To avoid overfeeding, accurate estimates of energy and caloric requirements of critically ill patients are required. Critically ill patients typically undergo a period of catabolism, which can be associated with significant body protein loss, depending on the severity of critical illness and the length of the catabolic process. Calculation of nitrogen balance can quantify the extent of catabolism and evaluate the efficacy of supplemental nutrition in these patients. Nitrogen balance is calculated by subtracting total nitrogen losses (urine, stool, insensible losses) from nitrogen intake (1 g nitrogen per 6.25 g of protein). The primary mode of nitrogen excretion is urinary, and 24-hour urine collection for urinary urea nitrogen (UUN) is the most common means of measurement. Fecal and insensible losses are typically small but estimated. UUN can be a poor estimate of overall nitrogen losses in burn patients, when urine output is low (<1L/d), in patients with acute or chronic renal failure and in patients with enteric fistulas where exceedingly high losses occur in the fistulae output.