EVALUATION OF EFFECT OF ISOMERIZATION ON CHEMICAL AND BIOLOGICAL ASSAY OF VITAMIN D - ANALYSIS OF FAT-SOLUBLE VITAMINS X

The reversible thermal isomerization of calciferols (D) to precalciferols(P), leading to a temperature‐dependent equilibrium in solutions, interferes with many chemical and biological analytical procedures for the evaluation of vitamin D products. Two quantities should be distinguished: the potential vitamin D content (precalciferol + calciferol) and the actual vitamin D content (calciferol only). Of these two, only the potential vitamin D content is of importance to the buyer of vitamin D. Biological vitamin D assays give the potential vitamin D content, provided sample and reference standard dilutions are equilibrated by heating simultaneously to be sure that all have the same P–D ratio. This equilibration is superfluous in most of the chemical determinations of the potential vitamin D content. The actual vitamin D content can be determined by chemical, but not by biological, assay. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company