# Fasting-Induced Changes in Serum Kynurenines Do Not Always Reflect Their Urinary Excretion

## TL;DR

Fasting increases most serum kynurenines but does not always increase their urinary excretion, suggesting complex regulation of these metabolites.

## Contribution

This study is the first to compare fasting effects on serum and urinary kynurenines, revealing discrepancies in excretion patterns.

## Key findings

- Fasting increased serum 3-HK, AA, PA, KYNA, and XANA but decreased KYN and QA.
- Urinary excretion increased for KYN, 3-HK, XANA, and QA but not for AA and PA.
- Serum KYN and QA decreases may result from enhanced urinary elimination.

## Abstract

Background: The effects of fasting on serum kynurenines (KYNs) have been reported; however, no data are available on whether fasting also modifies their urinary excretion. Kidney organic anion transporters are involved in KYNs excretion, suggesting that changes in serum levels may result from altered urinary elimination. Considering the important role of KYNs in regulating various physiological processes, it is crucial to understand the factors that determine their blood concentrations. The present study aimed to determine the effect of an 8-day fasting period on the concentrations of KYNs in both serum and urine. Methods: Thirteen participants underwent an 8-day fast. The exercise test was performed at baseline after an overnight fast and after 8 days of fasting. Results: Fasting increased the serum concentrations of 3-hydroxykynurenine (3-HK), anthranilic acid (AA), picolinic acid (PA), kynurenic acid (KYNA), and xanthurenic acid (XANA). Conversely, serum kynurenine (KYN) and quinolinic acid (QA) decreased, while 3-hydroxyanthranilic acid (3-HAA) remained unchanged. In urine, KYN, 3-HK, XANA and QA increased after fasting, whereas AA and PA did not change. Conclusions: In conclusion, these findings indicate that fasting generally increases serum kynurenines (KYNs), which are associated with enhanced urinary excretion, suggesting that fasting may stimulate their synthesis. In the case of anthranilic acid (AA) and picolinic acid (PA), their increase in serum does not influence their urinary excretion. Conversely, a decrease in serum KYN and quinolinic acid (QA) may result from enhanced urinary excretion.