MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 255 



DIGESTION EXPERIMENTS. 



The data of the metabolism experiments above described include statistics of the amounts 

 of nutrients consumed in the food and excreted in the feces. The difference between these 

 amounts represents the so-called digestible or available nutrients." The amount of each nutrient 

 thus made available divided bj the amount in the corresponding food is here taken as the coeffi- 

 cient of availability. 



Each metabolism experiment, therefore, includes a digestion experiment; furthermore, each 

 metabolism experiment or series of experiments was preceded by a digestion experiment, gener- 

 ally of 4 days' duration, during which the subject was outside the respiration calorimeter, but had 

 the same diet, and as nearly as convenient the same amount of muscular exercise, as in the metab- 

 olism experiment. We thus have for each metabolism experiment or series of metabolism 

 experiments two corresponding digestion experiments. While the chief object of the preliminary 

 experiment was to bring the body into approximate nitrogen equilibrium, the results, as bearing 

 upon the availability of the food, are of importance. 



The portions of protein, fat, carbohydrates, and ash not made available are eliminated in 

 the feces. The unavailable alcohol is eliminated through the kidneys, lungs, and skin, and was 

 determined in these experiments according to the method described beyond (p. 258). 



In what has been said about the availability of the different nutrients in food no reference has 

 been made to the availability of the energy. While it is commonly believed that all of the energv 

 of the available fats and carbohydrates is capable of use by the body, all of the energy of the 

 protein can not be so utilized. The nitrogen of the available protein is eliminated from the body 

 in the form of urea, uric acid, and similar compounds, carrying with them a certain amount of 

 energy. From the results of a considerable number of determinations of the ratio of the heat of 

 combustion of urine to the available protein it has been found that for each gram of the latter 

 there is lost in the urine an average of 1.25 calories of energy. This amount must therefore be 

 deducted from the energy of the available food in order to obtain the available energy of the 

 available protein. This is done by multiplying the number of grams of the latter by 1.25. and 

 deducting the product from the difference between the total energy in the food and that in the 

 feces. The difference gives the amount of available energy, which, divided by the total energy in 

 the food consumed, gives the coefficient of availability of the energy. 1 ' 



The proportions of the different nutrients digested and made available in any given case 

 depend upon the diet and the individual. So far as concerns the diet, the availability may vary 

 with (1) the kinds. (2) the amounts of food materials. (3) the method of preparation, and (1) the 

 accessories, including condiments, beverages, etc., and with the rest, alcoholic beverages. The 

 same diet may be differently digested by different individuals or by the same individual under 

 different conditions of health, physical activity, and nervous strain. 



The details of the digestion experiments with alcohol diet are given in Tables CV to CXV1I1 

 of the Appendix. 



Table 11 compares the availability of food in diets with and without alcohol and the availability 

 of the same diet with the same persons outside and inside the respiration chamber. In the first 

 case the principal difference is that of diet, the alcohol being the chief factor; in the second case 

 the differences are those of the physical and mental condition of the individual. The discussion 

 of the effect of alcohol upon availability of the nutrients of the diet is given on pages 256 to 

 l'5s. beyond. 



"For further discussion see page 256 beyond, and Repts. Storrs (Conn.) Alt. Exp. Sta., 1896, p. 103, and 

 1897, p. 154. 



"For further discussion of this subject Bee Atwater and Bryant, Kept. Storrs | Conn. ) Agr. Exp. Sta., 1S99, p. 

 96. See also discussion by W. I >. Atwater in Bui. 99 of the U. S. Dept. Agr., Office of Experiment Stations, Pro- 

 ceedings of the Association of American Agricultural Colleges and Experiment Stations, 1900, p. 112. 



