Impact of Feeding on Resting Metabolic Rate and Gas Exchange in Critically Ill Patients

Background: Feeding is understood to alter gas exchange and resting metabolic rate (RMR) in critically ill patients, but there are limited data describing such changes. Methods: A large collection of RMR measurements and energy intake was assembled in mechanically ventilated and critically ill patients. The data were used to explore differences in RMR, respiratory quotient (RQ), and carbon dioxide production (V̇co₂) related to feeding state. Results: A total of 708 RMR measurements were available. No significant thermogenic effect of feeding could be appreciated. When controlled for body weight, the difference was 4.2% (1946 ± 400 kcal/d for unfed vs 2028 ± 341 kcal/d for fed patients; P =.299), and this small difference was fully eliminated when body temperature was also controlled for. RQ increased as carbohydrate and energy intake increased, but there was much variation in the relationship. Among 215 overfed patients, only 22 had RQ ≥1.0, and among 9 patients whose carbohydrate intake alone exceeded RMR, only 4 had an RQ ≥1.0. V̇co₂ was influenced mainly by the volume of oxygen consumed and to a lesser degree by carbohydrate intake. Conclusions: No appreciable thermogenic effect of feeding occurs in continuously fed critically ill patients. This factor does not need to be considered in the assessment of resting energy needs. RQ is not a reliable indicator of overfeeding, and the ability of carbohydrate intake to alter V̇co₂ is small, although not absent.