 Objectives : The aim of this study is to evaluate the immune effects of enteral L-arginine administration in non surgical critically ill patients. These patients will be selected based on the decrease in nasal NO: directed immuno nutrition. The main objective is to demonstrate that L-arginine administration, as compared to placebo administration, increases nasal NO and enhances immune functions increase in HLA-DR expression on monocytes, modification of circulating Myeloid-Derived Suppressor Cells MDSC, decrease in IL-6, IL-17 plasmatic concentrations: stimulation of immune response. The secondary objective is to demonstrate the safety of L-arginine administration on organ failure and on the incidence of nosocomial infections. 