DIGEvSTIBILITY AND AVAILABILITY OF FOOD. 183 



the last meal immediately preceding each experiment and with 

 the last meal of each experiment. The total amount of feces 

 for each experiment was collected and prepared for analysis. 

 The food materials and feces were analyzed according to the 

 methods ordinarily followed in this laboratory. The urine 

 was collected for the preliminary period as well as for the 

 metabolism period. The bladder was emptied just before the 

 first meal w^s taken and all urine was collected thereafter dur- 

 ing the experimental periods and the da}^ following the last 

 period. The nitrogen and energy of the urine were deter- 

 mined, so that each digestion experiment includes also a ni- 

 trogen metabolism experiment. 



As explained above, the differences between the nutrients 

 in the food and the corresponding ingredients in the feces are 

 taken as representing the amounts of the nutrients digested 

 and made available to the body for the purposes of nutrition. 

 The amount of each nutrient thus made available divided by 

 the amount in the food gives the percentage or coefficient of 

 availability. 



Not all the energy of the available food is utilized by the 

 body. A portion of the protein of the food is excreted by 

 the kidneys not completely oxidized. The available energy 

 is therefore the energ}^ of the available nutrients less the en- 

 ergy of the materials excreted in the urine. For nearly all of 

 these experiments the quantity of energy in the urine has been 

 directly determined. This determination was interfered with 

 in some of the preliminary experiments and in them the energy 

 of urine has been calculated by multiplying the quantity of 

 available protein by the factor 1.25, it having been found in a 

 number of experiments with healthy men with normal excre- 

 tion by the kidneys that for ever}^ gram of protein absorbed 

 there was on the average 1.25 calories of energy in the unoxi- 

 dized materials of the urine. 



The tables and descriptions which follow give the details of 

 fifty digestion experiments which form part of the met- 

 abolism experiments. The first six of these were published in 

 a former report of the station, -'= but are repeated here because 

 the results as calculated according to revised data differ slightlj' 

 from those already recorded. The remaining experiments in- 

 clude all those not hitherto published. Besides those here given, 



* Report for 1S97, pp. 159-166. 



