250 VITAMIN D GROUP 



tion level. The attainment, of the normal le\-el in the otherwise normal 

 individual depends chiefly on an adequate intake of vitamin D} 



The availability of calcium to the plasma does not in itself determine its 

 level in the plasma, since introduction of calcium to the plasma in hypo- 

 parathyroidism results only in a temporary rise in the plasma calcium 

 concentration with a rapid re-establishment of the previous low calcium 

 level. In the ricketic child with hypocalcemia the administration of soluble 

 calcium salts in the absence of vitamin D will restore the normal serum 

 calcium level, but there will follow a simultaneous drop in inorganic phos- 

 phorus concentration. In the presence of adecjuate vitamin D intake, 

 normal calcium level is restored while normal inorganic phosphorus re- 

 mains unchanged, thus establishing a higher calcium phosphorus product. 

 Only toxic doses of vitamin D will raise this product still further. This 

 indicates the existence of a homeostatic mechanism which tends to keep 

 both ion concentrations at a level which is constant for the age group. 

 Vitamin D therefore influences the concentration of calcium in at least two 

 ways: by increasing absorption from the gastrointestinal tract, and by 

 raising the solubility product of calcium phosphate in the plasma, and 

 perhaps by regulating renal loss of calcium and phosphate either directly 

 or through the parathyroid glands. The mechanism seems to be as follows: 

 the vitamin D produces a rise in the serum calcium, which in turn depresses 

 parathyroid activity. Since the action of the parathyroids is to diminish 

 the renal threshold for phosphorus, their suppression raises the threshold 

 and elevates the serum phosphorus level.* 



Under normal conditions bone acts as a reservoir for blood calcium. Hast- 

 ings^ has demonstrated a rapid restoration of plasma calcium after re- 

 moval of calcium from the blood and the return of such decalcified blood 

 to the circulation. Presumably this is accomplished by solublizing calcium 

 from bone. It is possible that the parathyroid glands play an important 

 role in this process. It is postulated that bone mineral forms the solid phase 

 of a bone-tissue fluid blood plasma system w^hich is normally in equilibrium. 

 A reduction of plasma calcium concentration disturbs this equilibrium, and 



3 For a given animal there would seem to be two products, one that is maintained 

 in the absence of vitamin D, and another, higher one when adequate vitamin D is 

 available. The former may be attained with a normal calcium level and a low 

 inorganic phosphorus concentration or a low calcium with a normal inorganic 

 phosphorus. The level of one ion is at the mercy of the amount of the other that 

 enters the plasma. With adequate vitamin D the total product as well as the 

 concentration of the individual components are stabilized, and if disturbed by 

 excessive ingestion of one, equilibrium is rapidly re-established at the normal level. 



^ F. Albright and E. C. Reifenstein, The Parathyroid Glands and Metabolic Bone 

 Disease, p. 127. Williams & Wilkins Co., Baltimore, 1948. 



5 A. B. Hastings, New Engl. Med. J. 216, 377 (1937). 



