CHAPTER III. 



FEEDING EXPERIMENTS WITH ABIURET PRODUCTS 

 OF DIGESTION. 



The Value of Abiuret Products of Digestion. 



INTIMATELY connected with the question of the extent of protein 

 digestion and the form in which the digestion products are absorbed 

 are the feeding experiments with pre-digested protein. These impor- 

 tant experiments, first carried out by Loewi (259), have yielded valu- 

 able results. They have clearly demonstrated that a food, in which 

 the nitrogen consisted wholly of protein digestion products which no 

 longer gave the biuret reaction, was capable not only of maintaining 

 life but of keeping the animal in a state of nitrogenous equilibrium 

 and even of leading to a certain retention of nitrogen and a rise in 

 weight. An interesting fact which set at rest some of the objec- 

 tions to the complete breakdown of protein in the intestinal canal was 

 discovered in the course of Loewi's investigations. A calorimetric 

 estimation of I grm. of the digestion products used by Loewi in his 

 experiments was carried out by Rubner. This amount was found to 

 yield 4-599 calories, a figure very close to that for albumin. In 

 Loewi's experiments the rest of the animal's diet was made up of fat 

 and carbohydrate. He found that, if he fed the digestion product 

 with fat alone, no nitrogenous equilibrium resulted, but that this 

 took place as soon as carbohydrate was added. Lesser (251), who 

 was one of the earliest workers to repeat Loewi's work, was unable to 

 confirm it. Lesser used in his experiments both peptic and tryptic 

 digestion products, but was unable to get a positive nitrogen balance, 

 although the products acted as sparers of protein. In spite of Loewi's 

 bad results with fat alone Lesser omitted carbohydrate from his diets, 

 Henderson and Dean (184) were the first to use acid hydrolytic pro- 

 ducts in their experiments. They found that they got nitrogen re- 

 tention but were not at all certain that it indicated protein synthetic 

 action. As the result of the line of research opened up by the original 

 experiments of Loewi a great number of experiments have been carried 

 out mainly by Abderhalden and his co-workers in Germany and by 



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