Metabolism and excretion
Vitamin A and other fat-soluble vitamins (except vitamin K) differ from the water-soluble vitamins in that the body has an almost unlimited capacity to store them but only a limited capacity to excrete them. The major route ofvitamin A excretion from the body is through bile into the intestine, where some may be reabsorbed and some is lost in the feces. Most of the biliary vitamin A is in the form of retinoyl-glucuronides. In addition to glucuronides, a large number of different oxidation products ofretinoic acid are found in the urine; nevertheless, urinary excretion of vitamin A metabolites represents a very small daily loss of the vitamin. The possible significance of vitamin A metabolism in regard to its physiological functions is being actively investigated.
