Age-dependent differences in energy metabolism in the acute phase of critical illness

Objectives: Resting energy expenditure (REE) declines with age in healthy individuals, independent of the age-related decrease in lean body mass. The aim of this study was to evaluate whether this holds true in critically ill medical patients. Moreover, we assessed how measured REE compares with energy requirements calculated by prediction equations in different age groups.

Methods: In this retrospective cohort study, 200 critically ill medical patients with need for mechanical ventilation underwent indirect calorimetry within 72 h of admission after an overnight fast to determine REE. REE was adjusted for body weight (REEaBW). Patients were divided into age quartiles (I: 18-35, n = 21; II: 36-52, n = 43; III 53-69, n = 93; IV = 70-86 y, n = 43). Sex, Simplified Acute Physiology Score II, temperature at time of measurement, height, weight, and body mass index were assessed. We calculated energy requirements by Harris-Benedict and Mifflin-St. Jeor equations. Kruskal-Wallis test was used for group comparisons. Parameters that were significant in univariate regression entered the multivariate regression model.

Results: REE (P = 0.009) and REEaBW (P < 0.001) declined with age in our study population. Multivariate regression reveals age (R = -8.49 (95% CI -8.30- -1.83), P = 0.003), P = 0.004) and body temperature (R = 92.52 (95% CI 40.08-135.97, P < 0.001) as independent predictors for REE.

Conclusion: REE and REEaBW decrease with age in critically ill medical patients. Age and body temperature are independent predictors of both REE and REEaBW. Prediction equations underestimate energy requirements in critically ill medical patients.