Vitamin A

Vitamin A is a common name for a group of molecules that comprises Retinal, Retinol and Beta-Carotene. Humans have enzymes that can readily convert all of these forms back and forth.

Vitamin A has many uses in the human body, one of which is the use of Retinal as the Chromophore in Opsins. It is therefore required for all sight, and a deficiency of Vitamin A can have adverse effects on vision. However, a surplus of Vitamin A (such as absorbing lots of Beta-Carotene from eating lots of carrots), has not actually shown to be effective at improving vision in any regards. However, it is still a common ingredient in Pseudoscience Colorblindness cures.

The Chromophore used in human Opsins is actually Vitamin A1. Vitamin A2 may also be used as a Chromophore in some species, which leads to a red-shifted and broader Absorption Spectrum. Some species may regularly switch between these two Chromophores as they change environments to maintain some flexibility in their visual system. (Corbo 2021)