Numerous digestion experiments with a large variety of foods have abundantly established the fact that these materials differ greatly in their solubility in the digestive juices. This is an important matter, and one which should be well understood, for we must consider both the weight of the food eaten and its availability in determining its nutritive value. Variations in digestibility are caused primarily by variations in composition, therefore, we must deal Tundamentally with the susceptibility of the various single constituents of foods to the dissolving action of the several digestive ferments.

In this connection, we need to pay little attention to the mineral compounds. They do not undergo fermentative changes in the way that the carbon compounds do, but pass into simple solution either in the water accompanying the food, or in the juices with which they come in contact.

117. Digestibility Of The Proteins

As has been noted, protein is a mixture of nitrogenous compounds. The gluten of wheat contains at least five of these bodies, and other seeds as many. What is the relative susceptibility of these single proteins to ferment action either as to rapidity or completeness of change does not appear to be known. Some proteins are practically all digested by artificial methods, and probably are in natural digestion. It is a fact, however, that protein is much more completely dissolved from some foods than from others. That of milk and meat is practically all digestible, that of some grains very largely so, while with vegetables quite a large proportion escapes solution. Whether this is due to a differing degree of solubility on the part of the characteristic protein compounds of the various foods is not quite determined. The fact that highly fibrous materials show the lowest proportion of digestible protein suggests as an explanation that the nitrogen compounds of plant tissue are so protected by the fiber present that they escape the full action of the digestive juices. It is certain, however, that the protein in plant tissue is less fully digested than that from milk, meat, and eggs.

118. Digestibility Of The Carbohydrates

In the case of the carbohydrates, our knowledge of the relative susceptibility of the individual compounds to enzym action is more definite. First of all, the necessary modification of the sugars, which are already soluble, is slight, and they are wholly digested. In the second place, we have learned in two ways that the starches are wholly transformed to diffusible compounds, first by submitting them in an artificial way to the action of various diastatic ferments, and, second, by discovering a complete absence of starch or its products in normal human feces. We can say, therefore, that under normal conditions the unprotected starches, like the sugars, are completely digestible.

Digestibility must be considered, however, from the standpoints both of rapidity and of completeness. As to the former factor, starches from unlike sources exhibit some remarkable differences. Investigations by Stone, who submitted a number of these bodies to the action of several diastatic ferments, show that "this variation reaches such a degree that under precisely the same conditions certain of the starches require eighty times as long as others for complete solution." The potato starches appear to be acted upon much more rapidly than those from the cereal grains.

Other carbohydrates, cellulose and hemicelluloses, such as pentosans, galactan and mannan and related bodies, show great variations in digestibility according to their source, these variations ranging in observations by Swartz from 0 to 100 per cent. The extent to which these substances disappear from the alimentary canal appears to be dependent on their solubility and their susceptibility to attack by bacteria.

119. Digestibility Of The Fats

The extent of the digestion and absorption of the fats or oils is also not definitely known. If we were to accept the figures given for ether-extract in tables of digestion coefficients as applying to the real fats, we would believe that their digestibility varies from less than one-third to the total amount. It is unfortunately true that these coefficients mean but very little. The ether-extract from some foods is only partially fat or oil, as we have seen, and the inaccuracy of a digestion trial is still further aggravated by the presence in the feces of bile residues and other bodies which are soluble in ether, so that the difference between the ether-extract in the ingested food and that in the feces does not give accurate information as to what has happened to the actual fats. It seems very probable that pure vegetable fats and oils and all mixed animal fats are quite completely absorbed.

The foregoing statements make it plain that when the general composition of a food is known, it is possible to predict with a good degree of certainty whether its rate of digestibility is high or low. The larger the proportion of starch, sugar, milk, meat and eggs and the smaller the percentage of gums and fiber, the more complete will be the solution.