Abstract
<jats:p>Isolated REM sleep behavior disorder (iRBD) is among the most reliable prodromal markers of α-synucleinopathies, yet accessible biomarkers of phenoconversion risk are lacking and the fecal metabolome remains poorly investigated in this population. In this exploratory, cross-sectional study of 21 iRBD patients and 15 healthy controls, we characterized fecal samples through an integrated multi-omics framework. The fecal metabolome was profiled on two complementary mass spectrometry platforms: HS-SPME GC-EI-high-resolution MS, used for both untargeted volatilomics and targeted quantification of linear and branched short-chain fatty acids, and untargeted LC-MS/MS for the non-volatile fraction. Gut microbial community composition was characterized on the same material by 16S rRNA gene sequencing, and the metabolomic and microbial layers were combined by DIABLO multi-omics integration. Linear short-chain fatty acids were preserved, whereas branched-chain fatty acids were selectively increased in iRBD, indicating a shift toward proteolytic fermentation. Volatile indole was reduced, while p-cresol and 2-octanone were increased, paralleled at the LC-MS/MS level by attenuation of the tryptophan-indole axis, class-level bile acid depletion, and increased acylcarnitines. The microbiome was depleted of Blautia and Faecalibacterium and enriched in Oscillospirales. Integration resolved a coherent host-microbe signature discriminating groups with 84.8% cross-validated accuracy, positioning fecal metabolomics as a sensitive window into prodromal gut-brain interactions.</jats:p>