The basic molecule of vitamin A is retinol, or vitamin A alcohol (figure 1), which is present in foods and tissues mainly as esters combined with long-chain fatty acids such as palmitic acid. The richest food source is liver, with other animal and fish sources providing substantial amounts of the preformed vitamin. After absorption retinol is transported via the chylomicrons to the liver, where it is either stored as retinyl esters, or re-exported into the plasma as the free alcohol in combination with retinol-binding protein, from which it is delivered to critical tissue sites. Two important active metabolites are retinal (or retinaldehyde), the active element of visual pigment, and retinoic acid, an intracellular messenger that modulates cell differentiation. Figure 2 illustrates the conversion of all-trans-[3-carotene to substances with vitamin-A-related activity.