The primary factors in feeding premature infants are dependent on the development and maturation of digestion and absorption. The maturation of digestive and absorptive functions of carbohydrates, proteins, fats, minerals, and vitamins in the young premature infant were determined in relation to availability of hydrolytic enzymes, such as lipases, proteases, amylases, glucosidases, and lactase. The feeding is dependent on the ability of the premature infant to secrete salivary enzymes, gastric acid, pepsin, pancreatic exocrine enzymes, the presence of enterohepatic circulation, and the hydrolytic and absorptive capacity of the entercocyte. To evaluate the complexity of the gut maturation process, we proposed a unified concept where the ontogeny of the gastrointestinal system is the result of the following four major determinants: genetic endowment, intrinsic developmental and biological clock, endogenous regulatory mechanisms, and environmental influences. The developmental clock represents a predetermined temporal sequence of happenings in ontogeny that is inherently controlled. By 20 weeks of gestation, the anatomic differentiation of the fetal gut has progressed to the extent that it resembles that of a newborn. Secretory and absorptive functions, however, develop at different rates; the intestinal absorptive process is only partially available before 26 weeks of gestation, whereas gastric and pancreatic secretion is only basal and can be stimulated only partially even in the full-term newborn period. Regulatory mechanisms control the expression of the genetic endowment at various stages in gastrointestinal development. Neural-hormonal factors play major roles in the ontogeny of the gut. Adrenalectomy, hypophysectomy, and thyroidectomy delay the development of the gut. Administration of glucocorticoids or thyroxine at the critical stage in maturation causes early appearance of enzymes within the intestine. Other hormones that are potentially important in regulating gastrointestinal development include cholecystokinin, gastrin, secretin, which have trophic effects on the gastrointestinal tract, and insulin, insulin-like growth factors, and epidermial growth factor. The development of gastrointestinal secretory function, particularly in response to hormonal stimulation, has received considerable attention. The degree of response of the target cell is determined not only by the amount of effective hormone reaching it but also by the number and affinity of receptors on its surface. Human newborns have high levels of gastrin in their sera, yet have low acid output. Exogenous gastrin is an ineffective stimulant despite the presence of seemingly "anatomically developed" parietal cells. It seems that neither endogenous nor exogenous gastrin has an effect on the target cell. If one accepts the role of circulating gastrin levels in the regulation of its own receptor, one can hypothesize the absence of a regulatory effect of gastrin in the newborn period. It was shown that hormonal regulation of migrating activity by mot...

The ability of newborns to digest proteins, fats, and carbohydrates depends, to a large extent, on their level of exocrine pancreatic function. Building on the limited published data, we studied pancreatic enzyme activities in the duodenal fluid and the response of the exocrine pancreas to secretogogues in 15 premature and full-term infants at birth and at 30 days of age. We compared these findings to those obtained from identical studies of 17 children age 2 years and above. In addition, we measured the pancreatic exopeptidase, carboxypeptidase B, in relation to other pancreatic enzymes. The duodenal fluid of newborns and infants contained no amylase and negligible lipase. Carboxypeptidase B levels were also low compared to those in the older children. In contrast, chymotrypsin activity in infants was about 50% to 60% of level found in the older children. Trypsin activity, the highest of all enzymes measured, was about the same in both newborns and older children, with a transient increase at 30 days. Administration of pancreozymin had no effect on pancreatic enzymes in the duodenal fluid of newborns and a slight effect on 1-month-old infants. But by age 2 years, a full response of the pancreas to pancreozymin was evident. In infants and newborns, responses to secretin were poor. Thus, the secretory response of the human pancreas to secretogogues, absent or minimal at birth, is acquired during the postnatal period.

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A B S T R A C T The activities of intestinal sucrase and isomaltase are not detectable in rats before 15-16 days of age, but administration of corticosteroids precociously induces the activities of these two a-glucosidases. 9-day old rats were removed from their mothers, warmed in an incubator, and fed by constant infusion through gastrostomies. The basic diet was a soya preparation to which various sugars were added. When the diet contained 2% sucrose, diarrhea ensued for 48 hr, but subsided when intestinal sucrase and isomaltase appeared precociously. In animals fed sucrose, the activities of sucrase and isomaltase were markedly increased as compared to animals on carbohydrate-free diets (sucrase 2.41±0.23 vs. 0.63± 0.13 U, isomaltase 3.43±0.42 vs. 0.78+0.18 U). Maltase activity was doubled, while lactase was not altered significantly. The mitotic index of crypt cells, the depth of crypts, and incorporation of thymidine-3H into DNA were increased. In adrenalectomized rats, activities of sucrase and isomaltase were not detected nor induced by sucrose. Steroids given to adrenalectomized rats caused appearance of the enzymes; but if cortisone and sucrose were given together, there was synergism evidenced by a marked increase in activities (sucrase 7.2± 1.1 vs. 0.68±0.12 U). In contrast to observations in adult animals, the effect of sucrose on a-glucosidases in developing animals demands the participation -of the adrenal gland. INTRODUCTIONLactase is very active in the intestine of the newborn rat, but then gradually decreases so that by the end of the suckling period the activity is very low (1-4). In contrast, the activities of the intestinal a-glucosidases are either very low (maltase) or undetectable (sucrase and isomaltase) during the early suckling period (2, 3, 5). Sucrase and isomaltase appear by the 15th to 21st postnatal day, and attain adult values by the 30th postnatal day (2, 3, 5). Similar developmental patterns for the disaccharidases have been observed in the intestine of pigs (6), cows (7), and dogs (8) in contrast to the human in whom all of the intestinal disaccharidases are present and active before birth (9-11).Precocious development of intestinal sucrase and isomaltase in rats can be stimulated by injection of corticosteroids before the time these enzymes would normally appear (5). Simultaneously with these biochemical changes, the morphology of the intestine is altered so that it displays a more mature histological appearance, similar to that found in the adult animal (3).Recently it was observed that when adult humans were fed high concentrations of sucrose, there was an increase in the activities of intestinal sucrase and maltase (12). Likewise, when elevated dietary carbohydrate was fed to fasted adult rats for 24 hr, there was a twofold increase in the activities of sucrase and maltase (13,14). Neither adrenalectomy nor administration of steroids had an effect on the activities of the disaccharidases in the adult animal (14).In the present investigation, we have utilized the suckling ra...