BIOLOGY IN HUMAN AFFAIRS 



Vitamin D. Previous to 1922 nearly all infants and young 

 children in the north temperate zone suffered at least a 

 mild attack of rickets. In this condition the bones do not 

 calcify and, as a consequence, are subjected to deformity. 

 The cause of the disease was long a mystery, although it 

 had been recognized for more than one hundred years 

 that cod liver oil was a specific for its prevention or cure. 

 It was not until rickets had been produced experimentally 

 in animals that much progress was made in understanding 

 human rickets. 



Rickets develops in young animals when they are fed a 

 diet deficient in calcium, excessive in phosphorus, and 

 deficient in vitamin D — a substance which is abundant 

 in cod liver oil. Rickets also develops when the diet con- 

 tains little phosphorus, an excessive amount of calcium, 

 and a deficiency of vitamin D. In this disease the phos- 

 phorus content of the blood is markedly reduced, the 

 inorganic phosphate being as low as one-fourth the normal 

 eight milligram per hundred cubic centimeters of plasma. 

 Feeding cod liver oil tends to raise the phosphorus content 

 of the blood to the normal level. 



When the diet is deficient in calcium and contains exces- 

 sive phosphorus, the phosphorus is absorbed into the 

 blood but cannot be deposited in the bones because there 

 is no calcium to combine with it. This phosphorus is 

 excreted as phosphate to a considerable extent into the 

 intestine and carries with it some calcium irrespective of 

 the needs of the body for this element. Rickets so produced 

 tends to be complicated by tetany, a condition in which 

 the blood calcium is reduced. In the reverse of this con- 

 dition — that is, when a low phosphorus, high calcium 

 diet is fed — an excess of calcium is absorbed in the blood 

 but cannot be retained and is excreted into the intestine, 

 taking with it a certain amount of phosphorus irrespective 

 of the needs of the body for this element. This quickly 



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