20 BULLETIN 310, U. S. DEPARTMENT OF AGRICULTURE. 
The average amount of the water-free feces occurring as meta- 
bolic products in the ether extract is, accordingly, 9.89 per cent. 
The method of making the allowance is indicated by the following 
equations: 
9.89 X (weight of water-free feces) = metabolic products. 
(Total ether extract) — (metabolic products) = unutilized f at. 
(Utilized fat) -r- (total fat eaten) = per cent of digestibility. 
Applying these equations to the values which express the com- 
parative digestibilities of the fats studied, the coefficients of digesti- 
bility are increased to 97.3 per cent for lard, 93.1 per cent for beef 
fat, 87.6 per cent for mutton fat, and 97.1 per cent for butter. 
Should the percentage of metabolic products be based on the total 
weight of food as eaten, rather than on the weight of water-free feces, 
this value is found to be 0.27 per cent. The corresponding availa- 
bilities of the four fats, in the order studied, then become 97.3 per cent, 
93.0 per cent, 86.8 per cent, and 96.9 per cent. That these values 
should agree so closely with those based on the weight of water-free 
feces is in accordance with the theory proposed by Prausnitz, 1 that 
the amount of metabolic products occurring in the feces is directly 
dependent upon the character and quantity of the ingested food — 
that food materials do not leave residues for feces, but produce feces 
composed principally of metabolic products. 
GENERAL DISCUSSION. 
All the fats included in this series of experiments were well assimi- 
lated, the coefficients of digestibility ranging from 97 per cent for 
butter to 88 per cent for mutton fat. The average amounts of fat 
eaten per subject per day during these experiments were 90 grams of 
lard, 100 grams of beef fat, 53 grams of mutton fat, and 100 grams 
of butter. 
The average amount of protein consumed daily by the subjects was 
somewhat lower than that specified in dietary standards, but it was 
only limited by personal choice, as the subjects were permitted to 
eat as much of the wheat biscuit as they desired. Moreover, since 
only the fat portion of the diet was under consideration, it was not 
considered essential to maintain any special nitrogen level. 
The values for the digestibility of the carbohydrate content of the 
diets were 96, 97, 97, and 96 per cent, for all practical purposes 
identical with its digestibility in the ordinary mixed diet, for which 
the average value is 97 per cent. 2 
The average energy value available per man per day as calculated 
by the usual factors and the coefficients of availability found in the 
digestion experiments was 2,235 calories for the lard, 2,730 calories 
for the beef fat, 2,145 calories for the mutton fat, and 2,420 calories 
for the butter diet. These energy quantities would be insufficient for 
i Ztschr. Biol., 30 (1894), No. 12, p. 335. 2 Connecticut Storrs Sta. Rpt. 1901, p. 245. 
