Nutrition 



137 



ammonia, single amino acids, and special mixtures as of peptones. Similarly 

 some of them can synthesize either pyrimidine or thiazole (or both), and from 

 these thiamine; others can synthesize thiamine only, while the highly special- 

 ized flagellates and free-living ciliates must be supplied with thiamine in the 

 diet. 



The loss of ability to synthesize needed compounds is not always complete, 

 since animals in deficiency can be shown to synthesize some needed com- 

 pounds. When flagellates are transferred from one medium to a simpler one, 

 growth is often poor initially and good later, indicating metabolic adaptation 

 which may be either a change in enzyme pattern or selection of those in- 

 dividuals which initially had the greater synthesizing capacity. ^-- Mammals 

 normally obtain their nitrogen as protein, but, if minimal amounts of the 

 essential amino acids are supplied, mammals can use urea or ammonium salts 

 from which to synthesize amino acids and thus protein. The amount of pro- 

 tein normally consumed by man in our western civilization is much greater 

 than the amount required to supply the necessary amino acids, and protein is 

 an uneconomical source of energy. Certain amino acids act in specific processes 

 (e.g., arginine in growth), but are needed by man in gram amounts daily, as 

 compared with milligrams or micrograms of B vitamins. The amino acid and 

 vitamin requirements for growth are greater than those for maintenance of 

 adults. It is striking that the amino acids which are "dietary essentials" are 

 similar for the ciliate Tetrahymena and for the rat. 



It appears that the essential amino acids and B vitamins are similar in all 

 animals and that each one has its particular role in cellular metabolism. Some 

 of the B vitamins act as coenzymes for vital reactions in all aerobic cells; this 

 has been established for thiamine, riboflavin, niacin, pyridoxine, and panto- 

 thenic acid. Animals must either synthesize these essentials, get them from a 

 dietary source, or obtain them from symbiotic microorganisms which can 

 synthesize them. Insects and mammals with gastrointestinal tracts sterilized 

 of microorganisms have much higher vitamin requirements than those whose 

 gastrointestinal tracts contain the normal symbionts. k is difficult to state the 

 true metabolic requirement for a substance such as a water-soluble vitamin or 

 amino acid in a higher animal, even when microorganisms are eliminated, since 

 some of the substance is often synthesized, although not in sufficient quantity. 

 It is probable that higher animals can be stimulated to greater synthesis of B 

 vitamins by a lack of these and with an excess of the precursors in their diet. 

 Most of the B vitamins, then, because of their cellular functions, are metabolic 

 requirements of all animals and are dietary requirements of some. 



There are also many substances which are required by specific groups of 

 animals. The ciliate Tetrahyvtena requires guanine, whereas mammals can 

 synthesize it from adenine, and Tetrahymena requires a pyrimidine uracil, 

 which is not metabolized when it is fed to mammals but which is synthesized 

 by them. Apparently Protozoa and many insects and mammals do not need 

 fats, whereas a few insects and mammals need only one fatty acid, linoleic 

 acid. All insects require cholesterol; mammals and birds synthesize it. The fat- 

 soluble vitamins, vitamins A, D, E, and K, are strictly vertebrate requirements, 

 well known only for birds and mammals. Insects grow well in the absence of 

 these vitamins. Investigation of specific functions of fat-soluble vitamins, such 

 as that of vitamin A in visual purple synthesis, of vitamin D in calcium- 



