Digestion has an important relation to the nutritive efficiency of food, and to the physical welfare of the individual. On the one hand, only that portion of the food that is digested and absorbed can serve the purposes of growth and the maintenance of the vital functions, and on the other hand, bad digestion causes discomfort and disease.

120. Meaning Of "Digestibility"

'In discussing the factors that may influence the digestion of food, it is essential to understand clearly what is involved in the term digestion as it is used in science and in common speech. The term is made to include three elements, completeness of solution and absorption of the food nutrients, rate of digestion, and comfort of digestion. In science, the figures that are given for the digestibility of various foods refer to the completeness or extent to which the food is dissolved and transferred to the circulation. But different foods from which come the same proportion of undigested dry matter may differ materially in the rate at which they undergo digestive changes, and in this sense their digestibility is unlike. Again, when for any cause digestion causes discomfort, the sufferer declares that the particular food eaten is not digestible. As a matter of fact, the ultimate completeness of solution in the digestive fluids may not be influenced either by the rate or the discomfort attending the process. Among the numerous factors that may modify digestion, the following are among the most important: 121. Kind of food. - The kind of food, other things being equal, determines the completeness and also, we may believe, the rate of digestion. Investigation has shown, as already noted, that vegetable foods are less fully digested than those of animal origin, a fact due probably, to the inclosure of the protein and other nutrients in the fibrous tissue of much vegetable substance. It is entirely rational to claim, too, that this association of the nutrients with a cellulose framework retards digestion by protecting the protein and other compounds from attack. If no allowance is made for the metabolic products in the feces, the digestibility of the various classes of foods is calculated to be as follows: -

The individual nutrients differ in their susceptibility to attack by the digestive fluids. We have seen that potato starch is hydrolyzed more rapidly than that from the cereal grains. In the case of the fats the higher their melting point the more slowly they are likely to be decomposed and emulsified. It is probable that tallow requires a longer time for complete digestion than does butter or the salad oils, and may be less completely absorbed. This point may well be raised touching the digestibility of imitation butter that is made, in part at least, from the body fats of bovines and swine.

This influence of the melting point is quite clearly indicated by the following figures: - 1

1 " Metabolism and Practical Medicine," Von Noorden, Vol. I, p. 56.