It was fully appreciated by students of nutrition many years ago that the food analysis had its serious limitations, and the belief prevailed that the way to eliminate the defects of chemical technic, was to supplement the data obtained by chemical methods with information secured by actual trials to determine the digestibility of foods. The subject and the diet having been decided upon, samples of the food to be eaten were carefully analyzed to determine their content of protein, fat, ash, moisture and "nitrogen-free extract." The period of the experimental trial varied considerably, but in most cases it covered three days. It was customary to keep the subject on a milk diet during a fore period, so that the feces derived from the experimental diet would be different in appearance and could be accurately separated. The chemical constituents of the ingested food minus the amounts of each which appeared in the fecal residues, afforded the data from which the digestibility of the protein, fat and carbohydrate were calculated.

There were some errors in this procedure. In addition to the food substances which escaped digestion, a part of the nitrogen and salts eliminated from the intestine were derived from the secretions of the digestive glands, and from the cellular elements which came from the intestinal mucosa. The quantity of secretions eliminated from the tract depended in great measure on the content of indigestible cellulose in the food. This acted as a sponge, and retained liquids so that they could not be reabsorbed. An animal which is fed a diet containing just sufficient protein to meet its needs, may easily be made to suffer a nitrogen deficit by including in its food supply a liberal amount of cellulose or other indigestible carbohydrate such as are abundant in many vegetables.

In interpreting the results of digestion experiments "utilization" of the several components of the food was assumed to be measured by the extent to which they disappeared from the digestive tract. That such an interpretation is not always justified is readily understood. A diet of cereal grains will be well digested and absorbed, but the substances which reach the blood and tissues are not a satisfactory pabulum for the nutrition of the body. Some complexes which are indispensable to the body are present in too small amounts to admit of the utilization of the other useful ones which are abundant, but cannot serve a useful purpose unless they can be used to form definite structures, no unit of which can be dispensed with. Such a food supply may be well absorbed but not assimilated. Utilization is an unfortunate choice of a term in this sense for it is liable to be misinterpreted. Knowledge of the digestibility of food-stuffs is of value to students of nutrition, but the idea of quality in the array of digestion products must not be lost sight of.