AbstractObjectivesHand synovitis, a potentially modifiable pathological lesion, is common and associated with pain and hand OA; nevertheless, its pathogenesis remains uncertain. This study investigated the relationship between gut microbiota dysbiosis and hand synovitis prevalence and evaluated whether bile acids mediate the association.MethodsParticipants were derived from a community-based observational study. Synovitis in each hand joint was assessed using US. Gut microbiota was evaluated using 16S ribosomal RNA amplicon sequencing on faeces, and plasma bile acids were measured by HPLC mass spectrometry. We examined the relationship between gut microbiota dysbiosis and hand synovitis prevalence, as well as the extent to which bile acids were involved in the association.ResultsAmong 1336 participants (mean age: 63.2 years; women: 58.8%), 18.3% had prevalent hand synovitis (unilateral in 13.6% and bilateral in 4.7%). β-diversity, but not α-diversity, of gut microbiota was significantly associated with prevalent hand synovitis. Higher relative abundance of the genus Prevotella and lower relative abundance of the genus Blautia were significantly associated with the prevalence of hand synovitis. Similar associations were also observed for laterality and the number of joints affected by hand synovitis. The association between Prevotella and hand synovitis was partially mediated through its effect on tauroursodeoxycholic acid and glycoursodeoxycholic acid, the mediation proportions being 25.7% and 21.6%, respectively.ConclusionOur findings suggest that gut microbiota dysbiosis is associated with the prevalence of hand synovitis. Such an association appears to be partially mediated by plasma bile acids.