aging immunosenescence

# The Gut–Eye Axis and Ocular Health The emerging concept of a **gut–eye axis** recognizes that gut microbes and their products can affect the eye. Gut bacteria ferment fibers to produce **short-chain fatty acids (SCFAs)** (like acetate, propionate, butyrate) and modify bile acids (BAs). These metabolites enter the circulation and can reach the eye, influencing its immune environment and function ([pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC10488056/#:~:text=derived%20metabolites%20involved%20in%20counteracting,the%20bile%20acid)). For example, microbial dysbiosis – an imbalance in gut flora – has been linked to ocular diseases from age-related macular degeneration and uveitis to dry eye and glaucoma ([pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC10488056/#:~:text=Moreover%2C%20recent%20studies%20underline%20a,better%20management%20of%20these%20diseases)). In fact, a recent survey found that gut imbalance is associated with multiple eye conditions, and only a handful of early trials (four of 25 studies) have tested interventions like probiotics or fecal transplants on eye disease ([pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC10516887/#:~:text=ocular%20pathology,clinical%20trials%20may%20be%20warranted)). This gut–eye axis suggests that gut-derived SCFAs, BAs, and even inflammatory components (like LPS) could modulate ocular **immune tone** (the baseline immune status) and affect tissues like the trabecular meshwork (the fluid drainage filter) and intraocular pressure (IOP). ## Microbial Metabolites and Ocular Immunity ### Short-Chain Fatty Acids (SCFAs) **SCFAs** are fatty acids with fewer than six carbon atoms, mainly acetate, propionate, and butyrate, produced by gut bacteria digesting fiber. They **regulate immune responses** systemically ([www.frontiersin.org](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2024.1377186/full#:~:text=SCFAs%20can%20ameliorate%20immune,often%2C%20metabolites%20and%20inflammation%20go)) ([pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC10488056/#:~:text=derived%20metabolites%20involved%20in%20counteracting,the%20bile%20acid)). In the eye, SCFAs exert anti-inflammatory effects. In mouse models, injected SCFAs were detected in ocular tissues and *reduced* inflammation from endotoxin (LPS) exposure ([pubmed.ncbi.nlm.nih.gov](https://pubmed.ncbi.nlm.nih.gov/33617852/#:~:text=responses%20of%20the%20eye%20and,functions%20in%20the%20intraocular%20milieu)) ([pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC10488056/#:~:text=derived%20metabolites%20involved%20in%20counteracting,the%20bile%20acid)). This shows SCFAs can cross the blood–eye barrier via blood and calm intraocular inflammation. For instance, intraperitoneal butyrate in mice dampened LPS-induced uveitis, reducing pro-inflammatory cytokines and boosting regulatory T cells ([pubmed.ncbi.nlm.nih.gov](https://pubmed.ncbi.nlm.nih.gov/33617852/#:~:text=resp

The Gut–Eye Axis: Probiotics, Metabolites, and Intraocular Pressure

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