The need for certain components of food (i.e. protein) for critically ill children is not clear. It is important to have critically ill children fed adequately to prevent that their condition becomes worse or that recovery takes longer. Research methods used in the past to investigate the need for protein (Nitrogen Balance calculations), were not sensitive enough in severely ill children. The purpose of this study is to develop a new research method to determine the need for protein in severely ill children. In order to develop this new method, more information is needed on the way the body of these children uses protein in 24-hours. In the present study during 24-hours 8 children of age less than 18 years who are admitted to either the Pediatric ICU or the Cardiovascular ICU. Subjects will receive a standard nutrition, providing an age specific amount of protein (age ≤ 3: 2.52 protein g/kg BW.d; age 4-6: 1.8 protein g/kg BW.d; age > 10: 1.44 protein g/kg BW.d) via tube feeding. They will also receive a mixture of stable isotopes of amino to investigate protein behavior in the body (protein kinetics) both by infusion in their blood and together with the nutrition. Blood will be drawn every 60 minutes during the 24-hour period and the behavior of protein and the concentrations in blood of amino acids and urea will be measured. Urine will be collected to measure nitrogen balance. The investigators will compare the results of this nitrogen balance method with the results of the stable isotope method. PIM2, PRISM, SIRS criteria will be used to get information on the severity of illness of the subjects. Also body weight and length as well as body composition of the subjects will be measured at the start and after the 24-hour period. Body composition will be measured by Bioelectrical Impedance Spectroscopy. Endpoints of the study are net whole-body protein synthesis (protein balance), 24-hour pattern of protein balance, 24-hour urea production, 24-hour nitrogen balance, 24-hour contribution of arginine kinetics to whole body protein breakdown, 24-hour muscle protein breakdown, splanchnic amino acid extraction and plasma amino acid concentrations.