Substrate Utilization of Carbohydrates, Fat, and Protein in the Late Acute Phase of critically ill surgical patients: A retrospective longitudinal study.

Background and aims: Tailoring nutrition to measured energy expenditure (EE) and provision of adequate protein amounts, is considered gold standard for critically ill patients. There are currently no recommendations to measure specific substrate utilization or to adapt nutrition accordingly.

Methods: In this retrospective longitudinal study, we analyzed results of 316 simultaneous measurements of indirect calorimetry (IC) and urinary urea nitrogen (UUN) in 191 mechanically ventilated, critically ill patients, admitted to the surgical intensive care unit (SICU) in a tertiary medical center. We calculated substrate utilization, compared it to administered nutrition, investigated factors that may influence it and tested the added value of routine UUN measurements over IC alone.

Results: The mean total EE, measured using the indirect calorimetry module, was 1600±451 kcal/day The mean daily fat, carbohydrates and protein oxidation were 118.3±63.9 grams, 53.3±114.1 grams and 64.7±36.0 grams, respectively, accounting for 68.8% ± 31.4%, 14.2% ± 29.6% and 17.0% ± 8.6% of the total EE. Fasting and noradrenaline administration were the only factors that influenced substrate utilization, resulting in higher fat and lower carbohydrate utilization (p<0.045 for all). There were significant differences between percentage of substrate utilization and supply, with average differences of +28.5% for fat, -38% for carbohydrates and +1.9% for protein (p<0.006 for all).

Conclusions: SICU patients demonstrated very high fat utilization and lower than expected carbohydrate utilization, that differed significantly from amounts supplied. Further prospective studies are needed to elucidate the clinical impact of these findings.