Role of vitamin A on the ocular surface

Vitamin A is an essential fat-soluble vitamin that cannot be endogenously synthesized by the human body. Retinoic acid (RA) is the biologically active form of vitamin A. Utilizing both nuclear and non-nuclear receptor-mediated pathways, RA plays a crucial role in regulating various biological processes, including apoptosis, differentiation, and anti-inflammatory properties within the cornea and conjunctiva. In addition, RA has been demonstrated to exert a significant influence on anti-tumor mechanisms. Disruption of RA signaling can result in corneal defects, anophthalmia, and microphthalmia. However, the beneficial effects of RA are only observed when it is administered at appropriate dosages, and higher doses have an adverse impact. Ocular abnormalities are often early indicators of a vitamin A deficiency. The lacrimal gland secretes vitamin A onto the ocular surface, where it is metabolized into RA via two sequential steps. This article provides a comprehensive overview of how vitamin A is transformed and transported from the intestine to the ocular surface, ultimately contributing to the maintenance of the normal physiological function of the ocular surface.