CHEMISTRY OF DIGESTION AND NUTRITION. 289 



of energy to the body in the same sense as do carbohydrates and fats. When 

 any one of these three substances is used in a diet, the proportion of proteid 

 necessary for the maintenance of N equilibrium may be reduced greatly. Upon 

 the theory of circulating proteids, this is explained by saying that these sub- 

 stances are burnt in place of proteid, and that the proportion of this latter 

 material which undergoes the fate of circulating proteid is thereby diminished. 

 Actual experiments have shown that gelatin is more efficacious than either fats 

 or carbohydrates in protecting the proteid in the body, and it has been sug- 

 gested, therefore, that it may take the place, partly or completely, of the circu- 

 lating proteid, according to the amount fed. If this suggestion is true, we 

 may say that gelatin has a nutritive value the same as that of the proteids, 

 except that it cannot be constructed into living proteid. The relative value 

 of fats, carbohydrates, and gelatin in protecting proteid from destruction in 

 the body is illustrated by the following experiment, reported by Yoit. A dog 

 weighing 32 kilograms was fed alternately upon proteid and sugar, proteid 

 and fat, and proteid and gelatin : 



Nourishment (grams). Calculated destruction of flesh. 



Meat. Gelatin. Fat. Sugar. in body (grams). 



400 200 450 



400 250 439 



400 200 356 



Munk 1 has attempted recently to determine how far the proteids of food may 

 be replaced by gelatin. In these experiments a dog was brought into a condi- 

 tion of nitrogenous equilibrium upon a diet of flesh-meal, rice, and lard, con- 

 taining 9.73 grams of nitrogen. During the period this diet was continued the 

 animal, whose weight was 16.5 kilograms, was oxidizing in its body 3.7 grams 

 of proteid daily for each kilogram of weight. In a second period lasting four 

 days the quantities of rice and lard were the same as before, but the proteid in 

 its diet was reduced to 8.2 grams, which contained 1 gram of nitrogen ; the 

 balance of the nitrogen was supplied in the form of gelatin, so that in round 

 numbers only one-sixth of the required daily amount of nitrogen was given 

 as proteid. The result was that the animal maintained its nitrogen equilib- 

 rium for the short period stated. It was found that the experiments could 

 not be continued longer than four days, owing to the growing dislike of the 

 animal for the gelatin food. During the second period the animal was receiving 

 in its food and burning in its body only 0.5 gram of proteid daily for each 

 kilogram of weight, as against 3.7 grams upon a normal diet. It would not 

 be possible to substitute fats or carbohydrates for the proteids of the daily diet 

 to anything like the same extent without causing a consumption of some of the 

 store of proteid material within the body. 



Nutritive Value of Pats. The fats of food are absorbed into the lacteals 

 as neutral fats. They eventually reach the blood in this condition, and are 

 afterward in some way consumed by the tissues. The final products of their 

 oxidation must be the same as when burnt outside the body namely, CO 2 



1 Pftuger's Archivfiir die gesammte Phyaiologie, 1894, vol. Iviii. p. 309. 

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