The evidence presented in the foregoing chapters of this book deals principally with the energy transformations incidental to the ingestion of food. The experimental plan, while undergoing many changes in the decade in which the research has been in progress, nevertheless had, as its fundamental basis, a quantitative measurement of the energy transformations, either directly by means of the calorimeter or indirectly by calculation from data obtained for the respiratory exchange; in many of the experiments the nitrogen excretion was also determined. In several series of experiments a measurement was made of the increase in the energy output required for the mastication of food or the drinking of such liquids as water, coffee, and beef tea, but aside from the experiments in which the effect of mastication was studied, and a few calorimeter experiments in which the food materials were taken within the experimental period, the work of prehension and the external muscular work of eating were entirely excluded.

When possible, pure nutrients were used; if this was not practicable, as was only too frequently the case, a diet was employed in which a special nutrient predominated; thus a study could be made of the quantitative transformations following the ingestion of various kinds of food. It has been our plan to discuss in the foregoing chapters the experiments of each division of the research and thus in a way subsequent discussion is more or less of a repetition. It is perfectly legitimate, however, to recapitulate and attempt to correlate the findings for the several classes of food materials.

The only purely mechanical process studied was that of chewing. Although unfortunately the evidence is not definite in every case, nevertheless the general picture is sufficiently clear to state positively that mastication produces a distinct increase in the heat production. It has likewise been pointed out that, though contrary to the belief of some enthusiasts who advocate prolonged mastication for the more perfect digestion and assimilation of food material, the unused portion of modern food materials is, under normal conditions, extraordinarily small. The preliminary preparation of practically all of the food materials of civilized man removes in large part the indigestible portion; the energy content of almost any mixed diet may therefore be said to be absorbed to the extent of 90 per cent or over. Since pure carbohydrates are almost perfectly absorbed, it is probably safe to assume that with ordinary mixed diet approximately 95 per cent of the energy is actually absorbed.

The error in computing unabsorbed material from an analysis of the feces should again be emphasized. Fecal matter by no means consists wholly of undigested food material, but is made up in large part of bacteria, the residue of digestive juices, and the debris of the epithelial lining of the intestinal tract. It is the common custom to analyze fecal material and to consider the nitrogen obtained as a measure of the unabsorbed protein, the fat as unabsorbed fat, and the carbohydrate, although existing in small amounts, as unabsorbed carbohydrates. This is fundamentally wrong, although the method for determining the digestibility of food has been based upon these false premises practically ever since the introduction of food analysis. It can be seen that an absorption of 95 per cent on this basis would, when properly interpreted, mean an actual energy absorption of nearly 98 per cent; hence the advocates of excessive mastication must attempt to increase an absorption which is already 98 per cent of the total amount. This is obviously impossible and physiologically unsound. If we further consider the extra energy required for excessive mastication, it is more than probable that such slight increase in absorption as may possibly occur with an excessive comminution of food materials by prolonged chewing may be considerably more than offset by the additional consumption of energy required for mastication.

In the experiments on drinking liquids, such as water which is without nutritive qualities, beef tea which has a measurable amount of the stimulating extractives (creatine and allied compounds), and coffee which contains a slight amount of extractives and of caffein (a heart stimulant), the picture is again not uniformly clear. Sufficient experimental evidence has been accumulated, however, to state positively that the drinking of large amounts of water results in an actual increase in the total production. Beef tea, taken either hot or cold, likewise slightly increases the metabolism. Coffee produces a similar slight increment. While a logical explanation of the increase in metabolism due to coffee and beef tea might be found in their content of stimulating materials, such as caffein and creatine, it is difficult to explain the increase due to water on this basis, and it is not impossible, even in the absence of positive evidence, that we have to deal here with an internal mechanical process which may be directly associated with the secretion of the normally occurring large amounts of urine following excessive liquid ingestion.

In a final consideration of the results of drinking liquids which show, as a rule, a relatively small increase in metabolism due to this factor, we must again state that the experimental technique was by no means perfect at that period of the research, and that the defects in the baseline frequently vitiated many of the results. As was clearly brought out in the discussion of the basal metabolism, such variations have considerable significance when the basal values are used for comparison with values obtained in subsequent periods in which only small increments are found; it is thus especially important to secure accurate basal values for such experiments. Accordingly, in studying the results of the group of experiments under consideration, it is necessary to draw conclusions not from the detailed results, which were based in some cases upon defective values, but from the general picture presented. This general picture shows that the ingestion of a large amount of liquid, such as water, coffee, or beef tea, is followed by a measurable increase in the metabolism.

Following the ingestion of food materials a pronounced increment in the metabolism was almost invariably found. We may therefore disregard possible inaccuracies in the base-line and discuss the experiments on the general assumption that the basal values were determined with sufficient accuracy to warrant quantitative deductions from the increments actually measured, although we freely admit that whenever practicable a carefully determined base-line each day prior to the ingestion of food is highly desirable, if not, indeed, absolutely essential.

The observed increment in the metabolism as a result of the ingestion of food is in accordance with the experience of nearly all of the other investigators in this field. The increase was especially pronounced with protein, carbohydrates, and mixed diets, and less pronounced when diets with a preponderance of fat were used.