Enteral and Parenteral Nutrition

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ENTERAL AND PARENTERAL NUTRITION. The ascent of enteral and parenteral nutrition into a major therapeutic advance in the clinical care of critically ill patients as well as those with temporary or permanent loss of gastrointestinal function only occurred during the last thirty years of the twentieth century. Enteral tube feeding was first employed in the 1600s and was made popular in the medical profession by the famous English surgeon John Hunter at the end of the eighteenth century.

Three important developments accelerated the widespread acceptance of invasive feeding by tubes in the gastrointestinal tract (enteral nutrition) or in veins (parenteral nutrition). Milton Winitz and his colleagues developed chemically defined diets that provided all the essential nutrients in their predigested form. Protein and carbohydrates were provided as their basic building blocks, amino acids and glucose, with minimal amounts of fat sufficient only to meet basic requirements along with all the essential vitamins and minerals. Originally intended as low-residue diets for the American space program, they were made available, particularly by Henry Randall, via a feeding tube to surgical patients who could not be nourished with regular food by mouth. Later formulas were made more complex with protein present as peptides, which contain a number of linked amino acids, more complex carbohydrates, and larger amounts of fat. Subsequently Stanley Dudrick, Douglas Wilmore, Harry Vars, and Jonathan Rhoads administered similar predigested nutrients in concentrated form into large veins of six beagle puppies, who tolerated them well and experienced normal growth and development. Similar successes in a human infant and in malnourished surgical patients with gastrointestinal dysfunction led to widespread adoption of this life-saving and sustaining technique. Parenteral fat became available for intravenous administration as a component of parenteral formulas several years later. Finally, scientists recognized that protein calorie malnutrition (PCM) affected from one-quarter to one-half of all hospitalized adults and children (Bistrian et al., 1974; Bistrian et al., 1976), that PCM had a major impact on morbidity and mortality from an underlying disease, and that nutritional support by enteral or parenteral means could improve outcomes in malnourished or stressed individuals.

Initially a few interested individuals employed these techniques mainly in academic centers. However, enteral and parenteral nutrition have become essential components of care in critical care units, important adjuncts for many patients recovering from major abdominal surgery, and lifesaving methods for tens of thousands of individuals with permanent impairments of intestinal functions.

During the late twentieth century investigators developed a greater understanding of why PCM occurs, better methods and formulas for providing enteral and parenteral nutrition, and improved techniques for identifying the patients most likely to benefit. Whereas primary PCM develops as a consequence of inadequate intake of protein, energy, and often other essential nutrients, the PCM seen in hospitalized patients is largely due to the underlying primary disease. A process named the systemic inflammatory response develops following any major tissue injury, infection, or inflammatory disorder, such as inflammatory bowel disease or rheumatoid arthritis, and is a part of the body's innate defense system that supports the immune system and fosters healing. However, the response has potentially harmful side effects, including muscle wasting and severe PCM, if it is prolonged or severe, as when anorexia and gastrointestinal dysfunction limit the spontaneous dietary intake of food and increase the protein breakdown that are parts of the systemic inflammatory response.

The well-nourished individual can tolerate up to one week of illness without requiring invasive nutrition to avoid complications, usually infections or poor wound healing, from PCM. When malnutrition is a problem at the outset, feeding is helpful within three to five days. In the severely malnourished patient, defined by an unintentional weight loss of 20 percent or more, or the severely ill patient, especially one with a closed head injury, a major skeletal trauma, severe body burns, or a severe infection, feeding begun early can improve the outcome. The American Society for Parenteral and Enteral Nutrition developed specific guidelines for the roles of enteral and parenteral feeding that incorporate these principles (Klein et al.).

An important recognition that feeding critically ill patients, particularly if done incorrectly, could be harmful soon followed its broader application. The primary complications of parenteral nutrition include lung collapse, vein clots, infections related to the tube or as a consequence of poorer blood sugar control, or metabolic complications related to introducing all essential nutrients directly into a vein without modification by the gastrointestinal tract and the liver. With enteral nutrition the primary complication is intestinal intolerance as reflected in vomiting, diarrhea, or aspiration pneumonia. Although some have asserted that enteral feeding is more efficacious than parenteral feeding, a close look at the evidence suggests that they are probably equally effective but that enteral feeding would be preferred whenever possible. The potential complications of both modes of feeding can be minimized by assuring that trained, skilled individuals and teams use them only when indicated.

Subsequent developments included immune-enhancing diets with oils, such as fish oil and certain vegetable oils like flaxseed, containing omega-3 fatty acids; the amino acids arginine and glutamine; and nucleotides. Usually provided in combination, these nutrients, when added to a standard enteral formula, seem to improve outcomes by reducing infection rates and shortening hospital stays after major abdominal surgery for malnourished patients, patients with major traumas or burns, and patients in critical care units (Beale et al.). In addition the placement of feeding tubes by endoscope dramatically increased the number of patients who can receive enteral feeding. Placement into the upper small bowel improves tolerance to enteral feeding among critically ill patients, and placement into the stomach or small bowel with exit of the tube through the abdominal wall avoids surgical placement in less severely ill patients who need chronic feeding. The development and wide application of enteral and parenteral feeding was one of the major medical advances of the twentieth century.

See also Health and Disease; Medicine; Nutrients; Nutrition.


Beale R., D. Bryg, and D. Bihari. "Immunonutrition in the Critically Ill: A Systematic Review of Clinical Outcome." Critical Care Medicine 27 (1999): 27992805.

Bistrian B., G. Blackburn, E. Hallowell, and R. Heddle. "Protein Nutritional Status of General Surgical Patients." Journal of the American Medical Association 230 (1974): 838860.

Bistrian B., G. Blackburn, J. Vitale, D. Cochran, and J. Naylor. "Prevalence of Malnutrition in General Medical Patients." Journal of the American Medical Association 235 (1976): 15671570.

Dudrick S., D. Wilmore, H. Vars, and J. Rhoads. "Long-term Parenteral Nutrition with Growth, Development, and Positive Nitrogen Balance." Surgery 64 (1968): 134142.

Klein S., J. Kinney, K. Jeejeebhoy, D. Alpers, M. Hellerstein, M. Murray, and P. Twomey. "Nutrition Support in Clinical Practice: Review of Published Data and Recommendations for Research Directions." American Journal of Clinical Nutrition 66 (1997): 683706. Summary of a conference sponsored by the National Institutes of Health, the American Society for Parenteral and Enteral Nutrition, and the American Society for Clinical Nutrition.

Stephens R., and Henry Randall. "Use of a Concentrated, Balanced, Liquid Elemental Diet for Nutritional Management of Catabolic States." Annals of Surgery 170 (1969): 642667.

Winitz M., J. Graff, N. Gallagher, A. Narkin, and D. Seedman. "Evaluation of Chemical Diets as Nutrition for Man-in-Space." Nature 205 (1965): 741793.

Bruce Ryan Bistrian