Vitamins are organic substances needed in very small amounts for normal functioning of the body that must be provided in the diet. From early research, beginning with the identification of a fat-soluble growth factor in milk (vitamin A), four fat-soluble factors have been identified as the vitamins A, D, E, and K. These vitamins possess characteristics which, in addition to their solubility, are much different from those of the water-soluble vitamins. The water-soluble vitamins are vitamin C and B-complex vitamins or B-vitamins. All B vitamins, many nonvitamin organic factors (e.g., lipoic acid and ubiquinone), and minerals function as coenzymes, cofactors, and prosthetic groups in enzyme systems (also called holoenzymes). Both retinoids and carotenoids have vitamin A activity. The best known function for vitamin A is its role as a coenzyme with the protein opsin, found in rods and cones, the photoreceptor cells of the eye.