The possible fluctuations in the metabolism have likewise been shown for 35 subjects1 who were studied for periods of time varying from 5 days to 4 years and 5 months. In all cases the subjects were in the post-absorptive condition and with complete muscular repose. The increase in the oxygen consumption is used as an index of the fluctuations of the metabolism, with the value for the minimum daily average as a basis. The figures indicate that the oxygen consumption varied above the minimum from 3.5 per cent to 31.3 per cent, with an average variation of 13.9 per cent. The greatest variations were found as a rule with those subjects studied for the longer periods.

The results obtained with many of these subjects were considered in more detail in the preceding section. By reference to tables 37 to 45 it will be seen that while the average values showed usually no tendency to change materially during the months or years the subjects were studied, yet the values for the individual periods often fluctuated widely. With such fluctuations it would not be logical to use a basal metabolism determined on one day for comparison with the metabolism determined on another day, except possibly when the superimposed factor to be measured is of considerable size, as in severe muscular work.

With well-trained subjects of long experience, an average basal value may be considered as more reliable than those obtained with untrained subjects. Perhaps one of the best illustrations of the constancy of metabolism with a thoroughly well trained subject is that of the professional bicycle rider, M. A. M., studied by Benedict and Cathcart,2 whose metabolism was determined practically every morning for several months. (See tables 47 and 48.) The uniformity of the average metabolism for the day throughout this extended period is striking, to say the least, the variations in the metabolism being small. In fact, these particular experiments have been cited as conclusive evidence that when the base-line has once been fairly established it may, with propriety, suffice as a common base-line for subsequent use. But in physiological experimenting of this kind a subject is rarely so completely under control that he can be used daily for several weeks and even months in experiments with a respiration apparatus. Such conditions have never, we believe, been duplicated in experiments in which the influence of the ingestion of food had been primarily considered.

1Benedict, Journ. Biol. Chem., 1915, 20, p. 263, table 4. 2Benedict and Cathcart, Carnegie Inst. Wash. Pub. No. 187, 1913.

In studying a superimposed factor with a great increase in metabolism, such as that commonly occurring in severe muscular work experiments, the use of a common base-line is open to the least objection, but most factors have a less pronounced effect upon the metabolism than severe muscular work. Even with so constant a metabolism as that of M. A. M., it would be impossible to use an average basal value in many experiments with him on the influence of the ingestion of food, for the variations in the metabolism in the supposedly satisfactory collection of basal values were at times plus or minus 5 or 10 per cent, and the total effect of many processes of digestion fall well within this limit.

The constancy in the average metabolism shown in tables 37 to 45 confirm in practically every detail the general conclusions drawn by Gigon2 from the basal data obtained by him with the Sonden-Tigerstedt apparatus in Stockholm and the Jaquet apparatus in Basel, and subsequently by means of another apparatus employing a spirometer, Muller valves, and mouthpiece, in the Poliklinik in Basel. Since the data obtained with the Stockholm apparatus were exclusively confined to carbon-dioxide production, they can not be taken as comparable values for indicating constancy in the total heat production. Nevertheless it is important to note that, even on the basis of the figures presented by Gigon, variations of nearly 10 per cent are found, which far exceed in many instances the variations found in observations following the ingestion of the several foodstuffs.

Table 47. - Carbon Dioxide Produced At Different Times Of Day In Respiration Experiments; Subject M. A. M., In Post-Absorptive Condition And Lying On Couch

(Values per minute).

Average age, 29 years. Average body-weight (naked), 66.0 kilograms. Height, 177 cm.

1he experimental periods were usually 15 minutes in length and there was but one period in each half hour. 2Gigon, Munch, med. Wochenschr., 1911, 58, p. 1343.

1The experimental periods were usually 15 minutes in length and there was but one period in each half hour.

2Two other results (seventh period, 207 c.c, and eighth period, 202 c.c.) were obtained and included in the average for the day.

3Average of results obtained in ninth to twelfth periods, 208, 190, 201, and 202 c.c, respectively.

4The total number of periods in which the carbon dioxide was determined in the 54 experiments was 154.

Accordingly, in the final selection of a determined basal value, it is of fundamental importance that we should bear in mind the fact that in averaging a large number of experiments the tendency is for all of the irregularities to be eliminated. For a comparison with an average value obtained from a large number of food experiments it may be justifiable to use a basal value of this kind, but in a comparison with the results of only one or two food experiments the variations in the single periods must be taken into consideration.