Influence of Aging on Nitrogen Accretion During Critical Illness

Background: Aging adversely affects nitrogen accretion during health, but its effect during critical illness is unknown. Nitrogen balance (NB) response to varying protein intakes was compared between critically ill, older and younger patients. Methods: Adult patients admitted to the trauma intensive care unit, given enteral or parenteral nutrition, and who had a NB determination within 5-14 days after injury were evaluated. Patients with renal or hepatic disease were excluded. Patients were categorized as older (60 years) or younger (18-59 years of age). Data are given as mean +/- SD or median [interquartile range]. Results: Fifty-four older (69 [65, 77] years) and 195 younger (35 [27, 47] years) patients were evaluated. NB was blunted for the older patients with an observed trending improvement in NB from -13.5 +/- 5.5 to -5.6 +/- 8.8 g/d (P = NS) noted at 1.5-1.99 g/kg/d. NB improved from -22.2 +/- 8.2 to -11.8 +/- 9.9 g/d (P = .05) at 1-1.49 g/kg/d and modestly thereafter for each 0.5-g/kg/d increase in protein intake for the younger patients. Serum urea nitrogen concentration during the NB was highly variable but overall greater for the older patients (20 [14, 33] vs 15 [10, 20] mg/dL, P = .001). Conclusions: Improvement in nitrogen accretion was blunted at lower protein intakes in critically ill, older patients compared with younger patients. Individualization of protein intake is warranted.