Reference
Threonine
**Threonine** is one of the nine essential amino acids — meaning the human body cannot synthesize it and must obtain it from dietary protein. It is a polar amino acid carrying a hydroxyl group, and it is heavily used in the production of mucin, the maintenance of the intestinal barrier, and the assembly of connective tissue proteins.
What threonine does in the body
Threonine is incorporated into a wide range of structural and secreted proteins. It is particularly concentrated in mucins — the heavily glycosylated proteins that form the protective mucus layer lining the gut. Because mucin turnover in the intestine is rapid and continuous, the gut is a major consumer of dietary threonine, with a substantial fraction extracted before it reaches systemic circulation.
Beyond the gut, threonine contributes to collagen and elastin, the proteins that give skin, tendons, and other connective tissues their tensile properties. The hydroxyl side chain also serves as an attachment point for O-linked glycosylation and as a site that regulatory enzymes can phosphorylate, making threonine relevant to cell signaling as well as structure.
Threonine in food proteins
Threonine content varies by source. In a porcine collagen sample analyzed in Nutrients (2019), threonine was reported at 1.96 g/100 g, alongside isoleucine at 1.61 and leucine at 2.51 g/100 g — modest figures that, together with collagen’s complete absence of tryptophan, classify collagen as an incomplete protein (Paul C, Leser S, Oesser S. Nutrients 2019, PMID: 31096622).
Most intact plant and animal protein isolates supply threonine in adequate proportion relative to human requirements, so it is rarely the limiting amino acid. Potato protein isolate is one such complete source; its Digestible Indispensable Amino Acid Score has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID: 33133540). For a broader overview of the ingredient, see what potato protein is and how it is made.
Why threonine matters for protein quality
Because threonine is essential and disproportionately consumed by the digestive tract, a protein source’s threonine adequacy affects its overall quality scoring. A protein deficient in threonine would have its amino acid score reduced, regardless of total protein content. In practice, threonine is rarely the bottleneck — sulfur amino acids such as methionine more commonly limit plant proteins — but its role in gut and connective tissue maintenance makes consistent intake worthwhile.
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