Protein for the critically ill patient--what and when?

Critical illness is characterised by catabolism of the skeletal muscle that releases amino acids for protein synthesis to support tissue repair, immune defence and inflammatory and acute-phase responses. Protein requirements for these patients have generally been based on levels that result in the lowest catabolic rates or most favourable nitrogen balance. The definition of these levels, in particular, in relation to indexing to a measure of patient weight or lean body mass, is controversial. Furthermore, optimal nitrogen balance may not necessarily equate to best clinical outcome. There is some evidence that administration of specific amino acids may be advantageous at least during the early or most catabolic phases of illness, in order to support the specific amino acid requirements of the metabolic pathways activated by the injury or infection. Current widely used guidelines differ in the protein prescription they recommend and in the timing of administration in relation to intensive care admission. A pressing need exists for well-designed randomised trials that compare differing levels of protein or amino acid provision, and the timing of this provision, for their effects on clinical endpoints.