Enteral feeding formulas can be categorized based on their protein source. Intact protein feeds contain protein that has not been hydrolyzed and include Ensure, Osmolite, Jevity, and Promote. Semielemental feeds contain hydrolyzed proteins (small peptides) as their source of nitrogen and include Peptamen, Alitraq, and Perative. Elemental formulas utilize free amino acids (FAA) as their source of nitrogen and include Vivonex and F.A.A. Not all patients can utilize intact protein feeds. In patients with hypoalbuminemia (albumin <2.5) there is significant gut edema and an impairment of gut cell-wall proteins necessary for nutrient absorption. Use of elemental or semielemental formulas is associated with better amino acid absorption and insulin responses compared with intact protein formulas. Semielemental formulas may be better tolerated than either intact or elemental formulas with regard to diarrhea. Because elemental formulas are hypertonic they may also have a higher incidence of nausea, vomiting, and abdominal distension.

In general, critically ill patients are hypercatabolic and mildly hypermetabolic. The goals of metabolic support strategies are to preserve organ function, body cell mass, and protein synthesis. Of note, providing high levels of nonprotein calories does not prevent protein catabolism. Overfeeding can lead to increased production of carbon dioxide and interfere with weaning from mechanical ventilation. Current recommendations suggest initiating nutritional support with total calories of approximately 84 to 105 kilocalories/kg/d with protein content of 1.2 to 1.5 g/kg/d. Adjustment of nonprotein calories is based upon indirect calorimetry (metabolic cart) to ensure patients are not over- or underfed. Assessment of total protein needs and utilization should be based on laboratory parameters (e.g., prealbumin, transferrin) and nitrogen balance (urine urea nitrogen [UUN]).