Retinol then attaches to a serum carrier, retinol binding protein, for transport to target tissues. Retinol enters the absorptive cells of the small intestine, preferentially in the all-trans-retinol form. Topical retinoids include adapalene, isotretinoin, retinol, tazarotene, trifarotene, and tretinoin. These are known as retinoids and include retinaldehyde and retinol. The lipid droplets in the cell body store vitamin A as retinol ester. When eaten, beta-carotene is a provitamin, converted to retinol (vitamin A). Allantoin, lactic acid, retinol, and squalene, for example, can be vegan. Retinol is in the vitamin A family. For this reason, amounts of vitamin A are measured in Retinol Equivalents (RE). Non-polar molecules, such as retinol or lipids, are poorly soluble in water. Retinal is then reduced to retinol by the enzyme retinol dehydrogenase. Traditionally, RBP is synthesized within the liver with secretion being dependent upon retinol concentrations. Retinol dehydrogenase is an enzyme that is dependent on NADH. Retinol at normal doses is well tolerated.