The influence of gestational age and size on the absorption of D-xylose and D-glucose from the small intestine of the human neonate.

D-Xylose absorption (using the one-hour blood levels) and active D-glucose absorption (calculated from changes in transmural electrical potential difference) have been measured after intraduodenal infusion of the appropriate sugar, in groups of neonates who were full-term or preterm, and who were in addition either appropriately sized for gestational age, or growth-retarded in utero. The one-hour D-xylose blood levels were significantly lower in preterm infants, and were even lower in the growth-retarded group. In follow-up studies D-xylose absorption rose in all groups of infants over the first three weeks of postnatal life but there was no sign of 'catch-up' in the results of the growth-retarded infants. In the studies of D-glucose absorption the theoretical kinetic parameters 'apparent P.D.max' (equivalent to Vmax) and 'apparent Km' were calculated. Normal full-term infants had values for both parameters that were approximately two-thirds of published adult values. Malnourished neonates had significantly lower P.D.max and apparent Km, and preterm infants had a lower P.D.max, than the values obtained in those infants who were full-term and appropriately grown.