Nutrition 411: Selecting the Right Tube-Feeding Formula

  Because of the many different tube-feeding products on the market today, formula selection can be quite confusing. The task becomes more complicated if the patient has several comorbidities. Imagine trying to select just the right formula for a patient with diabetes, renal insufficiency, and a pressure ulcer who is overweight but has very little muscle mass and states he has a history of constipation. In order to have the best possible patient outcome, it is important to select the most appropriate formula, particularly if the patient is NPO and fully dependent on tube feeding to meet 100% of his nutritional needs. Clinicians should understand the basic product characteristics available before contemplating specific choices.

Caloric Density

  A standard or so-called “house” tube-feeding formula usually provides 1 calorie per 1 milliliter (mL) and a nutrient composition quite similar to what is recommended for healthy individuals. There are also formulas that provide anywhere from 1.2 to 2.0 calories per mL. Caloric density is an important consideration for patients with volume restrictions. For example, 1 L of a standard product provides 1,000 calories. If a patient is volume-restricted due to congestive heart failure, pulmonary insufficiency, impaired renal function, or other problem, the same 1 L of a more calorically dense product can provide 1,500 or even 2,000 calories. Calorically dense formulas often are used for overnight feedings or bolus feedings where the aim is to give the patient a large number of calories in a short amount of time. However, there are some trade-offs, such as an increase in osmolality.

Osmolality

  Osmolality is the measure of size and quantity of ionic and molecular particles within a given volume. The unit of measure is mOsm/kg of water. Isotonic means that a formula is within the normal physiologic range, approximately 300 mOsm/kg of water.¹ Generally, tolerance problems are minimized by using an isotonic formula. As more calories are packed into a given volume, more molecular particles are present; hence, the osmolality rises. These formulas are hypertonic and can be as high as 1,000 mOsm/kg of water or more. Patients will tolerate products differently depending on their specific medical condition and gut integrity, but a higher osmolality can mean a greater risk of tolerance problems such as osmotically induced diarrhea. Formulas can be diluted with water to decrease the osmolality but this is discouraged because today, patients are frequently fed using a closed delivery system. A more common approach to dealing with high osmolality formulas is to begin at a lower infusion rate and advance gradually toward the goal rate while monitoring gastrointestinal tolerance.

Delivery Systems

  Depending on how a tube-feeding product is packaged, infusion is via an open or closed delivery system. The open system utilizes either a large syringe or an open-top container for tube-feeding delivery. Products include flip-top cans, bottles, brick packs, or powder packages that require reconstitution with water. In the closed system, a container is prefilled with the sterilized tube-feeding product; the bottle then is spiked with tubing and attached to the enteral access device. The container usually contains at least 1 L of product and formula hang time is usually between 24 and 36 hours, as long as sterile technique is used.

Fiber Content

  Tube-feeding products vary in the amount and type of fiber they contain. If a patient is suffering from diarrhea or constipation, it is important to look at the fiber content of the formula. Sources of fiber in enteral formulas include both soluble and insoluble fiber. Insoluble fiber, also known as bulking fiber, does not dissolve in water. Soluble fiber dissolves in water and helps moderate bowel function. A balance between both types often is used; this minimizes concerns about the fiber clogging a feeding tube because of the increased viscosity from the soluble fiber. In addition, some formulas contain fructooligosaccharides (FOS). FOS are a type of prebiotic, a substance that helps maintain an optimal ecological balance in the gastrointestinal tract. FOS also may help control Clostridium difficile infections by restoring the gut microflora and creating an environment that inhibits growth of the pathogen.²

Formula Composition

  Another way to classify tube-feeding products is by the degree of digestion necessary before absorption. Before selecting a formula, the patient’s digestive and absorptive capacity must be assessed. A formula with the appropriate-sized molecules then can be ordered. There are three broad classes of products: polymeric, peptide-based, and modular. Polymeric formulas contain intact protein. Patients who have normally functioning digestive tracts should be able to digest and tolerate intact proteins. However, persons with impaired GI function may have difficulties with polymeric formulations. Peptide-based formulas may be better tolerated in these cases because the protein source is hydrolized protein, di- or tri-peptides, or free amino acids. Modular formulas supply only one macronutrient, usually protein or fat. Modules often are used to customize or adjust another formula. For example, in order to increase the protein content of a standard formula, a protein module can be added in the form of protein powder or liquid. Like adding water to a dilute a tube feeding, the practice of adding modular products to a closed system is discouraged because of infection control risks. The act of opening a closed delivery system defeats the original intent of the closed system. In addition, with the number of formulas available today, it is often simply a matter of selecting a different formula because one is available for almost every need.

Vitamins/Minerals/Electrolytes

  Each formula requires a different volume to meet 100% of the recommended dietary intake (RDI) of vitamins and minerals. The manufacturer’s information typically lists the number of calories and number of mL required to reach 100%. For example, a patient on Perative (Abbott Nutrition, Columbus, OH) would have to receive 1,155 mL or 1,500 calories to meet 100% of the RDIs.³ This issue is important when considering patients with certain diseases such as renal or hepatic insufficiency. Formulas specifically made for these diagnoses tend to have a lower amount of vitamins, minerals, and electrolytes. It is important to read the label if you are concerned about the amount of a certain vitamin or mineral.

Disease-specific Formulas

  Formulas are available to treat specific conditions such as pulmonary disease, impaired glucose tolerance, renal disease, immunocompetence disorders, and others. Although some practitioners may think disease-specific formulas have more to do with marketing than science, many of these products contain proprietary ingredients or nutrient compositions that make them markedly different from a standard or house formula. Several studies have been conducted comparing a disease-specific formula to a standard formula. In a 2009 study,⁴ two different protocols were used to measure postprandial glycemia and insulinemia. The first protocol was used in 22 subjects with diabetes who were fed a diabetes-specific or standard formula. In the second protocol, continuous glucose monitoring was used to assess glucose levels in 12 enterally fed patients with diabetes receiving the standard formula followed by the diabetes-specific formula continuously for 5 days each. End points included postprandial glycemia and insulinemia, glycemic variability (mean amplitude of glycemic excursions [MAGE]), mean glucose, and insulin use. The diabetes-specific formula reduced postprandial glycemia, mean glucose, glycemic variability, and short-acting insulin requirements. According to the authors, these results suggest potential clinical usefulness of a diabetes-specific enteral formula for minimizing glycemic problems in hospitalized patients. This reiterated the conclusion found in an earlier meta-analysis⁵ of 23 studies (comprising 784 patients) of oral supplements (16 studies) and tube feeding (seven studies). The majority of the studies compared diabetes-specific with standard formulas. The aim of this systematic review was to determine the benefits of nutritional support in patients with type 1 or type 2 diabetes. These authors concluded that short- and long-term use of diabetes-specific formulas as oral supplements and tube feedings were associated with improved glycemic control compared with standard formulas. They state that if such nutritional support is given long term, this may have implications for reducing chronic complications of diabetes, such as cardiovascular events. That said, in clinical practice some institutions support the use of disease-specific formulas while others do not. It is up to each practitioner to become familiar with the various products and the relevant literature in order to make an informed judgment that will produce the best outcome possible for each individual patient.

Practice Points

  Table 1 highlights some of the products currently available, including those most commonly used for patients with wounds. Most facilities follow a formulary, but the formulary should be reviewed annually and new products tried as they become available. Product representatives are always happy to supply samples for a trial period, so investigate some of the products you may not currently use. Employ critical thinking skills to evaluate the merits of each formula by reading the label, understanding the ingredients, and reading the research. Registered dietitians (RDs) are your resource for tube-feeding information — order a consultation when necessary. Selecting the formula is only the first step. Infusion rates, tolerance problems, hydration issues, body weight changes, and many other physiological and ethical issues also may need attention.

Coming next month: Dysphagia

Nancy Collins, PhD, RD, LD/N, FAPWCA, is founder and executive director of RD411.com and Wounds411.com. For the past 20 years, she has served as a consultant to healthcare institutions and as a medico-legal expert to law firms involved in healthcare litigation. Correspondence may be sent to Dr. Collins at NCtheRD@aol.com. This article was not subject to the Ostomy Wound Management peer-review process.