What is retinoids?

Human body gets its vitamin A from two classes of chemicals:
Retinoids
Carotenoids

The retinoids have been recognized for over 50 years for their profound impact in biological functions. The term retinoids refers to compounds whose names all start with ret: retinol, retinaldehyde, retinoic acid, and other chemically similar cofounds, both natural and synthetic.

These fat soluble substance are found in several foods of animal origin: liver and whole milk, eggs and butter. Retinoids give preformed vitamin A, the kind of nutrient that body can use right away.

The retinoids also can be synthesized by cleavage from the pro-vitamin beta-carotene.

Retinoid acid, for example, can substitute for vitamin A in maintenance of epithelial tissue and growth but is not adequate for vision or reproduction.

Retinoids also play significant roles in epithelial cell differentiation and immune function.

The standard unit for quantifying the biologic activity of the various forms of vitamin A and its precursors is known as a retinal activity equivalent (RAE).
What is retinoids?