At this point we may ask: What is the lowest metabolism? If in a normal state of nutrition the voluntary muscles of the body are so perfectly controlled that there is no visible movement, the muscles so relaxed as to diminish the muscle tonus, the pulse rate and the respiration rate depressed to the lowest point, and there is no food in the alimentary tract, and furthermore, if the subject is in deep sleep, we may expect to obtain the minimum metabolism for that particular subject.

The ideal conditions outlined for obtaining such a low metabolism are, as a matter of fact, not readily secured with the majority of subjects. If in studying the influence of a superimposed factor upon metabolism, the measured base-line can be relied upon as uniform, it is not necessary that the lowest metabolism be secured. In experiments which involve relatively slight changes in metabolism, however, the lower the metabolism which can be secured for the base-line, the greater will be the degree of accuracy in the percentage increase obtained as a result of the superimposed factor.

1Magnus-Levy, Arch. f. d. ges. Physiol., 1894, 55, p. 1; see especially p. 23. 2Benedict and Cathcart, Carnegie Inst. Wash. Pub. No. 187, 1913, p. 71. 3Benedict and Higgins, Am. Joum. Physiol., 1912, 30, p. 217.

Even when the basal value has been well established it does not necessarily follow that the metabolism of an individual will remain unchanged for an indefinite length of time, inasmuch as there will be changes in the composition of the body, particularly gains or losses of glycogen and fat; growth, climate, the season of the year, and such factors as temperature environment and various stimuli to the body may likewise have an effect upon metabolism. This question will be considered more at length in the discussion of the various methods for obtaining the basal metabolism.

Of the numerous factors affecting muscle tonus and nerve stimulation, great emaciation and the ravages of disease are distinctly of pathological rather than of physiological significance. In a number of pathological cases, when the metabolism is at a subnormal point through muscular atrophy and similar causes, there may be even less muscle tonus and minor muscular movement than with healthy persons in profound sleep. But these abnormal conditions need not be considered here.

It may be of considerable moment in this connection to note whether or not the increment above the base-line due to the ingestion of a definite amount of food is wholly independent of the absolute value of the base-line. For example, we will assume that the taking of a certain amount of food resulted in an increment of 25 calories during a period of 6 hours when the base-line was determined with the subject in complete muscular repose, in the post-absorptive condition, and lying awake. With the subject asleep, the base-line would unquestionably have been somewhat lower than that obtained with the subject awake. Have we any reason to believe that the increment due to the ingestion of food will be affected by this difference in conditions? Unfortunately our evidence is by no means clear on this point.

The particular problem studied in this publication is the absolute increase in the heat production caused by the ingestion of food. Aside from disease, the two principal factors which contribute to the depression of the base-line are sleep and fasting. It is conceivable that with a low base-line, such as would be found in deep sleep or during fasting, a greater increment would be obtained with a definite amount of food than with a higher base-line. On the other hand, it is possible that during sleep, and especially with a condition of under-nutrition resulting from fasting, the cells may be less susceptible to stimuli. In such a case the increment in the metabolism would obviously be less than when the subject is awake and in a normal state of nutrition.

Experiments primarily measuring the output of heat resulting from a definite amount of muscular work have shown that if the basal value is increased for any reason, either by previous alcoholic excess1 or by preceding diet,2 the increment in the heat production per unit of work is not measurably altered. This is in full conformity with the contention of Johansson and Koraen3 to the effect that the thermal processes accompanying food ingestion and those accompanying muscular work are entirely distinct from each other. The only striking illustrations in the literature of the opposite of this hypothesis are the observations of Durig,4 whose technique it is very difficult to criticize adversely; his results should therefore be considered as absolutely established facts. In Durig's Vienna experiments the basal metabolism was approximately 1 calorie per minute, while in the Monte Rosa experiments it was 1.26 calories per minute. The increment due to the ingestion of sugar was 0.268 and 0.306 calorie per minute in Vienna; on Monte Rosa with the same amount of sugar it was 0.206 and 0.115 calorie per minute. It would seem, therefore, as if with the higher baseline the sugar had a less stimulating effect.

For all practical purposes, however, we need not at present consider these special conditions, but may assume that if the base-line is determined under conditions of complete muscular repose, the increment measured will represent the true effect of the ingestion of food upon the metabolism irrespective of whether the subject is asleep or awake. The possible variations in the magnitude of this effect, due to the subject being either asleep or awake, call for experimental evidence, and as yet we have none at hand.

Basal Values Used In This Research

As the researches recorded in this publication have extended over a considerable period of time, namely, from 1904 to 1915, and this period has witnessed a rapid development of technique in all forms of metabolism measurements, it is not surprising that we find variations in the interpretation of the significance and importance of the base-line and in the method of studying the metabolism following food ingestion.

The experiments reported in this publication may be divided into three groups: First, those 24 hours in length; second, those approximately 8 hours in length; and third, those in which the individual periods were of short duration. The variations in the length of the period naturally resulted in a variation in the method of obtaining the basal metabolism. In the 24-hour experiments the basal metabolism was determined for each individual for one or more days and compared with 24-hour values determined on other days for the metabolism after food ingestion. In the earlier 8-hour experiments, the basal metabolism and the metabolism after food were determined on separate days, but later in the research the food was frequently given after several hours of fasting and the measurements continued for the remainder of the 8 hours. Thus, in these later experiments, the basal metabolism and the metabolism after food were determined on the same day. In the short-period experiments the basal metabolism was measured on the same plan as that used in most of the later 8-hour experiments, i.e., in several periods preceding the ingestion of food.

1Benedict and Murschhauser, Carnegie Inst. Wash. Pub. No. 231, 1915, p. 78.

2Ibid., pp. 80 and 93.

3Johansson and Koraen, Skand. Arch. f. Physiol., 1902,13, p. 251.

4Durig, Denkschr. d. Wiener Akad. d. Wiss., 1909, 86, p. 116.

Both the 24-hour experiments and the 8-hour experiments were carried out with respiration calorimeters, by means of which not only the carbon-dioxide production and the oxygen consumption could be measured, but also the heat production. The short-period experiments, in which the individual periods were approximately 15 minutes long, were made with respiration apparatus which gave measurements of only the carbon-dioxide production and the oxygen consumption. The heat production was calculated by the indirect method.

The main object in all these experiments was essentially the same, namely, to secure a constant base-line upon which could be superimposed the factor of the ingestion of food. With the 24-hour base-line it was necessary to assume that the metabolism was constant from day to day; with the earlier 8-hour base-line, that it was constant on different days; and in the experiments in which the base-line was determined on the same day as the metabolism after food (the later 8-hour experiments and the short-period experiments), that the metabolism was constant from hour to hour throughout the day. From a consideration of these plans of experimenting it is easily seen that the probability of constancy in muscular activity is not the same for all types of experiments. The advantages and disadvantages of each method may therefore be discussed more in detail in connection with the results obtained in the determinations of the basal metabolism.