAAS is still widely used in regulatory contexts.
Food Sources of Amino Acids
For complete EAA profiles:
- Eggs (best overall bioavailability profile)
- Meat, poultry, fish
- Dairy (casein and whey)
- Soy (tofu, edamame, tempeh)
- Quinoa
For plant-based EAA coverage through complementary proteins:
- Legumes + grains (lentils + rice, hummus + pitta)
- Nuts/seeds + legumes
- Whole grains + dairy (if vegetarian, not vegan)
See our guide to how much protein per day for recommended intakes, and what are macronutrients for the broader macronutrient picture.
Frequently Asked Questions
Do I need to take amino acid supplements? Most people who eat adequate total protein don't need individual amino acid supplements. BCAA supplements have a use case for athletes in fasted training, but for most people eating enough protein from whole foods, they're unnecessary.
Are all 9 essential amino acids in every meal? They don't need to be. The body maintains an amino acid pool throughout the day. As long as your overall daily diet includes adequate EAAs, timing within meals is less important (except for athletes optimising muscle protein synthesis, for whom leucine content per meal does matter).
What happens if you don't get enough essential amino acids? In the short term: reduced protein synthesis, muscle loss, impaired immune function. Chronically: growth retardation (in children), muscle wasting (sarcopenia), poor wound healing, immune deficiency. Specific deficiencies cause specific effects — tryptophan deficiency can impair serotonin production; lysine deficiency impairs collagen formation.
Are plant amino acids inferior to animal amino acids? The amino acids themselves are chemically identical regardless of source. What differs is the amino acid profile and bioavailability of the protein that contains them. Animal proteins generally have higher DIAAS scores, but adequate plant protein can meet all EAA needs with dietary variety and sufficient total intake.
What are BCAAs and do they work? Branched-chain amino acids (leucine, isoleucine, valine) are three of the nine essential amino acids with specific roles in muscle metabolism. BCAA supplements are popular in fitness communities. Evidence: they modestly reduce muscle soreness and fatigue during resistance training, but provide no benefit if total protein intake is already adequate. You're essentially paying for three amino acids when a complete protein source provides all nine.
Sources & References
- Moughan PJ. "Dietary protein quality evaluation in humans — the basis for FAO/WHO digestible indispensable amino acid score." Journal of Nutrition, 2021.
- Wolfe RR. "Branched-chain amino acids and muscle protein synthesis in humans: myth or reality?" Journal of the International Society of Sports Nutrition, 2017.
- WHO/FAO/UNU. "Protein and amino acid requirements in human nutrition." WHO Technical Report Series, 2007.
- Tomé D. "Digestibility issues of vegetable versus animal proteins: protein and amino acid requirements — functional aspects." Food and Nutrition Bulletin, 2013.
- van Vliet S, Burd NA, van Loon LJC. "The skeletal muscle anabolic response to plant- versus animal-based protein consumption." Journal of Nutrition, 2015.