Anthocyanins as Adjunctive Dietary Modulators of the Gut–Eye Axis: Bioavailability, Biotransformation, and Implications for Ocular Health

Retinal diseases such as age-related macular degeneration (AMD) and diabetic retinopathy (DR) are major causes of visual impairment and are closely associated with oxidative stress, inflammation, vascular dysfunction, and metabolic imbalance. Increasing evidence suggests that gut microbiota also contributes to retinal homeostasis, supporting the emerging concept of the gut–eye axis. In this context, dietary anthocyanins—with blueberry anthocyanins serving as a primary representative model—have attracted attention as potential adjunctive nutritional modulators of ocular health. However, their biological effects are strongly influenced by their limited bioavailability and extensive gastrointestinal metabolism. The objective of this review is to analyze the gastrointestinal fate of dietary anthocyanins and to discuss how their absorption, enzymatic transformation, and microbial biotransformation may influence ocular protection through the gut–eye axis. The review summarizes current knowledge regarding anthocyanin stability in the oral cavity, stomach, small intestine, and colon, as well as the formation of circulating phenolic metabolites generated by the host and through microbial metabolism. In addition, the molecular mechanisms through which anthocyanins and their metabolites may support retinal health are examined, including antioxidant, anti-inflammatory, vasoprotective, and neuroprotective actions. Overall, dietary anthocyanins, illustrated through the rich profile of blueberries, represent promising adjunctive compounds for supporting ocular health, although further clinical and mechanistic studies are still required.