

NUTRITION 269 



calories, although it contained 145 g. of protein, equivalent to 23*2 g. of 

 nitrogen. 



A second experimental period was undertaken in which the subject performed 

 hard work as mason and labourer for a term of ninety-five days. On a diet of 

 about 5,000 calories, with an average of 7 '22 g. of nitrogen per day, a slight loss 

 of nitrogen resulted, namely, 34 g. for the whole period. To get the nitrogen 

 minimum for hard work, the last ten days of the period may be taken, in which 

 nitrogen equilibrium was maintained 011 5'72 g. of nitrogen ( = 35'75 g. of protein). 



An important question is, naturally, whether this subject was in any way 

 the worse for this prolonged period of minimal nitrogen diet. It must be 

 admitted that he had lost a certain amount of nitrogenous substance, although 

 there was every evidence that his condition was just as good as at the beginning. 

 No period of recovery was necessary and, indeed, he was anxious to begin a new 

 experiment. 



Experiments were also made by Hindhede on himself and on a student with similar results. 

 The former gave a protein minimum of 16 g., with a calorie value of 2,650, doing light work. 

 The latter was doing moderate work on a diet of 3,700 calories and protein content of 25 g. 



It appears that we must admit that, for a strong healthy man, the protein food 

 actually necessary to replace wear and tear is very much less than that usually 

 assumed. It is interesting to notice that, as would be expected, the wear and 

 tear in hard work is greater than in moderate work, if we may judge by the 

 rise in the protein minimum from 25 g. in the latter case to 35 g. in the former. 

 But it is found to be the same fraction of the total intake in energy. 



Effect of Carbohydrate. In the experiments on feeding with the digestion 

 products of proteins already referred to, it may have been noticed that, while 

 Loewi (1902) was successful, certain other workers were unable to confirm his 

 results. Cathcart calls attention to the fact that, in Loewi's experiments, carbo- 

 hydrate was present to make up the proper calorie value, whereas in the 

 experiments that failed, fat only was used. Further, Cathcart himself (1909) 

 found that, if no carbohydrate was present in a nitrogen-free diet, creatine 

 appeared in the urine, whereas it was absent when carbohydrate was given. The 

 interpretation to be put on these experiments is that, in the presence of 

 carbohydrate, resynthesis of creatine into some cell protein takes place, so that 

 it would appear that some of the nitrogen lost in wear and tear can be made use 

 of again by the aid of carbohydrate. It seems, however, from the results of 

 Graham and Poulton (1913), that a repetition of these experiments is desirable, 

 although Cathcart himself, with Orr (1914), points out that they do not affect his 

 conclusions. 



Other experiments confirm the necessity of carbohydrates for the synthesis of 

 protein. It was shown by Hansteen (1899) that it applied to the higher plants 

 and by Felix Ehrlich (1911) that amino-acids were incapable of acting as sources 

 of nitrogen to yeast in the absence of carbohydrate. 



Maintenance. Certain evidence has already been referred to which suggests 

 that, in the wear and tear of active cells, it is not the whole of the large molecules 

 of the nitrogenous constituents of the protoplasmic system that are broken up. One 

 may state the fact either in the form that certain " side-chains " only of a giant- 

 molecule or "biogen" are disintegrated, or that certain chemical individuals, 

 forming part of the total reaction systems of the cell mechanism, are decomposed, 

 perhaps by subsidiary reaction. Reasons have been given above (page 19) for 

 regarding as doubtful the " biogen " view, and further evidence against it will be 

 found on page 498, but, in the present state of knowledge, decision is impossible. 



As to the fact that protoplasm itself does not break up, some additional 

 evidence may be mentioned here. M'Collum (1911, 2) feeds pigs for a sufficient 

 time on protein-free diet to obtain a constant ratio between the creatinine and the 

 total nitrogen output; the total nitrogen is then taken as being that due to 

 endogenous metabolism. The food protein to be tested is then introduced into the 

 food in quantity equivalent to the nitrogen excreted, an isodynamic portion of the 

 carbohydrate food being withheld. The experiments of most interest in the present 

 connection are those with zein and with gelatine. Zein contains neither glycine, 



