Metabolic fuel and hormone responses to fasting in newborn infants.

To examine why newborn infants frequently cannot maintain adequate levels of plasma glucose in the interval between delivery and the time they are first fed, circulating metabolic fuel and regulatory hormone concentrations were determined in 44 healthy infants at the end of an eight-hour postnatal fast. Plasma glucose fell below 40 mg/100 ml prior to eight hours in four of 24 term-appropriate-for-gestational-age (AGA), two of nine preterm-AGA, five of six term-small-for-gestational-age (SGA), and three of five preterm-SGA infants. Fuel and hormone patterns in the premature and SGA infants were not different from those found in term-AGA infants. Results in these neonates differed in two areas from the response to fasting seen later in life. In fasted term-AGA infants, ketones were low (beta-hydroxybutyrate 0.29 +/- 0.04 mM/liter) despite elevated concentrations of fatty acid precursors (1.4 +/- 0.07 mM/liter), and the group of infants studied failed to demonstrate the increase in plasma ketones with lower glucose levels (r = ".23, P = .07) which is found in older children. Levels of glucose precursors were two to three times higher in term-AGA infants (lactate 2.9 +/- 0.2 mM/liter; alanine 0.48 +/- 0.02 mM/liter) than levels found beyond the neonatal period and, in contrast to older children and adults, were not diminished in infants with lower plasma glucose (lactate, r = -.28, P less than .035; alanine, r = -33, P less than .02). These differences between the responses to postnatal fasting and those seen beyond the neonatal period suggest that the capacity for both hepatic ketone synthesis and gluconeogenesis is not fully developed at birth.