A Synbiotic-like Combination of Highland Barley β-glucan and Parabacteroides distasonis Modulates Diabetic Retinopathy through Gut-Retina Axis

Diabetic retinopathy (DR) is a significant cause of vision impairment in adults with diabetes, however, effective treatments remain limited. This study investigated the effects of synbiotic supplementation combining Parabacteroides distasonis (P. distasonis) and highland barley-derived β-glucan on retinopathy in a db/db mouse model of type 2 diabetes. In our experimental approach, male db/db mice were orally administered P. distasonis (2 × 10^8 CFU/mL), barley β-glucan (1% dietary supplementation), the combination of both, or PBS daily for 8 weeks. We assessed retinal morphology and vascular integrity through histological analysis, and examined gut microbiota composition via 16S rRNA gene sequencing. Serum bile acid profiles were analyzed using liquid chromatography-mass spectrometry, and gene expression in tissues was quantified by qRT-PCR. We observed that synbiotic treatment improved retinal thickness and attenuated vascular damage compared to untreated diabetic controls. The synbiotic intervention enhanced gut barrier integrity and altered gut microbiota composition. Furthermore, synbiotic supplementation altered serum bile acid profiles, elevating levels of ursodeoxycholic acid (UDCA) and tauroursodeoxycholic acid (TUDCA), which may play protective roles in retinal cells. Mechanistically, these changes were associated with activation of bile acid signaling pathways in the liver and gut, and reduced expression of inflammatory markers in the retina. Our findings suggest that targeted synbiotic intervention may offer a novel therapeutic approach for preventing or treating diabetic retinopathy through modulation of the gut-retina axis and bile acid metabolism.