354 - Multicellular Animals, Especially Man 



paired. 1 Significant amounts of vitamin K 

 are synthesized by the bacteria in man's 

 colon so that human deficiencies of this vita- 

 min are quite rare — except under certain 

 circumstances. In new-born infants, a defi- 

 ciency may arise before the bacterial flora in 

 the large intestine has reached full develop- 

 ment, and in adults it may arise in cases of 

 bile duct obstruction — since the bile salts are 

 necessary for a proper absorption of the fat- 

 soluble vitamins (A, D, E, and K). Thus many 

 hospitals now give fortifying doses of vitamin 

 K as a precaution against hemorrhage in 

 obstetrical cases, and in operations involving 

 the liver or the biliary tract. 



Origin and Nature of Vitamins. Modern 

 studies have made it clear that most, if not 

 all, of the compounds designated as vitamins 

 in animal nutrition are essential components 

 of protoplasm generally — in plants as well as 

 animals. In fact most of these compounds 

 play similar roles — as essential coenzyme 

 components of metabolic enzymes — in plant 

 and animal cells generally. However, typical 

 plants are able to synthesize their own essen- 

 tial compounds entirely from inorganic mat- 

 ter — and therefore vitamins are not listed 

 among the food requirements of the plant. 



Among saprophytic forms the power of 

 synthesizing essential protoplasmic com- 

 pounds varies widely, and there is good evi- 

 dence to indicate that a loss of synthesizing 

 power has played a significant role in the 

 evolution of different species. The ordinary 

 "wild" variety of the red baker's mold, Neu- 

 rospora, for example, displays maximum 

 growth on a medium that contains only one 

 of the generally recognized vitamins (biotin). 

 But if this mold is subjected to a suitable 

 dosage of x-ray — a treatment that induces 

 genie mutations (p. 510) — new varieties are 

 obtained that cannot grow unless the me- 

 dium contains one or more other vitamins, or 

 even the full assortment of vitamins neces- 

 sary in animals generally. In many cases, 



1 Several different compounds having an action simi- 

 lar to that of vitamin K have been discovered, and it 

 is possible that vitamin K is not a single substance. 



moreover, the experiments demonstrate a 

 definite relation between the mutation of a 

 single gene and the inability of the mold to 

 synthesize some one particular compound. 



It is early to generalize from such experi- 

 ments, even though considerable data are 

 beginning to accumulate. It seems likely, 

 however, that primitive organisms, much 

 earlier in geological time, generally possessed 

 more extensive synthetic powers. In plants 

 such powers of synthesis were completely 

 essential to survive, because plants take in 

 no organic foods, and such syntheses were 

 not eliminated by natural selection (p. 158). 

 Animals, on the other hand, feed extensively 

 upon the products of other organisms, and 

 consequently the capacity to synthesize or- 

 ganic compounds is less essential. Among 

 animals, therefore, the capacity to synthesize 

 many compounds was susceptible to elimina- 

 tion through natural selection. And even 

 today, although the vitamin requirements of 

 different animals are generally alike, what is 

 the vitamin in one animal is not necessarily 

 the vitamin of another. In the case of ascor- 

 bic acid, for example, this vitamin is essential 

 only in man and a few other species — such as 

 apes, monkeys, and guinea pigs. In other 

 words, scurvy, or its equivalent, is never en- 

 countered in a majority of species, because 

 most animals have not lost the metabolic en- 

 zymes by which ascorbic acid can be synthe- 

 sized in the cells. 



SUMMARY 



Only a carefully balanced diet will sus- 

 tain growth and health in man and other 

 animals. The daily energy expenditure must 

 be balanced by an adequate caloric content 

 in food; and the food must provide adequate 

 amounts of all substances needed in con- 

 structive metabolism. These structural re- 

 quirements include water, various essential 

 inorganic salts, at least a minimum of pro- 

 teins and natural fats, and a full assortment 

 of such vitamins as are -essential in the par- 

 ticular species; ■ 



