VITAMINS 



211 



The physiological action of vitamin D in preventing or curing rickets 

 is thought to be due largely to the fact that it promotes the absorption 

 of calcium from the food through the intestinal wall and into the blood 

 stream. In addition, some investigators feel that it has some direct 

 action, at present of an obscure nature, in promoting the actual deposi- 

 tion of calcium and phosphorus in the growing bone. It may be that 

 these beneficial effects result from an interaction of the vitamin with 



1 



Courtesy of the Wisconsin Alumni Kesearch Foundation. 

 Fig. 9-3. Rickets. 



alkaline phosphatase, a type of enzyme which is concerned with phos- 

 phorus absorption in the intestine and reabsorption in the kidney (a 

 process which reduces loss of phosphates in the urine). This enzyme 

 is especially active in bone where it breaks up organic phosphates of 

 the blood and thus provides inorganic phosphate (PO4-) for bone 

 formation. A water-soluble derivative of vitamin D has recently been 

 shown to stimulate the action of alkaline phosphatase very markedly, 



