336 CELLS, TISSUES, AND ORGANISMS 



nutritional substance in studies of anemia, was eventually shown to be 

 the transfer agent for the "single-carbon unit" in the biological synthe- 

 sis of the purine ring. 



The following is a list of the recognized vitamins: vitamins A, C, 

 D, E, and K; thiamine, riboflavin, nicotinic acid, vitamin Be, panto- 

 thenic acid, folic acid, vitamin B12, and biotin. Two other substances 

 sometimes included are choline and inositol. Most of the vitamins are 

 synthesized by bacteria or plants but not by animals. The inference is 

 that during evolution animals lost certain enzyme systems responsible 

 for the synthesis of vitamins, but animals are able to compensate for 

 this by eating food that contains the needed substances. 



Much work has been done in isolating, identifying, and synthesiz- 

 ing the vitamins and studying their functions. They often act as co- 

 enzymes in biochemical reactions related to growth, such as the forma- 

 tion of amino acids, purines, and other components of new tissues. It 

 is quite evident that it takes very little interference with the chemistry 

 of the body to produce a cessation of growth. One of the first things 

 that is noticed in vitamin deficiencies is a slowing of food intake, which 

 is sufficient in itself to stop growth. 



Since the higher animals have developed complex physiological 

 systems, such as endocrine glands and a central nervous system, it 

 seems paradoxical that they are unable to make certain substances 

 needed for everyday life. One might expect that a species that was un- 

 able to carry out such an important function would become extinct. 

 But predators and parasites are resourceful creatures, and they flourish 

 at the expense of their more industrious and thrifty victims. It is most 

 interesting to contemplate the nutritional history of the human race 

 with respect to the vitamins. In the primitive state, man consumed his 

 food raw and in the round: fish, insects, Crustacea, mollusks, the eggs 

 and fledglings of wild birds, edible plants, the flesh and entrails of ani- 

 mals. The contribution to the vitamin supply made by spoilage micro- 

 organisms should be added to the diverse list. Sometimes coprophagy 

 was practiced, and in many species of animals this makes an important 

 addition to the supply of B-complex vitamins and vitamin K. As man- 

 kind developed pastoral and agricultural habits, his food preferences 

 became more refined and sophisticated. This led to the appearance of 

 vitamin-deficiency diseases — beri-beri, scurvy, xerophthalmia, and pel- 

 lagra. The third chapter in this chronology took place in the past 50 

 years. A few inquisitive human beings isolated and synthesized the 

 vitamins. This finally enabled us to be independent of a few of our 

 enzymatic defects, but it did not change our parasitic and predatory 

 habits, for we proceeded to raise more domestic animals than ever by 

 adding synthetic vitamins to their food, and our per capita consump- 

 tion of meat and eggs is at an all-time high. 



