GENERAL CONSIDERATIONS CONCERNING THE METABOLISM OF THE INFANT Changes in the condition of the body cannot be observed directly, but can be calculated indirectly from three factors of metabolism. We have however, to make certain assumptions with regard to the carbon metabolism. The three factors are, the food, the excretion of urine and feces, and the gas metabolism (intake of oxygen, excretion of carbon dioxide and water vapor). The change in the body of a healthy breast fed baby (as shown in the gain of weight) rarely amounts to more than 40 Con. (1 1 ounce) in 24 hours. This value, like all values deduced from a comparison of differences, is rather uncertain and inexact. In the tenth week of life the figures for the ingestion and excretion amount to not less than (:in. (25 ounces), so that in the calculation of the metabolism all the errors of the metabolism accumulate themselves on this small value. The gaseous metabolism can only be determined by means of the respiratory apparatus of Pettenkofer. In the adult this arrangement is very likely to alter the normal conditions of life tint' of the experiment., and it always does so in the case of the infant.
The infant, whose conditions of life are changed by being put in the apparatus must he so arranged as to permit a separate collection of urine and feces without loss. very restless and fretful, and this exercises an influence on the metabolism. The gaseous metabolism is thus increased with an increased excretion of water vapor, which is still further encouraged by the almormallv high temperature of the air in the apparatus. Consequently, the urine is more scanty awl more concentrated than normal. Furthermore, the infant cannot remain in the apparatus more than twenty hours in twenty-four, as it must he removed at least five times to take food. This time is utilized to empty the vessels contain ing the urine and feces, to change clothes, bedding, etc. Thus for four very important hours, the gaseous metabolism could not be determined.
The successful and exact accomplishmet' of the experiment re quires from four to six days; it is very difficult, and beside complicated pieces of apparatus numerous well trained investigators have to he employed. In the course of the last few years, lleulmer and Rubner, with numerous co-workers, have conducted several of these metabo lism experiments on infants of different ages. Although the results were
very interesting, they were unable to obtain satisfactory information in regard to the conditions prevailing in the normal breast-fed infant, and particularly concerning its gain. This is due to several reasons. There was a constant error in the form of an excessive gaseous metabolism, and the small number of infants examined, with the short duration of the experiments, permitted too great a range of individual peculiarities.
There is another method for determining the values of metabolism, which can be designated as the stuti.qica/ roc/hod. This is of particular interest to the practicing physician, because he is able to apply it himself and it enables him to participate in the investigation of the subject. The results thus far obtained in this field have mostly been furnished by the work of the practitioner. For instance, during the total period of lacta tion the average quantity of the milk and its value in nutrient material were determined, as well as the average amount and composition of urine, and feces, and the gain. This was done without altering the con ditions of the baby's life. This method leaves the gaseous metabolism unknown, but it can be calculated with a high degree of certainty. The values involved are 100 Gm. (25 drams) or more, and are only a little influenced by the very insignificant uncertainties inherent to the deter mination of the three other factors of the metabolism.
The statistical method gives very reliable average values for the factors of metabolism in the normal breast-fed infant. A comparison of these average values with the results of the individual experiments makes it possible to recognize the influence of constant errors or of individual peculiarities, as well as the absence of such disturbances. For the reasons given above, such irregularities in the infant will hardly be avoided and therefore the results of the calculation will rarely coin cide with those of the direct observation. Rubner observed the metabo lism of two boys ten and eleven years of age in the respiration apparatus, and compared the results with those obtained according to the statistical method by Camerer (the elder) for boys of this age and found an unex pected similiari ty.