Fat-soluble vitamins
1982
District differences in nutritional function of the lipid-soluble (aqueous-insoluble) vitamins (A, D, E, and K) are reviewed. Vitamin D is a precursor of at least 1 hormone (1,25-dihydroxycholecalciferol), needed for calcium and phosphorus metabolic regulation. Vitamin K controls the synthesis of blood clotting factors, and may have additional functions as suggested by the finding of vitamin K-dependent proteins in certain tissues (e.g., bone). With the exception of vitamin A's established role in visual acuity, less is known on the mode of action of vitamins A and E than is known of D and K. Evidence suggests that vitamin E serves as a biological antioxidant against lipid peroxidation in membranes, while selenium, present with glutathione peroxide, provides a secondary defense against such peroxidation. Fat-soluble vitamin deficiencies from dietary inadequacies are rare in the US. However, since certain of these (vitamins A and D) are stored in body lipids, toxicity from high dietary intakes can occur. Synthetic analogs of these vitamins also are used. Vitamin A is used to treat acne vulgaris, to enhance immune function, and to prevent or treat epithelial cancer; D metabolites are used in therapy for vitamin D-resistant rickets and E in treating diseases having impaired vitamin E usage. (wz)
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