THE INTAKE OF MATERIALS AND ENERGY 59 



covering of the eyes. It is essential to the normal structure and functioning of all 

 epithelial tissues, and is also a growth factor. The first symptoms of vitamin A 

 deficiency are usually a hardening and roughening of the skin, and night blind- 

 ness, or inability to see under twilight conditions. Night blindness is directly 

 caused by lack of the vitamin, which is the precursor from which visual purple is 

 formed. This pigment in the retinal cells breaks down when exposed to light, 

 forming vitamin A and causing stimulation of the visual receptors. Normally it 

 is quickly rebuilt from vitamin A when light intensity decreases, but is only 

 slowly restored when the vitamin is deficient. 



Vitamin D, the antirachitic vitamin, is known to be a mixture of at least two 

 vitamins — calciferol, or D 2 , and dehydrocholesterol, or D 3 . Chemically these are 

 solid alcohols (sterols); the chemical formula of calciferol is C28H43OH. Both of 

 these D vitamins occur in animal oils but in the form of inactive precursors. 

 Ergosterol is the precursor of calciferol and changes into the active vitamin when 

 irradiated with ultraviolet light; the precursor of D 3 behaves similarly. When 

 the ultraviolet rays of sunlight fall upon human skin, D 2 and especially D 3 are 

 activated in the oil droplets in the dermis and become available for use by the 

 body. Fish-liver oils are rich in the D vitamins, which are also present in eggs, 

 butter, and milk but do not occur in plants. The D vitamins may be stored in 

 the body (chiefly in the liver) during the months of strong sunshine, forming a 

 reserve for winter use. 



Unless sufficient amounts of the D vitamins are present in the body, calcium 

 and phosphorus cannot be absorbed properly from foods nor sufficiently retained 

 in the body. These vitamins also maintain the concentration of phosphatase (an 

 enzyme causing phosphate deposition in bone) in the region of the bone-forming 

 cells. Deficiency results in a lack of calcium and phosphate, producing in children 

 the disease known as rickets, in which the bones become bent and deformed. 

 Prolonged excessive intake of the D vitamins may cause overdeposition of cal- 

 cium and phosphate and has even been known to produce death from partial 

 calcification of the kidneys. This is the only vitamin in which overdosage is 

 known to be injurious. 



Vitamin K, like vitamin D, is known to occur naturally in two forms — K x and 

 K 2 (empirical formulas C31H46O2 and C41H56O2). Two synthetic compounds 

 known as phthiacol and menadione possess similar properties and can substitute 

 for the natural vitamins. Ki is a light yellow oil, K 2 a yellow crystalline solid 

 at ordinary temperatures. Vitamin K in some way acts upon the liver, causing 

 that organ to produce a supply of prothrombin, a substance essential for the clot- 

 ting of the blood. The basic function of the K vitamins is, therefore, to prevent 

 hemorrhage, by maintaining the normal clotting power of blood. Human beings 

 normally get their supply of these substances either from the diet or by synthesis 

 in their own intestines by the intestinal bacteria. The latter source is probably 

 the most important. Both on this account and because ~K X is abundant in many 

 foods (especially leafy green vegetables), vitamin K deficiency is rare. 



The above three vitamins are the only ones of the fat-soluble group known to 

 be essential for human nutrition. Vitamin E (tocopherol) may eventually be 

 added to this list. This is the so-called " antisterility " vitamin (actually a complex 

 of at least 4 related substances), which occurs in the oils of plant seeds and in 



