Complete Protein
macronutrientA food source containing all 9 essential amino acids in adequate amounts.
Definition
A food source containing all 9 essential amino acids in adequate amounts. Animal proteins (meat, fish, eggs, dairy) are complete; soy and quinoa are plant-based complete proteins.
What Is a Complete Protein?
A complete protein is a protein source that provides all nine essential amino acids in sufficient quantities to meet human physiological needs. The nine essential amino acids — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — cannot be synthesized by the body and must be obtained from dietary protein.
Protein quality is commonly assessed by the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) or the more modern Digestible Indispensable Amino Acid Score (DIAAS), both of which compare a food's amino acid profile against human requirements and adjust for digestibility. A score of 1.0 (or 100%) represents a protein that meets or exceeds all essential amino acid requirements.
Common Complete Protein Sources
Most animal-derived proteins are complete:
- Eggs: often cited as the reference protein; PDCAAS = 1.0
- Dairy (milk, yogurt, whey, casein): all complete proteins; whey has a DIAAS of approximately 1.25
- Meat, poultry, fish: all contain the full essential amino acid profile
- Soy: the most notable plant-based complete protein; PDCAAS = 1.0
- Quinoa: a plant food with a favorable amino acid profile, though somewhat lower in lysine than animal proteins
- Buckwheat, hemp seeds, amaranth: plant sources with relatively complete amino acid profiles
Why It Matters
Consuming complete proteins ensures that no single essential amino acid becomes the limiting amino acid — the one present in the lowest amount relative to requirements — which would restrict the body's ability to synthesize proteins. For athletes seeking to maximize muscle protein synthesis, research indicates that meals containing at least 2–3 g of leucine from a complete protein source optimally stimulate muscle-building pathways.
Related Guides
Plant-Based Diet: Getting Complete Nutrition
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Egg Nutrition: The Complete Guide
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Related Terms
Protein
A macronutrient made of amino acids, essential for building and repairing muscle, organs, and tissues.
Amino Acid
The building blocks of protein.
Incomplete Protein
A protein source that lacks one or more of the 9 essential amino acids.
BCAA (Branched-Chain Amino Acids)
The three essential amino acids — leucine, isoleucine, and valine — with aliphatic side chains.
Lysine
An essential amino acid critical for collagen synthesis, calcium absorption, and antiviral immune function.
Methionine
An essential sulfur-containing amino acid that initiates protein synthesis and serves as a precursor for cysteine, taurine, and the antioxidant glutathione.
Tryptophan
An essential amino acid and precursor to serotonin (mood) and melatonin (sleep).
Threonine
An essential amino acid required for the biosynthesis of glycine and serine, important for collagen and elastin production and immune function.
Phenylalanine
An essential amino acid and precursor to tyrosine, dopamine, epinephrine, and norepinephrine (neurotransmitters).