This is a prospective, translational study on the splanchnic metabolism of methionine to determine the rates of the different pathways and synthesis rates of glutathione when critically ill children are fed by the enteral route, and the difference among various age groups. The study size will include 45 critically ill septic, pediatric patients (15 infants at 1 month-3 years of age, 15 children at 4-12 years of age and 15 adolescent at 13-19 years of age), males and females admitted to the pediatric intensive care unit (PICU) at Children's Medical Center, Dallas. The minimal subject's weight will be 4 kg. The number of subjects includes an expected drop out rate of about 20%, in order to obtain 15 patients with complete data in each group. Patients will receive nutritional support as per standard care. This study will yield important knowledge and may lead in the future to changes in the current practice on the management of critically ill pediatric patients in the PICU.