Neonatal intestinal metabolism.

In the presence of glutamine, glucose, and 3-hydroxybutyrate, glutamine is the preferred oxidative substrate in enterocytes of suckling and weaned rats. Our studies of changes in intestinal metabolism in the developing rat clearly indicate that the oxidation of substrates that enter the citric acid cycle in the form of acetyl-CoA such as glucose, fatty acid, and lipids is low during the suckling period and increases after weaning. In contrast, glutamine that enters the citric acid cycle in the form of 2-oxoglutarate is high during the suckling period and does not change during weaning. The control of the citric acid cycle appears to be the intramitochondrial [NADH]/[NAD+] ratio, which is high during the suckling period and low in the intestine of weaned rats. These studies demonstrate how evaluating changes in metabolism during a natural perturbation such as weaning can identify control mechanisms of metabolism. Finally, the changes in substrate oxidation during weaning are not controlled by the endogenous steroid burst that occurs at 16 days of age. Substrate oxidation changes only after weaning, suggesting that a change in diet is a significant factor in intestinal substrate oxidation.