214 VITAMINS 



Relationship of light and mineral content of diet to vitamin 

 D requirements 



Sunlight prevents rickets by the action of its Ultraviolet rays on some 

 provitamin in the skin, presumably 7-dehydrocholesterol. Practically 

 none of the effective ultraviolet rays penetrate ordinary window glass, and, 

 hence, much transmitted light possesses little or no antirachitic value. 



Calcification of growing bone, the process in which vitamin D is so 

 vitally concerned, consists primarily in the deposition of calcium and 

 phosphorus as a complex calcium phosphate in the organic matrix. No 

 amount of vitamin D in the diet, therefore, can take the place of these 

 inorganic constituents. Not only must the two elements be present in 

 adequate amounts, but their relative proportions influence greatly the 

 need for vitamin D. Rations employed in producing rickets experi- 

 mentally in rats are made especialty effective through the addition of 

 large amounts (three per cent usually) of calcium carbonate. Rickets 

 so produced is said to be of the low phosphorus type (common clinically) 

 since the inorganic phosphate content of the blood serum of animals on 

 such a ration is abnormally low. In human beings the normal amount 

 of inorganic phosphorus in the blood plasma is approximately 4 mg. per 

 100 ml., and the calcium content is 10 mg. per 100 ml. If either or 

 both of these values becomes low enough so that their product (milligrams 

 inorganic P X milligrams Ca per 100 ml.) drops to 30 or less, rickets 

 is almost certain to develop. A value of 40 or above is considered 

 normal. A low calcium type of rickets also occurs, and a third type in 

 which serftm calcium and phosphorus are both low is observed occasionally. 



Occurrence 



The most potent natural sources of D vitamins are the fish-liver oils. 

 Fish in which considerable oil is distributed throughout the body, e.g., 

 sardines and salmon, are the richest food sources. The concentration 

 in salmon averages approximately 12 /xg. per 100 g., while several other 

 fish are nearly as high. 



Next in order of concentration are egg yolk and butter with 4.6 and 

 2.3 fig. per 100 g., respectively. Although milk normally contains only 

 small amounts of the vitamin — reports vary from 0.008 to 0.106 fig. per 

 100 g. — it may be enriched by irradiation so as to contain 0.3-1.1 fig. 

 per 100 g., which is equivalent to about 135-400 LIT. per quart. Milk 

 produced by cows fed irradiated yeast also contains about 400 I.U. 

 per quart. Direct irradiation of the animals is without effect in increasing 

 the concentration of vitamin D above the normal level in milk. The 

 amount of vitamin D in a number of typical foods is indicated in the 



