That the young and growing individual experiences greater hunger than the adult or aged individual is common knowledge. This is an obvious biological correlation. In addition to the formation of new tissues, the young mammal is usually more active than the adult, and its body surface is larger in proportion to body weight, and hence (if warm-blooded) he loses proportionately more heat. All these factors call for greater amounts of food. There are a number of conditions that may operate to produce this greater hunger in the young, (i) There may be a greater rate of secretion of gastric juice, so that the. digestion in the stomach requires less time. This would give a more continuously empty stomach. (2) There may be relative hypermotility of the filled as well as of the empty stomach because of the actual youth of the stomach tissues, because of greater motor innervation from the brain (vagus tonus), or as the result of greater concentration of specific chemical stimuli in the blood correlated with the higher rate of metabolism. (3) There may be greater sensitiveness to afferent impulses on the part of the brain concerned in the hunger sensation. Either factor alone, or all combined, would result in greater frequency and intensity of hunger and appetite.
The first question to be determined is whether the stomach of the young exhibits greater hunger contraction than the stomach of adult and aged individuals of the same species. On the basis of work done on man and dogs this question can now be answered in the affirmative, at least for these two species.
The empty stomach of the newborn infant shows the periods of gastric hunger contractions before the infant has had any experience with food. The hunger periods are more frequent in the infant than in the adult; that is to say, the duration of motor quiescence of the empty stomach between the hunger periods is shorter. In the newborn and very young infant the quiescence of the empty stomach lasts from 10 to 60 minutes; in the adult usually from 1 to 3 hours.
This greater frequency of hunger in the child is also shown by the more rapid development of the hunger period after a previous meal. In an adult (after a full meal) gastric hunger contractions do not develop for from 4 to 6 hours. If the individual is lying in bed, the time is even longer. In the normal breast-fed infant the average time of appearance of a hunger period after a full meal is only 2 1/2 hours.
Pediatricians differ as to the incidence of hunger in infants, because up to the last few years there has been no certain objective criterion for the existence of hunger. This has led to varying views and practices in regard to the frequency with which infants should be nursed, some favoring an interval of 2 hours, some one of 2\ to 3 hours, and some a 4-hour interval between nursing. Since the contractions peculiar to the cardiac and fundal ends of the empty stomach are, in normal individuals at least, an objective index of hunger, the balloon method enables us to determine the time of onset of hunger in infants after previous feeding, and the answer to this question may aid in fixing the time that should elapse between the nursings of normal infants.
Our observations were made on thirty normal breast-fed infants, from 24 hours to 4 weeks old. In every case the infant nursed until it was satisfied. Introduction of the balloon into the stomach directly after nursing leads to vomiting of the balloon and some of the food. The balloon is readily retained from 45 minutes to 1 hour after feeding. Forty-five observations were made on the thirty infants. The average time between nursing and the appearance of the period of hunger contractions was 2 hours and 40 minutes, with a maximum of 3 hours and 30 minutes, and a minimum of 2 hours and 20 minutes.
Our tracings show that when the stomach is full of food the inflated balloon in the fundus reveals practically no contractions. As the stomach gradually empties, feeble tonus contractions appear, and increase in rate and intensity until they end in a period of typical hunger contractions from 2 1/2 to 3 hours after the previous nursing. This change of the tonus contractions of the fundic end of the stomach partly filled with food into the hunger contractions of the empty stomach has already been described. Our results with the balloon method confirm the X-ray observations of Pisek and Le Wald, Czerny, Leo, and others, who contended that the emptying time of a child's stomach is to 3 hours. The time required for emptying the stomach depends, of course, on the quantity and quality of the food as well as on the rate of the gastric secretion and the vigor of the gastric digestion peristalsis.
In this connection it may be of interest to note that the mammalian infant, except when under the strict control of a pediatrician feeds, on the whole, as soon as the hunger sensation is strong enough to be uncomfortable, provided food is at hand; and he thrives on this procedure; while the infant whose routine is ordered by the pediatrician nurses every 2, 3, or" 4 hours, irrespective of the onset or intensity of the periods of hunger. Of course, a certain period of rest for the gastric glands may be beneficial.
The infant's stomach shows feeble tonus contractions of the fundal end 1 hour after nursing. As the stomach discharges its contents these tonus undulations gradually increase in frequency and intensity until by the end of from 2§ to 3 hours these become transformed into vigorous hunger contractions. The time of onset of hunger contractions after previous feeding varies for each infant. In our present- series the minimum is 2 hours and the maximum 3 hours. In the normal individual the presence of vigorous hunger contractions is probably a biologic evidence that the stomach is in proper condition to receive food. If this is the case, the stomach of a normal infant is ready to receive food from 2 to 3 hours after the previous nursing.