L-threonine is an amino acid your body cannot make on its own. You have to get it from food, and it turns up in plenty of protein-rich sources such as meat, dairy, and legumes. Most people who eat a balanced diet already take in enough without trying.
As a supplement, L-threonine mainly supports protein synthesis. Investigators have looked into its effects on nervous system function, but those uses stay experimental and lack solid backing. No strong clinical evidence points to supplemental L-threonine for specific benefits like gut barrier support or collagen production.
The supplement comes as a capsule or powder. High doses can upset the stomach.