460 METABOLISM, NUTRITION, AND DIET 



pect that proteins could be metabolized into other forms, such as carbo- 

 hydrates and fats. Bernard long ago stated that protein was a glycogen- 

 former; that abundant glycogen was stored in the liver when flesh diet was 

 fed, and argued that protein was the source of the glycogen. The careful 

 work of a number of investigators has not obtained sufficient evidence to 

 clear up this question absolutely, but the weight of evidence is in favor of the 

 view that in the body sugar can be formed from proteins. Lusk has recently 

 shown that some of the amino acids must have been converted into dextrose 

 after deamidization in the body. Whether or not protein can be metabolized 

 into fat, and stored as such, seems at present an open question, notwith- 

 standing the immense amount of work expended in trying to solve the 

 problem. 



Cramer fed 450 grams of lean meat per day to a cat in a respiration 

 chamber for 8 days. The daily excretion of nitrogen was 13 grams, of 

 carbon 34.3 grams; calculating the amount of carbon in the food as 41.6 

 grams daily, this would leave 7 . 3 grams retained. This carbon might be 

 stored in the form of glycogen or as fat. Calculated as glycogen, it gives an 

 amount greater than an animal of that size could retain. Therefore, the 

 probabilities are that the carbon is deposited in the form of fat. 



In the examination of the fat formed in the larvae of blow-flies developing 

 in a quantity of coagulated blood, Hoffmann found ten times more fat than 

 existed in the blood. These experiments point in the direction of fat 

 formation from protein. 



The Effect of a Gelatin Diet. The albuminoid eaten in greatest 

 quantity is gelatin. Though gelatin closely resembles the protein mole- 

 cule chemically, it cannot replace entirely the protein of the food. As was 

 stated in the chapter on the Chemistry of the Body, gelatin is deficient in 

 certain amino acids, notably tryptophane which is not present at all. 

 It is probable that because of the absence of this "building stone," the 

 body tissues cannot reform their characteristic proteins for which this 

 amino acid would be an essential constituent. In other words, nitrogenous 

 equilibrium cannot be maintained on a diet consisting of gelatin, carbo- 

 hydrates, and fats. Gelatin, then, is a substance whose food value in part 

 is comparable to that of carbohydrates and fats, as the following experiments 

 will prove: On a diet of 500 grams of meat, without any gelatin, the subject 

 lost nitrogen to the equivalent of 22 grams of protein, but when 200 grams 

 of gelatin were added the subject gained 54 grams. In another experiment, 

 when the diet consisted of 2,000 grams of meat without gelatin, the gain 

 was the equivalent of 30 grams of protein, but when 200 grams of gelatin 

 were added the gain became 376 grams. The lack of a mixed protein food 

 value is proven by still a third experiment in which the diet consisted at 

 first of 200 grams each of meat and of gelatin; here the gain was the equiva- 

 lent of 25 grams of protein, but, when the meat was omitted and the gelatin 



