on is determined the amount, in pounds or grams, of each
nutrient consumed.[72] In like manner the nutrients in the indigestible
portion, or feces, are determined from the weight and composition of the
feces. The indigestible nutrients in the feces are deducted from the
total nutrients of the food, the difference being the amount digested,
or oxidized in the body. When the food is digested, the various
nutrients undergo complete or partial oxidation, with the formation of
carbon dioxid gas, water, urea (CH_{4}N_{2}O), and other compounds. The
feces consist mainly of the compounds which have escaped digestion. The
various groups of compounds of foods do not all have the same
digestibility; for example, the starch of potatoes is 92 per cent
digestible, while the protein is only 72 per cent. The percentage amount
of a nutrient that is digested is called the digestion coefficient.

In the following way the digestibility of a two-days ration of bread and
milk was determined: 773.5 grams of bread and 2000 grams of milk were
consumed by the subject. The dried feces weighed 38.2 grams. The foods
and feces when analyzed were found to have the following
composition:[62]

=====================================================================
COMPOSITION | BREAD | MILK | FECES[A]
---------------------------------------------------------------------
Water | 44.13 | 86.52 | --
Crude protein | 7.75 | 3.15 | 25.88
Ether extract | 0.90 | 4.63 | 18.23
Ash | 0.32 | 0.70 | 26.35
Carbohydrates | 46.90 | 5.00 | 29.54
Calories per gram | 2.450 | 0.79 | 5.083
=====================================================================

[Footnote A: Results on dry-matter basis.]

STATEMENT OF RESULTS OF A DIGESTION EXPERIMENT

=============================================================================
FOOD CONSUMED | WEIGHT | PROTEIN | ETHER | CARBO- | | HEAT OF
| OF | N x 6.25 | EXTRACT | HY- | ASH | COMBUS-
| MATERIAL | | | DRATES | | TION
------------------+----------+----------+---------+--------+-------+---------
| Grams | Grams | Grams | Grams | Grams | Calories
Bread |