Critically ill children have abnormal utilization of nutrients such as glucose, lipids and protein. Protein synthesis is increased mainly in the form of immune and signaling proteins, while muscle and structural protein synthesis is decreased. The metabolism of sulfur amino acids through the splanchnic area and specifically methionine and cysteine have not been investigated in critically ill septic children, despite that sulfur amino acids have important roles in thiol, antioxidant and epigenetic reactions. Methionine metabolism in sick children will be influenced by its rate of utilization through different pathways. Our study aims to investigate the metabolism of methionine and cysteine when both amino acids are given by the enteral route in critically ill septic children. The investigators are focused on the rates of transmethylation, remethylation and transsulfuration in critically ill septic children, and if the current standard nutrition maintains methionine nutritional balance and functional requirements in critically ill children fed by the enteral route.