Benefits
Collagen and Elastin Synthesis
L-Threonine is one of the amino acid building blocks the body uses to make collagen and elastin, the proteins in skin, tendons and connective tissue. That is basic biochemistry, not a tested effect. No human study of threonine supplements and skin, hair, nails, joints or tendons is cited on this page, so there is no evidence that taking extra does more than eating enough protein.
Mucin / Gut Mucosal Layer
L-Threonine is a major component of mucin glycoproteins — the protective mucosal layer lining the GI tract. The cited research is entirely in pigs: a feeding study in growth-restricted weanling piglets found more intestinal mucin, and a laboratory study used pig intestinal cells in a dish. No human study has tested this.
Immune Function
Threonine is used to build antibodies, and a severe shortage impairs antibody production, which is rare on a protein-adequate diet. The immune findings cited here come from piglets and from pig intestinal cells in a dish, not from people. No cited study shows that extra threonine improves immunity in a well-fed person.
Studied in a Rare Inherited Nerve Condition (Not a Consumer Benefit)
This is medical research, not a benefit for healthy people. In 1991 a double-blind crossover trial gave 4.5 to 6.0 grams a day of L-threonine to 18 people with familial spastic paraparesis, a rare inherited condition that causes muscle stiffness. Signals were modest, the trial was tiny and more than 30 years old, and it never became standard care, which is still physical therapy and prescription medicines. Nothing here means a supplement treats any disease.
Glycine Pathway: Theoretical Only
Because the body can convert threonine into glycine, researchers once explored it in serious neurological illness. No trial supporting that is cited on this page, and no dietary supplement treats, slows or prevents a neurodegenerative disease. Anyone facing a neurological diagnosis should be guided by a neurologist.
Mechanism of action
Protein Synthesis
Essential amino acid required for ribosomal protein synthesis. Hydroxyl side chain provides O-glycosylation sites in mucins and other glycoproteins.
Glycine Pathway
L-Threonine can be metabolized to glycine via threonine dehydrogenase + serine hydroxymethyltransferase. This is a proposed, unproven explanation for why threonine was once tested in neurological research. It has not been confirmed in people, and no cited study measured glycine receptor activity in humans.
Mucin Synthesis
Threonine is heavily represented in mucin protein backbones (alongside serine and proline) — the heavily-glycosylated proteins forming the protective mucosal layer in the GI tract and respiratory tract.
Lipotropic Effects
Older animal nutrition work describes threonine as a lipotropic factor alongside choline, methionine and inositol, meaning diets short of it led to fat building up in the liver. No study cited on this page tested this in humans, and none tested a threonine supplement for liver fat.
Clinical trials
Double-blind, placebo-controlled crossover trial of L-threonine at 4.5 to 6.0 grams a day (Growdon 1991, Clinical Neuropharmacology). The dose was lower than the 7.5 grams a day quoted elsewhere for this use.
18 people with familial spastic paraparesis, a rare inherited neurological condition. No healthy volunteers were studied.
Modest changes in muscle stiffness in some participants, in a group of only 18 people and more than 30 years ago. The result was not consistently replicated and never became standard care, which remains physical therapy and prescription muscle relaxants such as baclofen or tizanidine. This is the only human trial of L-threonine supplementation cited anywhere on this page.