Vitamin A

Beta-carotene, retinol, and retinal all show reliable vitamin A activity. Retinal and retinol exist in chemical equilibrium in the body and have equivalent antixerophthalmic activity. Retinal combines with the rod pigment opsin in the retina to form rhodopsin, which is needed for visual dark adaptation. Vitamin A prevents growth retardation and preserves the integrity of epithelial cells. Normal adult liver stores are enough to meet the body's vitamin A needs for about two years. Vitamin A is readily absorbed from the gastrointestinal tract, where it is also synthesized from beta-carotene. Its absorption depends on bile salts, pancreatic lipase, and dietary fat. It is carried in the blood to the liver by the chylomicron fraction of the lymph and is stored in the liver's Kupffer cells, mainly as retinol palmitate. Normal serum vitamin A levels are 80 to 300 units per 100 mL (a plasma range of 30 to 70 mcg per dL), and carotenoid levels are 270 to 753 units per 100 mL. The normal adult liver contains roughly 100 to 300 micrograms per gram, mostly in the form of retinol palmitate.