Postprandial stability and individual variation in plasma indole metabolites linked to microbial tryptophan catabolism after standardized feeding

Gut microbiota-derived indoles are increasingly recognized for their roles in metabolic, neurological, and inflammatory disorders, making them an active area in microbiome research. However, the temporal and post-prandial stability of indole levels has not been tested. In this study, plasma samples were collected from eight healthy male participants who had fasted overnight and consumed a standardized breakfast. Four tryptophan metabolites were measured at the end of the fasting period and at seven postprandial timepoints (0.5–8 h) to measure kynurenine (KYN), indole-3-acetic acid (IAA), indole-3-propionic acid (IPA), and indolelactic acid (ILA). For validation, an independent cohort ( n = 18, male) with samples (2 per individual) collected 8 weeks apart was included to validate individual consistency. All four metabolites exhibited significant inter-individual variability, but no postprandial or temporal fluctuations were observed. Strong correlations between timepoints 8 weeks apart validated temporal stability. Spearman's correlation showed moderate associations between KYN and ILA and between IAA and ILA. To assess correlations between microbial function and plasma indole levels, samples were stratified into two groups: high (High_indole) and low (Low_indole) based on median plasma indole concentrations. Shotgun metagenomic sequencing revealed that enzymes involved in microbial tryptophan catabolism, such as tryptophanase, aldehyde dehydrogenase, tryptophan aminotransferase, indolelactate dehydrogenase, indoleacetamide hydrolase, and indolepyruvate decarboxylase, were more abundant in the High_indole group. These genes were primarily contributed by species such as Akkermansia muciniphila , and Bacteroides spp. These results highlight the temporal stability of plasma indole metabolites and the need to account for individual variation when investigating their circulating levels.