Vitamin A

Vitamin A occurs in three forms: retinol, retinal, and retinoic acid. Retinol is the alcohol form of vitamin A. Precursors of vitamin A, the carotenes, occur as orange-yellow pigments, mainly in green leaves and to a lesser extent in corn.

The classical function of vitamin A is its role in night vision, and is why night blindness is a classical vitamin A deficiency symptom reported in horses. Vitamin A functions in cell differentiation by the regulation of gene expression via nuclear retinoic acid receptors, and as a result plays crucial roles in reproduction and embryogenesis. Additionally, vitamin A is important for maintaining the innate and adaptive immune response to infection.