In two of the Middletown calorimeter experiments, slight increments in the heat output were observed; the third showed a slight decrease. The data in tables 52 to 54 also show that the increments in the values for the gaseous metabolism approximated those found for the heat production. Thus the work of chewing gum performed by these subjects during a period of approximately 4 hours usually produced on the average a slight increase over the basal metabolism.

In the five experiments made with the chair and bed calorimeters in Boston, the time of actual chewing was considerably shorter than in the Middletown experiments, being approximately 1 1/4 to 2 1/2 hours in duration. In two of the experiments there was practically no variation in the metabolism; the other three experiments showed an increment of 5, 9, and 16 per cent, respectively. In one of these experiments, that with T. M. C. on January 7, 1911, the increment in the heat production was actually somewhat less than the increment noted in the gaseous metabolism, yet it points towards a true increase in the metabolism. In the bed-calorimeter experiment with V. G. on December 19, 1910, in which the high increment of 16 per cent was obtained, the increase in the heat output was somewhat higher than that shown for the carbon-dioxide production, which was but 8 per cent, and for the oxygen consumption, which was but 7 per cent. It is thus not impossible that errors in the measurement of the heat production may account for the abnormally high percentage increase in this factor. Disregarding the heat value obtained in this experiment with V. G. and substituting that obtained for the gaseous metabolism of about 8 per cent, the increment in the calorimeter experiments due to chewing gum will average approximately 3 per cent, with wide variations which include three negative values.

Measuring the metabolism with a respiration apparatus during the chewing of gum has certain technical difficulties which at first were thought to be insurmountable. By using nosepieces instead of a mouthpiece and giving careful instructions to the subjects, it was possible to make five experiments with four subjects. In all of the respiration experiments it was necessary to obtain the heat output by the indirect method of computing it from the gaseous metabolism. In considering the results of the chewing experiments, therefore, it is especially important to note any possibility of loss of carbon dioxide or intake of oxygen through the mouth during chewing, for naturally any leakage of air into or out of the mouth during the periods of observation would cause a disturbance in the measurements of the metabolism. If the results recorded show similar changes in the values for the carbon-dioxide production and oxygen consumption, it may be taken as an indication that there was little, if any, leakage of air through the mouth. In a number of instances it was impossible to obtain an accurate measure of the oxygen consumed, owing to carelessness on the part of the subject in opening the mouth during chewing. This was especially true with the subjects V. G. and J. J. C. On the other hand, F. G. B. and T. M. C. took especial care to prevent such losses and the respiratory quotients found indicate that there was no disturbance and no appreciable leak. Hence we may properly assume that there was an actual increment in the metabolism which was measured with a considerable degree of accuracy.

The summary of the results of the respiration experiments given in table 68 shows in all cases measurable increases in the metabolism, these varying from 11 to 27 per cent. In the experiments in which the oxygen measurement was lost, due to the carelessness of the subject in opening the mouth while chewing, the carbon-dioxide measurements were used for computing the heat output. The calorific value of carbon dioxide used was the one corresponding to the respiratory quotients found prior to the chewing period, or in the few cases when quotients after the chewing period were available, an average of the two sets of quotients was used. It is of course possible that when the mouth was opened during chewing there was an increase in the carbon-dioxide excretion as a result of an excessive ventilation of the lungs. If this were the case it might account for the increase in the heat output attributed to the chewing, since in these cases the carbon-dioxide production was the only factor of metabolism available. In the experiments with F. G. B. and T. M. C. it is reasonably certain that there was no such loss through the mouth; the heat values could therefore be computed from the oxygen consumption.

The data secured in the respiration experiments show that as a result of chewing gum the basal metabolism may be increased on the average approximately 17 per cent. The diversity of results in the calorimeter experiments may be partly explained by the fact that the experiments were carried out over a considerable period of time, and the total increment formed a relatively small proportion of the total heat measured. The conclusion is warranted, however, that chewing gum results in a positive increase in the metabolism of from 10 to 17 per cent. Although an analysis of the chewing gum shows that from 62 to 69 per cent of carbohydrates was present, it is certain that this small amount of gum - i. e., 3 to 30 grams - had no influence upon the metabolism.

Supplementary evidence in regard to the work of mastication was obtained in two experiments in which the subjects vigorously chewed a rubber stopper. Both experiments were made with the respiration apparatus, unfortunately with the less reliable subjects J. J. C. and V. G. The increase in the metabolism was essentially the same as that found with the other subjects in the gum-chewing experiments, namely, 18 and 14 per cent, respectively, or 16 per cent on the average, thus verifying completely the more carefully planned experiments on chewing gum. It would appear from the data obtained in this study that the work of mastication, such as would be involved in chewing gum or a rubber stopper continuously, may temporarily require an increment in the metabolism of approximately 17 per cent.

In general agreement with the rise in metabolism due to chewing gum and a rubber stopper, the pulse rate is found to have increased in nearly all of the experiments. In the calorimeter experiments in Boston the pulse rate increased 9 to 10 beats per minute in all except those with J. J. C, March 25, 1910, and V. G., January 2, 1911. In the experiments with the respiration apparatus an increase was found in practically all of the experiments, the increase in the averages ranging from 3 beats per minute with F. G. B. on May 5, 1911, to 25 beats with V. G. on January 5, 1911.

At this point we are certainly justified in calling attention to the relation of the measured increase in the metabolism to the question of prolonged mastication. One of the arguments which has been advanced is that such mastication insures the absorption of a larger proportion of material from the ingested food. The fallacy of this reasoning is clearly shown when it is seen that digestion experiments have established the fact that with ordinary mastication from 90 to 95 per cent of the total energy of foodstuffs is completely absorbed.

The common method of making digestion experiments is to determine the protein, fat, and carbohydrates in the food eaten, and to calculate or determine its heat of combustion. The quantities of the same factors are determined in the feces and the ratios of the differences between these two series of values to those of the food itself are reported as the coefficients of digestibility. This method of determining the digestibility is based upon the archaic conception that feces consist primarily of the undigested residue of food. As is now known, undigested food forms but a small part of the feces and the ratio is in fact much higher than the commonly stated proportion of 90 to 95 per cent. Even on the basis of the older interpretation, however, the possibility of increasing the digestibility or availability of foodstuffs by extreme mastication seems very small. Furthermore, when we find that this prolonged mastication demands an excess heat production of approximately 17 per cent above the basal value it is easily seen that any advantage gained from a possible increase in the digestibility of the food is more than compensated by the increase in the heat production. The conception of an increase in the digestibility and in the utilization of the energy of foodstuffs as a result of prolonged mastication thus finds no support in fact.