Feeding the gut has several advantages over parenteral nutrition (PN), especially in critically ill patients. Enteral nutrition (EN) helps preserve the structural and functional integrity of the gut. Structural integrity of the gut is maintained by increasing mucosal mass, stimulating epithelial proliferation, and by maintaining villus height. Functional integrity is supported by the EN through its effects on maintaining tight junctions between the cells and therefore preventing translocation of bacteria. Enteral feeding stimulates the blood flow to the gut and also helps in production of a variety of endogenous agents like cholecystokinin, bombesin, and gastrin, all of which have trophic effects on the epithelial cells. Feeding also stimulates immunoglobu-lin A, which coats bacteria and prevents their adherence to the mucosa, which, in turn, prevents the initiation of the inflammatory cascade. In the setting of major trauma or insult, alimentary exclusion may impair the functional and structural integrity of the gut and can result in bacterial translocation and hence significant systemic infection.
Critically ill patients and patients with major trauma benefit the most from EN. In the critically injured patient, increasing evidence suggest that enteral feeding is important in reducing septic complications such as pneumonia and intra-abdominal abscess. A meta-analysis of eight prospective studies of trauma patients and general surgery patients has concluded that septic complications occurred less commonly in patients with enteral feeding compared to those receiving PN support (Kudsk, 2002).
Nutrition is an important adjuvant therapy for IBD patients to help improve malnutrition. More recent data suggest a role for nutrition as a therapeutic agent using enteral formula fortified with antiinflammatory cytokines such as transforming growth factor-ft. There is a separate chapter on PN and EN (see Chapter 54, "Enteral and Parenteral Nutrition").
Timing ofEN appears to be important. Instituting enteral feeding within 24 to 48 hours after severe trauma has beneficial effects, whereas delaying 4 to 5 days may be too late to gain these benefits. It is our usual practice to begin tube feeding into the stomach or SB in intensive care unit patients within 24 to 48 hours of admission. Many patients will ultimately recover and eat normally. Patients with problems such as serious head injury, however, may ultimately need to have a PEG placed for long term nutritional support.
The Harris-Benedict formula or a simplistic equation (such as 25 kcal/kg/d) is useful to estimate the amount of calories needed. Once the goal rate has been reached, evaluation with nitrogen balance or indirect calorimetry can help to achieve optimal feeding rate. In general, stressed patients should receive 1.5 to 2 g protein/kg/d with 30 to 35 nonprotein kcal/kg/d, whereas nonstressed patients should receive 1.0 to 1.5 g protein/kg/d and 25 to 30 nonprotein kcal/kg/d.
Several specialized enteral formulas are now available to meet the needs of a variety of patients. Different formulas contain varying degrees of protein, carbohydrate, and fat, depending on the patient's underlying disease process and requirements. Standard formulas contain 50 to 55% carbohydrates, 15 to 20% protein, and 30% fat, the caloric density is > 1 kcal/mL, and the osmolality is close to isotonic (between 280 and 350 mOsm/L). Carbohydrates are in the form of oligosacchrides and polysaccharides. Most of enteral formulas do not contain lactose, avoiding the most common disaccharidase deficiency. Proteins are derived from whey, meat, soy isolates, and various caseinates. Fats are usually supplied by vegetable oils and medium chain triglycerides. Numerous other lipids such as fish oils, borage oil, and structured lipids may be substituted. Fiber is added to some formulas for avoidance of diarrhea. Elemental diets and semi-elemental small pep-tide formulas maybe useful in selected patients with poor SB nutrient absorption. As mentioned, there is a chapter devoted to PN and EN (see Chapter 54).
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