For many persons some vitamin D in addition to that ordinarily obtained by exposure to sunshine is necessary for the most efficient utilization of calcium and phosphorus. The vitamin acts by increasing the amounts of these substances available for the mineralization of bones and teeth. Measurement of the requirement of vitamin D presupposes the ingestion of a diet adequate in all other respects and particularly containing ample amounts of calcium and phosphorus; it should include also recognition that the human requirement of vitamin D (or of any other nutritional factor) includes both the quantity necessary to prevent obvious clinical pathologic changes and an amount which promotes a condition of normal nutrition and health.
The requirements of vitamin D may be defined as those amounts which, with ample intakes of calcium and phosphorus and a diet otherwise adequate, insure sufficient retention of calcium and phosphorus to permit (a) normal growth and
JEANS PC, STEARNS G. THE HUMAN REQUIREMENT OF VITAMIN D. JAMA. 1938;111(8):703–711. doi:10.1001/jama.1938.72790340005011