Amino acid

Building proteins is the best-known role, but not the only one. A review in the journal Amino Acids describes amino acids as cell signaling molecules and regulators of gene expression, and as precursors for hormones and other small nitrogen-containing substances [1]. Its examples of amino acid products include nitric oxide, glutathione, taurine, thyroid hormones and serotonin [1]. The review groups arginine, cysteine, glutamine, leucine, proline and tryptophan as "functional" amino acids that regulate key metabolic pathways [1].

Levels in the body matter too. The same review notes that raised levels of some amino acids and their products in the body, such as ammonia and homocysteine, are factors in disease [1]. For supplements, a 2026 review explains that upper levels of safe intake are set from no-observed-adverse-effect levels found in studies [2].

Several compounds on NootropicsWork are single amino acids. Tyrosine is studied because it is the precursor to the neurotransmitters dopamine and norepinephrine, according to a systematic review of 15 studies [3]. Not every amino acid ends up in protein: a review describes L-theanine as a nonproteinic amino acid specific to tea [4].

When a page calls a compound an amino acid, that describes its chemistry. It says nothing on its own about whether a supplement of it changes an outcome, which is what the graded evidence on each compound page covers.

• L-theanine: an amino acid from tea that is not used to build proteins [4].
• Dopamine: one of the neurotransmitters made from tyrosine [3].