Reference
Limiting Amino Acid
**Limiting amino acid** is the essential amino acid present in the smallest amount relative to the human requirement for it, which caps how much of a dietary protein the body can use for tissue synthesis.
How it works
Protein synthesis requires all nine essential amino acids together, in proportion. If one is in short supply, synthesis stalls at the point that amino acid runs out — the rest are deaminated and used for energy or stored, not built into new protein. The single scarcest essential amino acid therefore sets the ceiling, regardless of how abundant the others are. This is why the limiting amino acid governs a protein’s biological quality more than its total amino acid content.
How it sets quality scores
Protein quality scores are built directly on this concept. The amino acid score compares a food’s amino acid profile against a reference pattern; the lowest ratio — the limiting amino acid — becomes the score. In PDCAAS, that lowest amino acid score is then corrected for the protein’s digestibility, with values capped (truncated) at 1.00 (Schaafsma, Journal of Nutrition, 2000, PMID:10867064). A protein with a zero amino acid score yields a net protein utilization of roughly 25%, meaning four times the minimal amount would be needed to meet requirements (Hegsted, FAO/WHO). Wheat gluten, limited by lysine, scores around 0.25 on PDCAAS, while egg protein reaches 1.00 — both established FAO/WHO reference values.
Common limiting amino acids by food group
The limiting amino acid is predictable by plant family. Cereal grains are typically limited by lysine. Legumes are usually limited by the sulfur amino acids methionine and cysteine. In a survey of new pea genotypes, lysine was the most abundant amino acid at an average 7.9 g per 100 g protein, while methionine plus cysteine averaged only 2.6 g per 100 g protein — a chemical score of 46% (Foods, 2024). Combining complementary sources, such as a lysine-rich legume with a methionine-richer grain, raises the overall score. Collagen is a separate case: it lacks tryptophan entirely (0.00 g per 100 g), making it an incomplete protein under PDCAAS (Nutrients, 2019, PMID:31096622).
Relevance to potato protein
Potato protein isolate contains all nine essential amino acids and reports an amino acid score of 65% (PMID:34507729), among the higher values for a single plant source. Its DIAAS has been reported as high as 100 (Herreman et al., Food Science & Nutrition, 2020, PMID:33133540), placing it in the range of animal proteins for adults. For a deeper look at the source itself, see our guide on what potato protein is.
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