Optimizing protein in critical illness: Is g/fat-free mass the key?

One potential adverse effect of protein loading in critically ill patients is elevated blood urea nitrogen (BUN) concentrations. Along with kidney function, we hypothesized that muscle volume also affects the BUN trajectory with protein administration.

A post-hoc analysis was conducted of a prospective historical control study in which intensive care unit (ICU) patients were assigned to protein delivery targets of 0.9 and 1.8 g/kg/day over 10 days. Patients who received renal replacement therapy were excluded. Femoral muscle volume FMV was evaluated by computed tomography on ICU admission. Daily BUN and protein delivery were recorded throughout the 10 days.

Eighty-two eligible patients were analyzed. The BUN trajectories were slightly higher in patients with a lower muscle volume. The relative ratio of total protein delivery over 10 days to FMV showed a positive correlation with the BUN concentration on day 10 (r = 0.37, p = 0.0007); while this relationship was not significant with protein intake to actual body weight (r = 0.22, p = 0.068). In the linear mixed-effects model, protein intake per FMV was significantly associated with higher blood BUN concentrations (coefficient 0.07, 95 % CI 0.01 to 0.13, p = 0.02). Simultaneously, age and baseline creatinine were also significant predictors of BUN concentrations.

In critically ill patients, the relative ratio of total protein delivery to muscle volume was associated with BUN elevations. Protein targets may be more appropriately set based on muscle volume, for example, by using the grams per fat-free mass calculation.

The present study is registered at the University Hospital Medical Information Network-clinical trials registry (UMIN000040290, Registration date: May 7, 2020); https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000045970.