210 VITAMINS 



5000 international units are: ^ lb. of butter, % lb. of carrots, 1 to 1^/^ 

 oz. of spinach, or 1-2 teaspoonfuls of cod-liver oil. 



The international unit (I.U.) is defined as the vitamin A potency of 

 0.6 microgram (/xg.) of pure beta-carotene. Pure vitamin A has a bio- 

 logical potency of one unit in about 0.25 /xg. Thus 5000 I.U. would 

 correspond to only 1.2 mg. of true vitamin A, but 3 mg. of beta-carotene 

 would be needed to supply this number of units. The other carotenoid 

 precursors of vitamin A are only about one-half as effective as beta- 

 carotene, and so about 6 mg. would be required if the total vitamin A 

 supply were to come from this source. The above recommended daily 

 allowances are based on the assumption that approximately two-thirds 

 of the total vitamin A value of the diet will be contributed by one or 

 more of the precursors and one-third by vitamin A itself. 



A greatly excessive intake, for example, several million units taken 

 over a period of a few days or weeks, is apt to produce symptoms of 

 toxicity such as nausea and vomiting, headache, peeling of the skin, and 

 general prostration. The amounts necessary to cause such sickness, how- 

 ever, are so much greater than the normal requirements of the body 

 that in the ordinary course of events no such disorder should ever arise. 



VITAMIN D 



Physiological function 



Lack of vitamin D manifests itself largely in the form of a disease 

 known as rickets, and for this reason it is sometimes called the antirachitic 

 vitamin. The most obvious symptoms of rickets are bowed legs, enlarged 

 joints, and malformation of the head and chest. These abnormalities 

 result from improper calcification of the growing bones, as may be readily 

 detected by comparing X-ray photographs of rachitic and normal bones. 

 Analyses of the bones of rachitic rats show that the ash content on 

 the fat-free dry basis may be as low as 25-30 per cent as compared with 

 approximately 50 per cent for the bones of normal rats of the same age. 

 Such bones are necessarily weak and eventually assume the character- 

 istically bowed shape. Enlargements occur also where the rib bones join 

 the cartilages, and to this chain of bead-like formations has been given 

 the name "rachitic rosary." Weak abdominal muscles, together with 

 malformation of the chest, are responsible for a pot-bellied appearance. 

 Since bones and teeth are so closely related chemically, it is not surpris- 

 ing that rachitic children also have defective teeth. It is hardly necessary 

 to state that growth is retarded with the onset of rickets and eventually 

 ceases. The disease is seldom fatal, but it predisposes the individual to 

 other diseases, of which broncho-pneumonia is said to be most com- 

 mon. 



