Reference
Complete Protein
**Complete protein** is a dietary protein source that supplies all nine essential amino acids in amounts sufficient to meet human requirements. A protein lacking adequate quantities of even one essential amino acid is classed as "incomplete," because the deficient amino acid limits how much of the protein the body can use for tissue synthesis.
The nine essential amino acids
The body cannot synthesize nine amino acids — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — so they must come from food. A complete protein contains all nine in ratios close to what the body needs. Animal proteins such as whey are complete, providing all nine essential amino acids in a single source.
Of the nine, leucine is the primary amino acid trigger for muscle protein synthesis, which is one reason protein quality is judged on amino acid composition rather than total protein content alone.
How completeness is measured
Two scoring systems quantify protein quality. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) rates a protein against human amino acid requirements, adjusted for digestibility, on a scale capped at 1.00 — values above this ceiling are truncated to 1.00 (Journal of Nutrition, Schaafsma, 2000, PMID:10867064). The Digestible Indispensable Amino Acid Score (DIAAS) is a newer measure of amino acid availability and can exceed 100%. High-quality animal proteins such as milk, whey, egg, and casein reach this 1.00 ceiling on PDCAAS, as does soy protein isolate.
Why most plant proteins are incomplete
Plant proteins generally have lower quality scores than animal proteins because most are deficient in one or more essential amino acids. Wheat gluten, for example, scores around 0.25 on PDCAAS, while egg protein scores 1.00 under the established FAO/WHO reference values. Peas are typically limited by the sulfur amino acids methionine plus cysteine, which average a chemical score of only 46% (Foods, 2024). The Academy of Nutrition and Dietetics notes that a varied assortment of plant foods eaten across a day can together supply all essential amino acids (Journal of the American Dietetic Association, Craig & Mangels, 2009, PMID:19562864).
Potato protein as a plant exception
Potato protein isolate is a notable single-source plant protein that approaches completeness. Its PDCAAS is among the highest of any vegetable protein source, and DIAAS for potato protein isolates has been reported as high as 100% (Food Science & Nutrition, Herreman et al., 2020, PMID:33133540). A 2020 trial found that 25 g of potato protein isolate stimulated muscle protein synthesis at rest and after resistance exercise in young women, consistent with a high-quality protein (Nutrients, 2020, PMID:32349353).
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