Outside-the-Gut Nutrition

The circulatory system is one of 13 primary physiological and anatomical systems that are challenged during the course of our lives. The understanding and importance of the circulatory system relative to parenteral nutrition (PN) support became apparent only within the past four decades, although there is evidence of research in this area from the 1600s. Stanley Dudrick, often considered the “father of parenteral nutrition,” provided the first contemporary documentation that adequate nutrition intravenously could support growth, development, and metabolism among beagle puppies (Dudrick et al., 1968).

Consensus within the medical community in the 1960s was that feeding patients parenterally was impossible, impractical, and unaffordable. The early parenteral products needed to provide energy (adequate calories) for typically hypermetabolic patients as well as for those who were unable to consume nutrients orally, and thus were “starving” because of their significant clinical conditions. Before the development of fat emulsions, dextrose was the primary energy source in these products. The tremendously high volume required to deliver adequate energy mandated central access to the vascular system. Thus, then and in some contemporary medical settings, the superior vena cava with a blood flow of 2 L/min, is the route of choice for direct cannulation. This central venous catheter approach, first described in the early 1950s, is not without its complications, however, including infections, thrombosis, and phlebitis issues, and mechanical hurdles (Ge et al., 2012). Today, peripherally inserted central venous catheter (PICC) lines are commonly used to infuse patients with appropriate nutrition support. Long-term PICC has several complications, particularly venous thromboembolism (Chopra et al., 2013; Johansson et al., 2013).

The adage, “if the gut works, use it,” is the standard of care. In the absence of a functioning gut, medical management includes parenteral nutrition in order to reduce risks of nutrient insufficiency and energy inadequacy, all of which contribute to morbidity and mortality. The formulation of a “complete” nutrition support matrix involves a spectrum of components. Product formulations typically include carbohydrates, lipids, amino acids, and micronutrients, carefully prepared to meet each patient’s unique metabolic needs. For example, about 2 g of carbohydrate/kg bw is required to avoid hypoglycemia or hyperglycemia, which can contribute to increased incidence of mortality among critically ill patients. Intravenous lipids, now in the form of emulsions with an array of long-chained and medium-chained fatty acids, administered at or below 1 g/kg bw provide energy and modulate inflammatory processes and numerous other well-established functions. A balanced blend of amino acids at 0.8–2 g/kg bw, in conjunction with adequate energy, is provided in an attempt to maintain or achieve nitrogen balance (Canada et al., 2009; McClave et al., 2009). Amino acids and dextrose contribute significantly to PN osmolality. Importantly, the addition of micronutrients markedly increases the osmolality, due to their low molecular weight and the relatively high requirements by patients (Singer et al., 2009).

A significant innovation for PN was the introduction of lipid emulsions via intravenous administration. Clinical evaluations transitioned from the initial soy to current blends of medium-chain triglycerides, olive oil, and fish oil. Achieving the appropriate fatty acid mixture with emulsifying agents yielded an assortment of fat emulsion products now available in the United States (mostly soybean oil–based) and alternatives available outside of the United States (many mixtures with soy, medium-chain triglycerides, olive oil, and fish oil). Clinical evidence indicates these kinds of emulsion products may be successfully administered as part of a regimen to reduce systemic inflammation among patients with mild to moderate inflammation (e.g., malnourished, AIDS, malignancy, elective surgery) and those presenting severe inflammation (e.g., severe sepsis, burns, and shock) (Vanek et al., 2012; McClave et al., 2009).

The initial research by the English architect Sir Christopher Wren using a goose quill and pig bladder demonstrated that the intravenous infusion of wine and ale produced similar results as the oral intake of alcohol (Annan, 1939). Decades of investigations and numerous clinical studies since that time have successfully demonstrated that parenteral nutrition support of patients is, contrary to earlier medical dogma, possible, practical, and affordable with respect to improving health outcomes for patients of all ages.

References included in this article are available from the authors.

Roger Clemens, Dr.P.H., CFS,
Contributing Editor
Chief Scientific Officer,
Horn Company, La Mirada, Calif.
[email protected]

Heather Rodriguez, R.D.,
Contributing Editor 
Clinical Dietitian, Peterson Regional Medical Center, Kerrville, Texas
[email protected]