Malnutrition has a well defined effect on pulmonary physiology and pulmonary dysfunction may lead to nutritional depletion. Nutritional support is used as a part of the routine therapeutic armamentarium for patients with acute and chronic respiratory insufficiency. Since nutrients have pharmacologic properties and physiologic actions, these have to be considered, and tailored to the type of pathophysiology and stage of respiratory failure. Status of respiratory drive, respiratory muscle strength, air trapping, parenchymal inflammation, body and lung water and circulatory competence are all relevant. Nutritional support increases metabolic demand and respiratory drive with progressive improvement in respiratory muscle and parenchymal function. Carbohydrate may increase CO2 production and RQ, especially in injured or septic patients, which may result in increased ventilatory demand. Intravenous fat emulsions are an accepted alternative substrate; recent work suggests that their quantity and quality may affect pulmonary vasomotor tone and pulmonary inflammation due to changes in eicosanoid metabolism. These properties may have implications to both severe chronic diseases (i.e. cystic fibrosis) and acute respiratory failure. Amino acid infusions, especially the BCAA-enriched formulas, result in a dose-dependent increase in ventilatory drive and response to hypercapnia; detrimental effects may occur in states with high respiratory drive. In acute phase, calories are supplied in the 1-1.2 REE range; once the patient has stabilized this is increased to 1.4-1.7 REE. Glucose is supplied in the lower range of the obligatory need (2-4 g · kg-1 · day-1) and lipid, which has an equivalent nitrogen-sparing effect, is infused 12-24 h daily to provide 50-60% of daily calories. Amino acids are infused at 1-2 g protein · kg-1 · day-1. A good response to a proper nutritional regimen in respiratory insufficiency is accompanied by diuresis and a rise in serum albumin in addition to normalization of respiratory drive and muscle endurance.