Compares effects of free and protein-bound tryptophan on metabolites in rats, indicating differences in metabolism and potential supplementation strategies.
Tryptophan (Trp) is essential for homeostasis, but direct intake may cause risks. This study compared metabolic kinetics between free Trp and protein-bound Trp from soy protein isolate (SPI) and sheep whey protein (SWP). A novel UPLC-TQ/MS assay was built to quantify Trp and its metabolites simultaneously: 5-hydroxytryptophan (5-HTP), kynurenic acid (Kyna), indole-3-acetic acid (IAA), and indole-3-propionic acid (IPA). Pharmacokinetic profiling showed free Trp peaked rapidly in rats. Tissue distribution exhibited distinct enrichment: Trp in heart and adipose, 5-HTP in kidney and epididymal fat, Kyna in kidney and brown fat, and IAA/IPA in brain. Relative to free Trp, SPI and SWP delayed metabolite release and boosted microbial Trp conversion to IAA and IPA. SPI activated the 5-hydroxytryptamine pathway, raising plasma 5-HTP and fecal IAA; SWP favored the kynurenine pathway, elevating plasma Kyna and fecal IPA. These findings highlight differential outcomes of free versus protein-bound Trp supplementation.
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Zhu et al. (2026) studied this question.
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