<p>Beta-carotene, retinol, and retinal have effective and reliable vitamin A activity. Retinal and retinol are in chemical equilibrium in the body and have equivalent antixerophthalmic activity. Retinal combines with the rod pigment, opsin, in the retina to form rhodopsin, necessary for visual dark adaptation.<br />
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Vitamin A prevents retardation of growth and preserves the epithe-lial cells&#39; integrity. Normal adult liver storage is sufficient to satisfy two years&#39;requirements of vitamin A. Vitamin A is readily absorbed from the gastrointestinal tract, where the biosynthesis of vitamin A from beta-carotene takes place. Vitamin A absorption requires bile salts, pancreatic lipase, and dietary fat. It is transported in the blood to the liver by the chy lomicron fraction of the lymph. Vitamin Ais stored in Kupffer cells of the liver mainly as the palmitate. Normal serum vitamin A is 80-300 Units per 100 mL (plasma range is 30-70 mcg per dl) and for carotenoids 270-753 Units per 100 mL.The normal adult liver contains approximately 100 to 300 micrograms per gram, mostly as retinol palmitate.</p>