<unk>  is an essential human nutrient. It exists not as a single compound, but in several forms. In foods of animal origin, the major form of<unk>  is an alcohol (retinol), but can also exist as an aldehyde (retinal), or as an acid (retinoic acid). Precursors to the<unk> a provitamin) are present in foods of plant origin as some of the members of the carotenoid family of compounds.
All forms of<unk>  have a Beta-ionone  ring to which an isoprenoid chain is attached. This structure is essential for<unk> ctivity.
retinol, the animal form of<unk> , is a yellow, fat-soluble,<unk> ith importance in vision and bone growth.
other retinoids, a class of chemical compounds that are related chemically to<unk> , are used in medicine.
Vitamin A
Vitamin C
Vitamin B6
B vitamins
Vitamin D
Thiamin
Thiol
Cholic acid