 Background : A meta-analysis has demonstrated the beneficial effect of immuno nutrition in surgical patients, leading to half reduction of incidence of nosocomial infections HEYLAND DK, JAMA ; 2001. This beneficial effect seems to be related to L-arginine content of formula. In medical intensive care, such an improvement has not been shown, in spite of similar impairment of immune response, which could be due to a more heterogenous population. Our hypothesis is that this beneficial effect could be observed in selected patients of medical intensive care units. L-arginine is a semi-essential amino acid that is the precursor of nitric oxide NO synthesis. NO is involved in immune response regulation and has antimicrobial properties, notably into airways where it can be measured in exhaled gas. A decrease in exhaled and nasal NO has been demonstrated in critically ill patients, which may suggest an impairment of its production. 