[Fat-soluble vitamins: biotransformation]

Fat-soluble vitamins are essential for growth and normal functions of human body. These vitamins are generally absorbed from the gastrointestinal tract only when bile is present. Recently, it is recognized that fat-soluble vitamins have hormone-like functions in the human body. The carotene (provitamin A) is a remarkably potent source of vitamin A, and beta-carotene is the prominent type of carotene in nature. It plays an essential role in the function of the retina. With regard to the biotransformation, more than 90% of the intake of performed vitamin A is in the form of retinol esters, usually as retinyl palmitate. Vitamin A is readily absorbed from the normal gastrointestinal tract, and the absorption of the vitamin is related to that of lipid. Prior to entering the circulation, the hepatic retinyl ester is hydrolyzed and retinol itself is released from the liver. In the blood, vitamin A is present mainly as retinol associated as a protein complex. Normal calcium and phosphate metabolism is dependent on proper levels of vitamin D, and blood levels of these ions are influenced by gastrointestinal absorption, metabolism, and renal excretion of vitamin D. Both vitamin D2 and vitamin D3 are absorbed without esterification from the small intestine. Most of the vitamin appears first in lymph and primarily in the chylomicron fraction as a lipoprotein complex. Bile is essential for adequate intestinal absorption and deoxycholic acid is the most important constituent of bile.(ABSTRACT TRUNCATED AT 250 WORDS)