METABOLISM 623 



36. Protein Utilization. By "protein utilization" is meant the 

 percentage of the ingested protein which is actually absorbed and 

 assimilated. 



This may be determined by the following procedure: Ingest any diet of 

 known nitrogen content for a period of three days 1 (see table, page 602). Collect 

 all feces from the diet making the "separations" as directed on page 620, using 

 carmine as the initial "marker" and charcoal as the final "marker" or vice 

 versa. Preserve the feces as directed on page 621. Mix the total feces thor- 

 oughly and determine the nitrogen by the Kjeldahl method (see Chapter XXVII and 

 note on page 622). The approximate nitrogen utilization may be calculated as 

 follows : 

 (Food nitrogen Feces nitrogen) X 100 



Food nitrogen = Approximate percentage nitrogen utili- 



zation. If it is desired to ascertain the actual percentage of the ingested ni- 

 trogen which has been utilized by the body we must make a correction for meta- 

 bolic nitrogen. In doing this proceed as follows : Ingest a non-nitrogenous diet 

 as described on page 622 for a period of two days, using sufficient agar-agar to 

 insure a daily fecal output which shall approxunateln weight that obtained when 

 the regular protein diet was ingested. 2 Separate and preserve the feces as 

 directed on page 620. Mix thoroughly and analyze for nitrogen according to 

 the Kjeldahl method (see Chapter XXVII and note on page 622). Calculate the 

 actual percentage utilization of the diet as follows : 

 [Food Nitrogen (Fecal nitrogen metabolic nitrogen)] X 100 



= Actual per ' 



nitrogen 



centage nitrogen utilization. If urinary nitrogen is determined the above data 

 enable us to prepare a nitrogen balance (see Experiment 41, page 625). 



37. Influence of Defective Mastication pn Food Residues in Feces. Rapid 

 eating accompanied by defective mastication leads to the appearance of relatively 

 large macroscopic food residues in the feces. Under some conditions, however, pro- 

 tein utilization (see above) may be as satisfactory during food "bolting" as when 

 the food is very thoroughly masticated. 3 This problem may be studied by the 

 following method: 



(a) Ingest a diet containing meat and be certain to masticate the diet very 

 thoroughly. Collect a stool, examine macroscopically; mix carefully and examine 

 microscopically (see page 233). 



(6) Ingest a diet similar to that employed in above experiment (a). "Bolt" 

 the food, i.e., ingest it practically without mastication. Examine the feces as 

 above. Note the difference in the macroscopical and microscopical findings under 

 (a) and (b). 



If the nitrogen of food and feces is determined we may calculate the protein 

 utilization (see Experiment 36). By the additional determination of urinary 

 nitrogen, we may prepare a nitrogen balance (see Experiment 41, page 625). 



1 See note 4, p. 622. 



2 It is frequently difficult to so regulate the agar-agar intake as to secure the proper fecal 

 output. In such an event the proper value for metabolic nitrogen must be obtained by cal- 

 culation. For example if 89.1 grams of feces were excreted per day on the protein diet, and 

 166.5 grams per day (with a nitrogen value of 0.5 gram) when agar was employed, the 

 actual value for metabolic product nitrogen may be obtained by the following proportion, 

 assuming that the content of metabolic nitrogen is proportional to the weight of feces ex- 

 creted: 89.1 : 166.5 : : * : -5- x = 0.268 gram metabolic nitrogen per day. 



3 Foster and Hawk: Jour. Amer. Chem. Soc., 37, 1347, 1915. 



