Standard artificial feeding is not suitable for patients with acute respiratory failure due to the increase in CO2 production it entails, effect of carbohydrate metabolism. Randomized use of an enteral diet comprising 55% of fats in 10 patients with chronic pulmonary disease during an acute phase, receiving mechanical ventilatory support, achieved a rapid decrease of VCO2 value, 243 to 215 ml.min-1, while it increased to 250 ml.min-1 with the control diet. These variations are metabolic and not ventilatory in origin for they are not accompanied by changes in plasmatic total CO2 rate.