A major criterion for categorizing EN products is whether they contain more intact (polymeric) macronutrients or their macronutrient ingredients are present in simpler forms (oligomeric). Standard EN formulas are polymeric formulas; these are appropriate for most patients. Oligomeric formulas should be reserved for patients with GI dysfunction.
Polymeric formulas typically have low osmolality of 300 to 500 mOsm/kg. These formulas usually supply essential vitamins and minerals in amounts similar to the Adequate Intakes or Recommended Dietary Allowances (RDA) for these nutrients when the formula is delivered in amounts adequate to meet macronutrient requirements of most patients. Many polymeric formulas are inexpensive relative to oligomeric formulas. Most polymeric formulas are lactose-free and gluten-free, as are most modern tube feeding products. Products designed to be used as oral supplements generally are polymeric and often have sucrose or other simple sugars added to improve taste.
The oligomeric formulas are also known as chemically definedformulas. This class of formulas can be subcategorized based on whether the formula contains all free amino acids (elemental formulas) or peptides (peptide-based) as the protein source. Some formulas contain a combination of free amino acids and small peptides. Actually, dipeptides and tripeptides are absorbed more efficiently than free amino acids. Oligomeric formulas may be better tolerated than polymeric formulas for patients with defects in GI function and may be particularly useful with severe pancreatic dysfunction or significantly decreased GI surface area (e.g., short bowel syndrome).
Oligomeric formulas typically are more expensive than polymeric formulas and have higher osmolality because they contain more osmotically active particles. However, osmolality of these products usually does not exceed 700 mOsm/kg, a value less than that of many oral medications or a regular diet. In the past, there was concern that higher-osmolality EN formulas could cause GI intolerance, particularly diarrhea. This led to dilution of formulas by half or more with water and gradually increasing both the strength and rate of formula administration. This practice is unnecessary and serves only to delay attainment of goal nutritional support. Sometimes enteral feedings are diluted to deliver extra water required by the patient; this practice generally is discouraged because of potential risk of formula contamination. Instead, it is better to give extra water as boluses through the tube. If medications are administered through the feeding tube, generous amounts of water should be used to flush the tube before and after each medication; this practice helps provide extra fluid and prevent problems with occlusion of the tube.
Oligomeric formulas usually are less palatable than polymeric formulas and are not designed for use as oral supplements. Many of the oligomeric formulas provide some fat calories as medium-chain triglycerides (MCTs), a fat source that is more readily absorbed and metabolized than long-chain triglycerides (LCTs) typically found in polymeric formulas. The MCTs do not require bile salts or pancreatic enzymes for absorption. Some of the elemental formulas contain a low proportion of fat (less than 10% of total calories), which makes them useful in certain situations where fat needs to be restricted. The carbohydrate source in oligomeric formulas is also less complex than in polymeric formulas, consisting of oligosaccharides rather than hydro-lyzed starch.
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