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Incomplete Protein
**Incomplete protein** is a dietary protein source that does not supply adequate amounts of one or more of the nine essential amino acids (EAAs) relative to human requirements. Because the missing or under-represented amino acid limits how efficiently the body can use the rest, it is also called the limiting amino acid.
Limiting amino acids: cereals and legumes
The concept rests on a simple constraint: muscle and tissue synthesis can proceed only as far as the scarcest essential amino acid allows. Two food groups illustrate the pattern. Cereal grains — wheat, rice, corn, oats — are typically limited in lysine. Legumes — beans, peas, lentils — are generally limited in the sulfur amino acids methionine and cysteine.
The numbers are specific. In an analysis of pea (Pisum sativum) genotypes, the limiting amino acid was the sum of methionine plus cysteine, averaging only 2.6 g per 100 g of protein — a chemical score of 46% (Molecules, 2024, PMID 39519674). Collagen is incomplete for a different reason: it lacks the essential amino acid tryptophan entirely, measured at 0.00 g per 100 g, which categorizes it as an incomplete protein under the PDCAAS method (Paul et al., Nutrients, 2019, PMID:31096622). Wheat gluten, limited in lysine, scores roughly 0.25 on PDCAAS.
Protein complementation
Foods with opposite limiting amino acids can be combined so that one supplies what the other lacks — a principle known as protein complementation. Grains low in lysine pair with legumes rich in it; legumes low in methionine pair with grains that carry more. The pairing does not need to occur in the same meal. The Academy of Nutrition and Dietetics states that plant protein can meet requirements when a variety of plant foods is consumed and energy needs are met, with an assortment eaten over the course of a day providing all essential amino acids and ensuring adequate nitrogen retention in healthy adults (Craig & Mangels, Journal of the American Dietetic Association, 2009, PMID:19562864).
Relevance to potato protein
Most single plant sources are incomplete, which is why protein quality scores generally run lower for plant than for animal proteins. Potato protein isolate is an exception among vegetable sources: its PDCAAS is among the highest of any plant protein, and its DIAAS has been reported as high as 100 (Herreman et al., Food Science & Nutrition, 2020, PMID:33133540). This profile means it does not depend on complementation to deliver a usable spread of essential amino acids — the reason it is treated as a standalone source rather than one half of a pairing. For background on how the isolate is produced and used, see what potato protein is.
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