In addition to the great thoughts of Alchemist and Nawal, mines are shown below… In summary life is about BALANCE and the balance theory necessitates calories diversion as explained below. The concept is the same among Adults and Pediatrics.
The source of provided calories to meet Energy Expenditure must be diversed between Dextrose and Lipid as the oxidative capacity of both dextrose and fat in Pediatrics (and in adults) is limited. Therefore, around 60% of calories must be provided as Carbohydrate (not more than 70% and not less than 20%). In adult, the maximum oxidative capacity is 5mg/kg/min and if you do a simple calculation, this accounts to 60% of total needed calories. The minimal dextrose that needs to be provided to an adult patient even with Glucose intolerance is 120-150g per day; this accounts to 2 mg/kg/min or 25% of total calories. Extra dextrose calories above 5mg/kg/min or 60-70% of total calories leads to Liponeogenesis (de novo lipogenesis) thus huge production of CO2 (RQ = VO2/CO2 more than 8).
Lipid is much safer source of calories than carbohydrate if the clearance capacity is assured. As said by Alchemist, lipid produces lower CO2 upon oxidation (Respiratory Quotient is 0.7 compared to 1 with Dextrose). Lipid does not cause electrolytes derangement like dextrose as fat stimulates little insulin release. However, Dextrose is much more efficient source of calories as it stimulates more Insulin that is important for the shift of intracellular ions; therefore synthesis of a balanced cell. Extra lipid beyond 1.5g/kg/day or 60% of total calories lead to Lipogenesis (Fatty liver disease)
Minimal dose of lipid to prevent Essential Fatty Acid deficiency is 10% of total calories or around 0.25g/kg/d. We make it 40% to support Dextrose in the formation of cell membrane that is phospholipid in nature and to replete the wasted calories from the catabolic/metabolic process.