MO043IDENTIFICATION OF INDOXYL SULFATE PRODUCER PHENOTYPES BY ORAL TRYPTOPHAN CHALLENGE TEST: A MICROBIOTA-BASED PERSONALIZED NUTRITIONAL APPROACH
Authors: Lin Ting-Yun , Hung Szu-Chun 
Published: June 2020

Abstract

Background and Aims
Indoxyl sulfate (IS), one of the most representative gut-derived uremic toxins, has been implicated in the pathogenesis of cardiovascular disease in patients with chronic kidney disease (CKD). However, current approaches to IS reduction are not optimized. IS is converted from indole, a gut bacterial metabolite of dietary tryptophan, by two hepatic enzymes CYP2E1 and SULT1A1 (Figure 1A). Given that the interplay between microbes and host is associated with various phenotypes of medical importance, we hypothesized that the effects of a particular diet on individuals differ from person to person. In the present study, we measured individualized IS production using a fixed-dose loading of tryptophan. We aimed to characterize variable IS producer phenotypes and identify person-specific host and microbiome features associated with the variability.

Method
We developed an oral tryptophan challenge test (OTCT) to determine the individual IS producing capacity. Plasma and urine samples were collected at baseline after overnight fasting, and were then collected at 4, 8, 12, 24, 36 and 48 hours after oral administration of L-tryptophan at a dose of 2 gm. All participants were on a low tryptophan diet during the pharmacokinetic study of IS production following OTCT. Positive (above baseline) components of the area under the curve (AUC) were calculated to indicate IS production because of the variable nature of the baseline IS. Information on dietary patterns was obtained using a semiquantitative food frequency questionnaire (FFQ) validated for assessment of major nutrients in the diets of Taiwanese vegetarians and omnivores. Tryptophan and IS concentrations were quantified by high performance liquid chromatography. CYP2E1 and SULT1A1 single nucleotide polymorphism genotyping was analyzed by TaqMan PCR assay. The gut microbiome was assessed by metagenomic next-generation sequencing.

Results
We enrolled 40 healthy volunteers to undergo the OTCT. The mean age of the participants was 33 ± 7 years; 27.5% were men (n=11), and 40% were vegetarians (n=16). Participants were stratified into quartiles of baseline-adjusted AUC of IS. High interpersonal variability in IS production following OTCT was observed (Figure 1B). The 10 participants in the highest quartile (Q4) were defined as high IS producers. The other 30 participants in the Q1, Q2, and Q3 were defined as low IS producers (Figure 1C, 1D). The high IS producers were all female (n=10; P=0.025) and more likely to be omnivores (n=8; P=0.136). Otherwise, there was no difference in the age, body mass index, blood pressure, eGFR, fasting blood glucose, lipid profiles, and baseline plasma IS concentrations between the 2 groups. The high and low IS producers were different with respect to the principal coordinate analysis of β-diversity of the gut microbiota. A prediction model was constructed based on the individual IS producing capacity as contributed by the gut microbiome features and host genotypes.

Conclusion
We found high variability of IS in the response to OTCT across individuals, suggesting that universal dietary recommendations may have limited utility. High IS producers should avoid consuming foods that contain high levels of tryptophan. Therefore, the OTCT can be used to identify IS-producer phenotypes and may serve as a personalized dietary guidance and optimize food choices. Potential clinical applications for the OTCT require further studies to validate its use in patients with CKD.