DIGESTION EXPERIMENTS. 291 



collection impracticable. We were able, however, to obtain six or seven 

 digestion periods with most of the men in Squad A, these ranging from 

 3 to 16 days each. The first was October 1 to 4, when the subjects 

 were on normal diet; subsequent periods on reduced diet were from 

 October 8 to 12, October 17 to 21, October 31 to November 4, Novem- 

 ber 12 to 18, December 10 to 15, and finally, a period of a little over 

 two weeks, January 14 to 30. This schedule was followed by prac- 

 tically all of the subjects except Pec, whose habits of defecation were 

 so abnormal as to make it impossible for him to carry out this program 

 satisfactorily. With Squad B one digestion experiment was made, 

 January 15 to 23, when the men were on a reduced diet. 



The common interpretation of digestion experiments is based upon 

 a fundamentally erroneous conception that the feces represent pri- 

 marily undigested food. The presence of visible portions of undigested 

 material in feces naturally leads to this belief, but chemical and micro- 

 scopic analysis shows this is far from the case, and that the feces con- 

 sist in large part of bacteria, intestinal debris, and residues of digestive 

 juices. Consequently it is clearly erroneous to determine the digesti- 

 bility of any given nutrient by the comparison, for example, of the 

 amount of protein in the food and the amount of protein in feces. In 

 our experiments no attempt was made to determine the so-called 

 bacterial nitrogen or the so-called metabolic nitrogen; but since our 

 work was primarily a matter of relative comparisons, we have adhered 

 to the archaic form and present our data for these digestion experi- 

 ments in terms of the utilization of nitrogen and the availability of 

 energy in food for digestion periods. The nitrogen in feces is sub- 

 tracted from the nitrogen in food and the remainder considered as 

 utihzed nitrogen, and the percentage of the total ingested is recorded 

 as the percentage of nitrogen utilized. Similar treatment is given the 

 data for energy, except that not only the calories in feces are de- 

 ducted from the energy in the food, but likewise the calories in the 

 urine. The calories in the urine are computed by multiplying the 

 nitrogen in the urine by the factor 8.0. The total net calories are thus 

 the total calories in food less those in feces and urine. The percentage 

 of the total net calories compared to the total energy in the food 

 ingested is expressed as availability of energy. 



It is a matter of considerable regret that experimental evidence on 

 the character of the feces has been for so many years neglected. Even 

 the present available data regarding this are very fragmentary and no 

 rational method for the comparison of food intake with the several 

 ingredients of the feces is as yet universally employed. One fre- 

 quently sees the statement that feces are formed during fasting. What- 

 ever may be the case with animals, it is certain that during the 31-day 

 fasting experiment at the Nutrition Laboratory with man no evidence 

 was obtained of fasting feces. On the other hand, evidence was ob- 



