Potential Association Between Glucosamine Supplementation and Gut Microbiota Composition in Middle-Aged Japanese Adults: A Cross-Sectional Analysis Using 16S rRNA Sequencing

Background: This study examined potential associations between daily glucosamine (GlcN) use and the gut microbiota. Methods: In a cross-sectional sample of 200 Japanese adults aged 50–59 (100 regular GlcN users; 100 age- and sex-matched non-users), fecal 16S rRNA profiles were analyzed. α-diversity (richness, Shannon index) and β-diversity were compared; predominant genera (>1% mean abundance) were evaluated, and differential abundance was tested using ANCOM and LEfSe. A subgroup analysis assessed normal-BMI participants (18.5–25.0). Results: Overall α-diversity and β-diversity did not differ between users and non-users. Of the >150 genera detected, 22 exceeded 1% mean abundance, and their profiles were virtually identical across groups; neither ANCOM nor LEfSe identified taxa with significant differential abundance. In the normal-BMI subgroup, GlcN users showed lower species richness and a reduced prevalence of rare taxa (<10,000 total reads), with a decrease in Christensenella, which is consistent with prior intervention studies. Given reports linking Christensenella to lower frailty in specific contexts, these findings suggest a potential GlcN-related effect relevant to joint health. Conclusions: Habitual GlcN use was not strongly associated with broad changes in gut microbial diversity or common taxa in middle-aged adults, although it might have some effect on minor taxa. Longitudinal, controlled studies are warranted to confirm these findings and clarify mechanisms.