Human milk has a varying calorific value dependent largely on the amount of fat present. Thus Schlossmann1 finds that the calorific value per liter of nineteen samples of milk from 19 women averages 719 calories, with a maximum of 876 and a minimum of 567. The milks having the largest fuel value contained 5.2 to 5.1 per cent, of fat, while that having the lowest contained only 1.8 per cent.

The amount of the child's metabolism is dependent on his size. Rubner states that a baby weighing 4 kilograms produces 422 calories, an adult weighing 40 kilograms, 2106 calories, but that the metabolism per unit of area is the same.

Rubner and Heubner2 summarize their results on the metabolism of differently conditioned children as follows:

The metabolism in all these cases was essentially the same per unit of area.

In the last case the very noticeable amount of muscle movement and crying while the child was in the respiration apparatus increased the metabolism. Further details regarding this case give a very complete picture of the metabolism of an infant. The child weighed 4.06 kilograms at birth, and about 10 kilograms at the time of the experiment when five and a half months old. He was given his mother's milk.

1 Schlossmann: "Zeitschrift fur physiologische Chemie," 1903, xxxvii, 340.

2 Rubner and Heubner: "Zeitschrift fur ex. Pathologie und Therapie," 1905, i, 1.

The first day of the experiment the child was very uncomfortable on account of his new environment. The last day he was given only a small quantity of tea, and was therefore in a state of practical starvation. The carbon dioxid excretion on these days was as follows:

The diet on the second, third, and fourth days consisted of 1258 grams of human milk per day containing:

Total nitrogen................................... 1.99 grams.

Fat.......................................... 37.73 "

Milk-sugar.................................... 85.5 "

Of the total nitrogen only 1.63 grams were contained in true protein, the rest being in nitrogenous extractives. The percentage composition of this milk is given on page 399. Its actual nutritive value was 634.5 calories.

The balance sheet of the respiration experiment showed the following daily result:

Metabolism Of An Infant

The infant was nearly in calorific equilibrium during the period of milk ingestion. There were 634.5 available calories in the milk and 660.5 calories produced in the metabolism.

The quantity of the protein metabolism was extremely small, being 9.6 grams according to the usual method of computation. The milk contained protein to the extent of 7 per cent, of its total calorific content. Of this only 5 per cent, was metabolized and 2 per cent, was added to the body. The metabolism of an infant may therefore be maintained on a diet in which 5 per cent, of the energy is supplied by protein and 95 per cent, by fats and carbohydrates.

The specific dynamic action of the milk was almost negligible, the metabolism being approximately the same during the period of feeding as during that of starvation. Curiously enough, the protein metabolism was the same on days of milk ingestion as in starvation. The "wear and tear" quota was covered by a "repair" quota of equal amount. (See p. 282).

This child gained normally in weight before and after the respiration experiment, but during that time struggling and crying prevented fat addition to the otherwise well-developed normal infant.1

Schlossmann and Murschhauser2 note that, whereas during the first and second days of fasting an infant may eliminate 16 and 15 milligrams of urinary nitrogen per kilogram of body weight, return to a normal diet results in the elimination of only 8 milligrams per kilogram. This illustrates the avidity with which, under favorable conditions, all available protein is used for growth.

W. Camerer, Jr.,3 showed that a breast-fed infant nine months old may ingest 480 calories in the milk, produce 420 calories in metabolism, and add 60 calories to his body, or 15 per cent, of the energy content of the diet. In this case 40 per cent, of the protein intake was added to the growing organism.

Rubner and Heubner4 have reported a respiration experiment on a child seven and a half months old nourished with modified cows' milk. The intake was 682.8 calories, the metabolism 593.2, leaving 89.6 calories, or 12.2 per cent, for addition to the child's organism.

It is remarkable that a child's intuitive appetite should determine the ingestion of nutriment necessary to cover the energy requirement of his organism, and a small addition for normal development. A reduction of 15 per cent, in the intake of food would bring his prosperous growth to a standstill.

1 Heubner: "Jahrbuch fur Kinderheilkunde," 1905, lxi, 430.

2 Schlossmann and Murschhauser: "Biochemische Zeitschnft," 1913, lvi, 355. 3 W. Camerer, Jr.: "Zeitschrift fur Biologie," 1902, xliii, 1.

3 Rubner and Heubner: Ibid., 1899, xxxviii, 345.

Heubner1 says that the average normal infant requires 100 calories per diem per kilogram of body weight for normal nutrition during the first three months of his life, 90 calories during the second three months, and 80 and less thereafter. The energy content of the food should never sink below 70 calories per kilogram, which is about the maintenance minimum.

The so-called "scientific feeding" of infants is unworthy of the name unless the calorific requirement is carefully considered. From lack of this knowledge babies are frequently systematically starved.

It is evident from this discussion that the fundamental, basal metabolism of the infant cannot be determined during long periods in which crying is an ever-entering factor. Schlossmann and Murschhauser,2 for example, have found that the metabolism of an infant may double during an hour of movement when the baby would not be quieted, but cried intensely. The resting metabolism of this child five months old was estimated at 859 calories per square meter of surface in twenty-four hours. The same authors3 have shown that a change of environmental temperature between 220 and 170 C. has no influence upon the heat production of the infant. Hasselbalch4 in 1904 investigated the metabolism of newborn babies and established the fact that the respiratory quotient of the child at birth was about unity, which indicates that the earliest source of its energy requirement is derived from stored glycogen. This was confirmed by Bailey and Murlin, who also showed that on account of insufficient nourishment the respiratory quotient fell to the fasting level within twenty-four hours.

1 Heubner: "Berliner klinische Wochenschrift," 1901, xxxviii, 449.

2 Schlossmann and Murschhauser: "Biochemische Zeitschrift," 1910, xxvi, 14.

3 Schlossmann and Murschhauser: Ibid., 1911, xxxvii, 1.

4 Published in Danish; English translation by F. G. Benedict and Talbot in "The Physiology of the Newborn Infant," 1915, Carnegie Institution Bulletin 233.