ed material is
excreted in the faeces. The thoroughness of digestion is determined
experimentally by weighing and analysing the food eaten and the faeces
pertaining to it. The difference between the corresponding ingredients
of the two is commonly considered to represent the amounts of the
ingredients digested. Expressed in percentages, these are called
coefficients of digestibility. See Table II.

Such a method is not strictly accurate, because the faeces do not
consist entirely of undigested food but contain in addition to this the
so-called metabolic products, which include the residuum of digestive
juices not resorbed, fragments of intestinal epithelium, &c. Since there
is as yet no satisfactory method of separating these constituents of the
excreta, the actual digestibility of the food is not determined. It has
been suggested that since these materials must originally come from
food, they represent, when expressed in terms of food ingredients, the
cost of digestion; hence that the values determined as above explained
represent the portion of food available to the body for the building of
tissue and the yielding of energy, and what is commonly designated as
digestibility should be called availability. Other writers retain the
term "digestibility," but express the results as "apparent
digestibility," until more knowledge regarding the metabolic products of
the excreta is available and the actual digestibility may be
ascertained.

Experimental inquiry of this nature has been very active in recent
years, especially in Europe, the United States and Japan; and the
results of considerably over 1000 digestion experiments with single
foods or combinations of food materials are available. These were mostly
with men, but some were with women and with children. The larger part of
these have been taken into account in the following estimations of the
digestibility of the nutrients in different classes of food materials.
The figures here shown are subject to revision as experimental data
accumulate. They are not to be taken as exact measures of the
digestibility (or availability) of every kind of food in each given
class, but they probably represent fairly well the average digestibility
of the classes of food materials as ordinarily utilized in the mixed
diet.

5. _Fuel Value of Food._--The potential energy of food is commonly
measured as the amount of heat evolved when the food is completely
oxidized. In the laboratory this