Current parameters to assess nutritional status in critically-ill patients are useful to evaluate nutritional status prior to admission to the intensive care unit. However, these parameters are of little utility once the patient's nutritional status has been altered by the acute process and its treatment. Changes in water distribution affect anthropometric variables and biochemical biomarkers, which in turn are affected by synthesis and degradation processes. Increased plasma levels of prealbumin and retinol -proteins with a short half-life- can indicate adequate response to nutritional support, while reduced levels of these proteins indicate further metabolic stress. The parameters used in functional assessment, such as those employed to assess muscular or immune function, are often altered by drugs or the presence of infection or polyneuropathy. However, some parameters can be used to monitor metabolic response and refeeding or can aid prognostic evaluation.