Effect of protein intake on ventilatory drive

Previous studies have demonstrated that if isotonic amino acid infusions were administered at a rate that approximated normal daily protein requirements, a leftward shift of the minute ventilation·Pa(CO2) relationship occurred. This study examined the effect of the administration of parenteral nutrition, at a fixed caloric intake and two levels of nitrogen (N) intake, on the ventilatory response to CO₂ in nutritionally depleted patients. The intent was to determine whether increasing protein intake from normal to twice normal requirements would result in a further enhancement of the ventilatory response to CO₂. Eight patients with nutritional depletion (greater than 10% weight loss) were studied. The resting energy expenditure (REE) was measured during administration of 5% dextrose, using principles of indirect calorimetry. Each patient received parenteral nutrition for a 2-week period. Two diets were examined for a 1-week period each: 1) a high N intake - 15 mg nitrogen per kcal REE (approximately 21 g/day), or b) a low N intake - 7.5 mg nitrogen per kcal REE (approximately 11 g/day). The initial diet was assigned randomly. Total energy intake was set at 1.35 x REE as measured during administration of 5% dextrose solution. Nonprotein calories were administered as 50% glucose and 50% fat. Breathing patterns at rest and during inhalations of 2 and 4% CO₂ were analyzed using a canopy-computer-spirometer system. With an increased nitrogen intake there was a significant reduction in resting arterial Pa(CO2) from 39.9 to 37.6 mmHg (P < 0.05) with no significant change in pH. The relationship between V̇ and Pa(CO2) observed during inhalation of CO₂ showed a marked leftward shift (P < 0.01), indicating an increased ventilatory sensitivity to CO₂. These data indicate that increasing the protein component of a fixed caloric intake will enhance the ventilatory response to carbon dioxide.