ABSTRACT
When the vitamins were originally discovered they were isolated as chemically or physically defined fractions or factors from selected foods, but their exact chemical composition was seldom known. The designation of vitamins by letters was not systematically pursued in many cases, particularly when functions ascribed to given factors were discovered to be due to other substances. For both vitamins and minerals, when expressed on an energy basis, metabolic requirements are most often of the same order of magnitude from one species to the next. Vitamins A, D, E, and K are unlike the water-soluble vitamins because of their lipid solvent solubility, diverse nonenzymatic functions, sequestration in lipid vacuoles and adipose, and greater risk for toxicity. Like other fat-soluble vitamins, dietary vitamin D is absorbed after micellar solubilization in the upper intestine. Dietary vitamin D deficiency reduces calcium absorption, secondarily increasing PTH, and the risk for osteopenic bone disorders.
