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Normal postnatal glucose homeostasis is established by increased glucose production and glucose utilization.1 Factors that promote glucose production and release into the circulation include catecholamines and glucagon, which activate glycogenolysis. A high glucagon-to-insulin ratio, which induces synthesis and activity of the enzymes, is required for gluconeogenesis. Once normal feedings are established, glycerol and amino acids continue to fuel gluconeogenesis while dietary fatty acids activate the enzymes responsible for gluconeogenesis. Additionally, galactose derived from hydrolysis of milk sugar (lactose) in the gut increases hepatic glycogen production for sustained between-feeding hepatic glucose release from glycogen breakdown. Feedings also induce production of intestinal peptides, or incretins, that promote insulin secretion. Insulin decreases hepatic glucose production and increases glucose utilization for energy production and storage as glycogen. These opposing conditions of glucose production and utilization continue in response to normal feed-fast cycles, regulating normal plasma glucose concentrations.2

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Hypoglycemia

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Glucose is the major source of energy for organ function. All organs use glucose, and glucose deficiency leads to impaired cardiac performance, cerebral energy failure, hepatic glycogen depletion, and muscle weakness.3 Cerebral glucose metabolism accounts for as much as 90% of total glucose consumption in the newborn. Thus, maintenance of glucose delivery to all organs, particularly the brain, is an essential physiological function.4 Although alternate fuels can substitute for glucose metabolism, concentrations of these substances often are low in newborn infants, especially preterm infants. Newborns, therefore, are especially susceptible to hypoglycemia when they are exposed to conditions that impair glucose homeostasis during the transition from intrauterine to extrauterine life.

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Definition

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Hypoglycemia ought to be defined as any glucose concentration below the lower limit of the normal range of blood or plasma/serum glucose concentrations.5 This concentration, however, is uncertain, controversial, and variably defined. Early statistical evaluations in term infants defined hypoglycemia as a blood glucose concentration below 35 mg/dL, or a plasma glucose value below 40 mg/dL; even lower concentrations were used to define hypoglycemia in preterm infants. Such statistical definitions, however, have limited biological or clinical significance, as physiologically hypoglycemia is present when the concentration of glucose in the plasma produces glucose delivery rates that are inadequate to meet essential requirements for glucose utilization. Such requirements vary considerably among infants and over a broad range of glucose concentrations. Definitions of normal and hypoglycemic glucose concentrations also depend on postnatal feeding practices and when birth glucose concentrations are measured. Glucose concentrations are higher, for example, when feedings are initiated soon after birth than when feedings are delayed for several hours. Postnatally, the blood glucose concentration normally decreases to its lowest value between 1 and 3 hours after birth, followed by a progressive increase to greater than 50 to 60 mg/dL by 12 to 24 hours (Fig. 51-1).6

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Figure 51-1.
Graphic Jump Location

Plasma glucose concentrations during the first week ...

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