Introduction

Glucose is an important, if not the sole, source of energy metabolism in the fed state for brain and other nervous tissue, red blood cells, renal medulla, and retina. Assimilation of diet-derived glucose is necessary to provide glucose per se for these tissues, to serve as a source of nonprotein energy, and to stimulate normal rates of insulin secretion required to adequately suppress protein degradation and excessive lipolysis, and to stimulate protein synthesis. Carbohydrate contributes approximately 40% of the energy intake in infants ingesting human milk or cow milk-based formulas, and lactose provides perhaps the sole source of diet-derived glucose in human milk and about 50% of the diet-derived glucose in preterm formulas.

Dietary carbohydrate is assimilated via the intestine and colon in humans of all ages, but in the preterm newborn or young infant with defective function of the small intestine, bacterial fermentation of dietary carbohydrate is an especially quantitatively important metabolic pathway for enteral carbohydrate assimilation. This process may have both beneficial and adverse effects on the infant. Figure 22.1 summarizes carbohydrate assimilation by the gut. Lactose, like other dietary sugars fed to newborn infants (such as glucose polymer), is digested in the small intestine but also may undergo some fermentation in the colon. Glucose and galactose, derived from lactose digestion, are absorbed in the small intestine, enter the portal vein, and then undergo uptake by the liver, where galactose is almost quantitatively removed by the combined processes of conversion to glucose or incorporation into glycogen.