Threonine

What Is Threonine?

Threonine is an essential amino acid with a hydroxyl group on its side chain, classifying it as a polar, aliphatic amino acid alongside serine and tyrosine. The body cannot synthesize threonine and must obtain it through dietary protein. It was the last of the eight classic essential amino acids to be identified, discovered in 1935 by William Cumming Rose. The Recommended Dietary Allowance (RDA) for threonine in adults is 20 mg per kilogram of body weight per day. Threonine is particularly concentrated in skeletal muscle, the central nervous system, and immune tissues.

Key Functions

Threonine participates in a diverse array of biological functions:

• Protein structure: Threonine's hydroxyl group can be phosphorylated and glycosylated (attachment of carbohydrate chains), making it a critical site for post-translational modifications that regulate protein function, cell signaling, and cell-surface recognition.
• Mucin glycoprotein synthesis: Threonine is enriched in mucins, the glycoproteins that form the protective mucus lining of the gastrointestinal and respiratory tracts. Adequate threonine intake is essential for maintaining gut barrier integrity and mucosal immunity.
• Glycine and serine production: Threonine can be catabolized to produce glycine and serine, amino acids important for collagen synthesis, glutathione production, and one-carbon metabolism.
• Immune function: Threonine supports the production of immunoglobulins (antibodies) and is important for the function of the thymus gland and T lymphocytes.
• Liver fat metabolism: Threonine plays a role in hepatic lipid metabolism. Deficiency has been associated with fatty liver, as it is required for the synthesis of phosphatidylcholine, which facilitates lipid export from the liver as VLDL particles.

Food Sources

Threonine is found in most complete protein sources. Particularly rich sources include:

• Beef (cooked): approximately 1.3 g per 100 g
• Chicken breast: approximately 1.1 g per 100 g
• Cottage cheese (low-fat): approximately 0.7 g per 100 g
• Lentils (cooked): approximately 0.5 g per 100 g
• Pumpkin seeds: approximately 0.7 g per 100 g
• Quinoa (cooked): approximately 0.3 g per 100 g

Threonine is considered the second limiting amino acid in cereal-based diets after lysine, making it especially relevant for populations whose diets rely heavily on wheat, rice, or maize without adequate complementary protein sources.