The energy metabolism rate, an important parameter in severe clinical polytrauma and burns, is evaluated here using mass conservation law and simple thermodynamic considerations. The expressions for the energy and fat metabolism rates are evaluated utilizing easily measurable quantities: oxygen consumption, CO2 production, and nitrogen excretion rates. The derived equations are identical for the fasting and postabsorptive states, independent of feed composition. A new criteria for distinguishing between the fasting and postabsorptive states is suggested. These equations should help in evaluating energy metabolism rate for patients receiving continuous parenteral nutrition.