Nutrition 121 



been grown very slowly in a mixture of the same ten amino acids required by 

 rats, except for double the amount of arginine; however, they grow better in 

 a mixture of twenty amino acids''' (Table 21). The cockroach Blatella grows 

 at about half the normal rate in amino acid mixtures simulating casein in 

 composition (Noland, unpublished). The roach matures on s\'nthetic diets 

 dehcient in both tryptophane and methionine, which are evidently synthesized 

 by intestinal microorganisms at a rate sufiicient for slow growth. '"- Inability 

 to grow some insects on amino acid mixtures may be due partly to improper 

 mixtures and partly to inadequate salts in the diet. The amount of protein in 

 the diet of growing insects is important; for example, the ants of genus 

 Pheidole on a high protein diet tend to become soldiers, on a low protein diet, 

 workers. ^^^ 



Ihe amino acid requirements of mammals ha\e been carefully in\'estigated 

 by omitting \'arious amino acids from a food mixture "^- ''•'' and observing 

 growth (rats) and weight and nitrogen balance (man, dog). Results are given 

 in Table 21. Arginine is synthesized by the rat but not at a rate sufficient for 

 the demands of normal growth. Cystine is also needed and can be synthesized 

 only if adequate methionine is present, the synthesis apparently using the 

 sulfur of methionine. Glycine is also needed but can be synthesized from 

 simpler substances. Observations on dogs '"' indicate that arginine is not 

 needed for maintenance of adults, but that the other nine amino acids as 

 required for growing rats are needed by adults. Man requires eight amino 

 acids, whereas growing rats require ten. The absolute amounts required vary 

 with the caloric intake, being less with high calories. Man requires most of 

 the essential amino acids are supplied, and the conversion does not depend on 

 hence recommended amounts are 0.5 to 2.5 gm. per day. In general D-amino 

 acids cannot be substituted for L-amino acids, although some of the phenyl- 

 alanine and methionine can be in the D-form.^^'' The amino acid requirements 

 of the chick differ slightly from those of mammals (Table 21). Glycine and 

 arginine are required; citrulline but not ornithine can be substituted for 

 arginine; and tyrosine is needed if the ration is low in phenylalanine. '■^- "*• ^ 



The nitrogen requirement of young growing animals is much greater than 

 that of adults. In man, an infant requires four to hve times as much as an 

 adult on a unit/ weight basis. A trout needs 14 or more per cent of protein in 

 its diet for normal growth. "'* Calves fail to grow on a diet containing only 

 4.4 per cent protein, but when urea is added to give nitrogen equivalent to 

 about 16 per cent protein they grow well, presumably by conversion of urea 

 to protein by the symbiotic microorganisms in the rumen. ''■^' ^^ Rats can use 

 glycine, urea, and even ammonium salt for nitrogen if minimal amounts of 

 the essential amino acids are supplied, and if the conversion does not depend 

 on intestinal organisms. ^''^ 



Autotrophic organisms can use inorganic sources of nitrogen, such as nitrate, 

 ammonia, and sometimes atmospheric nitrogen, but with metabolic evolution 

 the abilities of synthesis decreased and specific amino acids are required by 

 ciliates and probably by all metazoans. Claims for requirements of peptones 

 and whole proteins cannot be accepted until extensive tests with balanced 

 mixtures of amino acids are made. The specific amino acid requirements 

 differ somewhat for various animal groups, but the ability to use exclusively 

 inorganic sources of nitrogen was lost early in animal evolution. Possibly 



