Vitamin A is an important fat-soluble vitamin that occurs in various chemical forms. The different forms of vitamin A can be converted from one form to another with the help of specific enzymes. Its two main biologically active forms are 11-cis-retinol and all-trans-retinoic acid. 11-cis-retinol’s primary role is in visual perception. All-trans retinoic acid is an essential signaling molecule in cell differentiation, proliferation, and apoptosis and is involved in gene transcription. It is crucial for reproduction, embryonic growth and development, normal brain function, immune system functioning, bone development, hair and nail growth, and RNA synthesis in epithelial cells, including the conjunctiva and cornea.

One of the most important functions of vitamin A is its involvement in visual phototransduction as a key part of photopigments, the first molecule in the process of transforming photons of light into electrical signals. In this process, large quantities of vitamin A in the form of 11-cis-retinal are being isomerized to all-trans-retinal and then quickly recycled back to 11-cis-retinal. A complex machinery of transporters and enzymes is involved in this process (i.e., the visual cycle). Any fault in the machinery may not only reduce the efficiency of visual detection but also cause the accumulation of toxic chemicals in the retina.

In conclusion, the normal amount of vitamin A is essential for good vision, but too much or too little can be bad. Vitamin A needs to be supplemented in night blindness due to vitamin A deficiency but should be avoided in Stargardt’s disease (STGD1, also known as fundus flavinoma or ABCA4-retinopathy), where toxic products of vitamin A accumulate excessively.

Sajovic, J., Meglič, A., Glavač, D., Markelj, Š., Hawlina, M., & Fakin, A. (2022). The role of vitamin A in retinal diseases. International journal of molecular sciences, 23(3), 1014. [Link]