62 FEEDS AND FEEDING 



practically pure protein is fed, the loss of nitrogen can be checked only 

 if the supply is far in excess of the waste from, the starving body. It is 

 assumed that this increase in nitrogenous waste when protein is fed in 

 such large proportion is due to a flooding of the tissues with amino 

 acids, the products of protein digestion, and a consequent stimulation 

 of the activities of the body cells. (11, 49) However, the food protein so 

 decomposed is not entirely lost to the animal. Not only may it be burned 

 as body fuel, thus saving the body fat, but, after the splitting of the 

 nitrogen from the molecules of protein or of amino acids, the non-nitroge- 

 nous residue which remains may be converted into glucose and finally 

 into glycogen or fat. Carnivora, or flesh-eating animals, have lived for 

 long periods on washed lean meat, consisting chiefly of protein, with 

 only a small amount of fat and a trace of glycogen. Since plant tissue 

 is rich in carbohydrates, such experiments have not been possible with 

 the herbivora, or plant-eating animals. 



93. Protein required for maintenance. The preceding discussions have 

 pointed out the functions of protein in the body under various condi- 

 tions. Let us now pass to a question of much practical importance the 

 amount of protein required to maintain animals at rest, when fed along 

 with sufficient carbohydrates or fats to meet the needs of the body for 

 fuel. When enough of these nitrogen-free nutrients is supplied, the 

 amount of protein required to prevent loss of nitrogen from the body 

 is much less than where the ration is nearly pure protein. In trials with 

 dogs Voit 7 found that from 2.6 to 3.3 Ibs. of lean meat per day was 

 required to check the loss of protein from the body when lean meat was 

 fed alone practically an exclusive protein diet. When carbohydrates 

 or fat was added, only one-half to one-third as much lean meat was 

 needed. Since the digestible portion of the crude fiber and likewise of 

 the pentosans can serve as body fuel (48), these nutrients to some degree 

 decrease the waste of nitrogen in the same manner as does a supply of 

 the more easily digested sugars and starch. 



Experiments show that a pound of carbohydrates has somewhat greater 

 protein-sparing action than a pound of fat, a surprising fact when we 

 remember that, on burning, fat produces over twice as much energy as 

 do carbohydrates. (76) Evidently there is no relation between the fuel 

 values of these nutrients and their protein-sparing power. This is perhaps 

 because glucose, which is needed by the body, is formed with difficulty 

 from fat. 8 



By feeding rations ample in carbohydrates and fat, some investigators 

 have succeeded in reducing the requirement of nitrogenous matter to 

 slightly more than the normal nitrogen waste of the body during starva- 

 tion. At the Pennsylvania Station 9 Armsby found in experiments with 

 steers, covering 70 days, that from 0.4 to 0.6 Ib. of digestible protein 

 daily per 1,000 Ibs. of live weight was sufficient to maintain the nitrogen 



'Ztschr. Biol., 5, 1869, p. 352. e Principles of Animal Nutrition, 1903, p. 142. 



8 Skand. Archiv. Physiol., 14, 1903, p. 112. 



