Reference
Tryptophan
**Tryptophan** is an essential amino acid that the human body cannot synthesize and must obtain from food. It serves as the metabolic precursor to serotonin, melatonin, and the vitamin niacin, and it is the least abundant essential amino acid in nearly all dietary proteins.
What tryptophan does in the body
As an essential amino acid, tryptophan is incorporated into structural and functional proteins, but it is best known for its secondary roles. A fraction of ingested tryptophan is converted into 5-hydroxytryptophan and then serotonin, a neurotransmitter, with serotonin in turn serving as the precursor to melatonin. A separate metabolic route, the kynurenine pathway, converts tryptophan into niacin (vitamin B3). Because tryptophan supplies both protein synthesis and these signaling and vitamin pathways, dietary intake is divided across several competing demands.
Why tryptophan is the least abundant essential amino acid
Across most food proteins, tryptophan is present in the smallest quantity of any of the nine essential amino acids. This scarcity makes it a frequent candidate for the limiting amino acid — the essential amino acid in shortest supply relative to requirement, which caps how efficiently the rest of a protein can be used. Its low abundance is also why tryptophan is sometimes underreported: it is partially destroyed during the acid hydrolysis used in standard amino acid analysis and requires separate methods to measure accurately.
The clearest illustration is collagen. Collagen contains no tryptophan at all — a measured content of 0.00 g per 100 g — which classifies it as an incomplete protein under the PDCAAS method (Nutrients, 2019, PMID 31096622). Because of this single missing amino acid, only about 36% of total daily dietary protein can be replaced by collagen peptides while still meeting indispensable amino acid requirements (Nutrients, 2019, PMID 31096622).
Tryptophan and potato protein
Unlike collagen, potato protein isolate supplies all nine essential amino acids, including tryptophan, which is part of why its Digestible Indispensable Amino Acid Score has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID 33133540). A complete amino acid profile means no single essential amino acid — tryptophan included — is missing to limit the protein’s usefulness. For broader context on the source and composition, see what potato protein is and how it is made.
Tryptophan also travels alongside leucine in research on protein quality. In endurance-trained adults, the high-leucine, tryptophan-containing protein alpha-lactalbumin raised myofibrillar protein synthesis 13%±5% more than an equal-nitrogen collagen dose, and only alpha-lactalbumin significantly increased plasma leucine and tryptophan relative to collagen (Med Sci Sports Exerc, 2020, PMID 31895298).
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