PROTEIN AS A SOURCE OF ENERGY. 



81 



As a single striking example there may be cited an experiment by Jordan, 1 in 

 which the protein supply of cows, beginning with a liberal ration, was gradually 

 diminished to about one-half and then gradually increased again to the original 

 amount. The following table shows the average nitrogen balance of cow No. 12 

 of the second series of experiments, the daily results being grouped into periods 

 as indicated. 



Average daily nitrogen balance of cows — Jordan. 



Date. 



Number 

 of days. 



Nitrogen 

 digested. 



Nitrogen 

 of milk. 



Nitrogen 

 of urine. 



Gain by 

 body. 







Grams. 



Grams. 



Grams. 



Grams. 



Jan. 30 to Feb. G 





186.6 



81.7 



87.0 



+17.9 



Feb. 6 to 16 



10 



18-5.2 



81.4 



87.5 



+16.3 



Feb. 16 to 26 



10 



161.6 



77.5 



81.9 



+ 2.2 



Feb. 26 to Mar. 8 



10 



130. 8 



74.0 



56.5 



+ .3 



Mar. 8 to 18 



10 



117.2 



66.6 



43.7 



+ 6.9 



Mar. 18 to 28 



10 



143. 6 



69.6 



61.8 



+12.2 



Mar. 28 to Apr. 7 



10 



171.4 



71.6 



89.2 



+10.6 



Apr. 7 to 14 



7 



185.7 



71.9 



104.4 



+ 9.4 



The amount of milk protein, like the total milk solids, diminished in quite a 

 normal way with the advance in lactation, while the percentage of protein in 

 the solids remained about the same. On the low protein rations of the middle 

 periods there seems to have been some falling off in the amount of milk protein 

 produced (and of the total milk solids as well) in comparison with what might 

 have been expected on an unchanged ration, but the difference is small, except 

 in one or two periods where the protein supply reached the lowest limit. Aside 

 from this the principal effect of the variations in the amount of digestible pro- 

 tein supplied was to increase or diminish the amount of urinary nitrogen, which, 

 as the table clearly shows, rose and fell with the supply of nitrogen in the feed. 



PROTEIN AS A SOURCE OF ENERGY. 



This increased katabolism of protein, however, is not to be re- 

 garded as a total loss of so much food material. The manner in 

 which surplus protein is disposed of is rendered clear by a considera- 

 tion of the chemistry of protein katabolism. Proteins are resorbed 

 from the digestive tract in the form of comparatively simple cleav- 

 age products, chiefly amino-acids, and the body uses these nitroge- 

 nous cleavage products as building stones out of which to reconstruct 

 body proteins broken down in the vital processes. As has just 

 been shown, however, this necessary demand is relatively small, wmile 

 the mature animal has lost the capacity which it had during growth 

 of building up large amounts of new protein tissue. When the 

 blood is, so to speak, flooded with these amino-acids in high protein 

 feeding, some increase in the formation of body protein appears to 

 result, as will be shown immediately, but this consumes a relatively 

 small proportion of the nitrogenous matter and lasts for only a 

 limited time. It is obviously an advantage to the organism, there- 

 fore, to be able to dispose of the surplus nitrogen. This it accom- 

 plishes by splitting off the NH 2 group and excreting it in the form 



1 New York Agricultural Experiment Station. Bulletins 132 and 197. 

 8489°— Bull. 143—1 2 6 



