Enteral nutrition (EN) refers to the delivery of nutrients via the gastrointestinal tract. It aligns with physiological processes, helps maintain the structural and functional integrity of the gastrointestinal system, is cost-effective, simple to use and monitor, and is associated with fewer complications. Because of these advantages, it is the preferred method of nutritional therapy. The indications for enteral nutrition depend on the patient's gastrointestinal capability to absorb and utilize the nutrients provided, as well as the ability of the gastrointestinal system to tolerate enteral formulas. As long as these two conditions are met, enteral nutrition can be used for patients who cannot consume food orally for reasons related to their disease or treatment or for those unable to meet metabolic needs solely through oral intake.
Categories of Enteral Nutrition Formulas
Enteral nutrition formulas can be classified into non-elemental formulas, elemental formulas, modular components, and disease-specific formulas.
Non-Elemental Formulas
Also referred to as polymeric formulas, these formulas use intact proteins or protein isolates as nitrogen sources. They are nearly isotonic, have a more palatable taste, and are suitable for oral or tube feeding. Non-elemental formulas are convenient to use and generally well-tolerated. They are suitable for patients with normal gastrointestinal function and are the most commonly used type of enteral nutrition formula.
Elemental Formulas
Elemental formulas are nutrient mixtures consisting of amino acids or peptides, glucose, fats, minerals, and vitamins. They are characterized by precise nutrient composition, comprehensive nutritional content, direct or near-direct absorption without the need for digestion, minimal residue, and the absence of lactose. However, they are less palatable. These formulas are appropriate for patients with partially compromised gastrointestinal digestive or absorptive function, such as those with short bowel syndrome or pancreatitis.
Modular Components
These formulas include enteral nutrition products that focus on providing specific nutrients or nutrient groups. They are used to supplement or fortify complete enteral nutrition formulas to meet the specific nutritional needs of certain patients. Common types include protein modules, fat modules, carbohydrate modules, vitamin modules, and mineral modules.
Disease-Specific Formulas
These formulas are designed to meet the specialized needs of patients with particular conditions or diseases, including diabetes, liver disease, cancer, pediatric disorders, lung disease, kidney disease, and trauma.
Enteral nutrition formulas are available in powder or liquid forms. Clinical selection should consider the specific characteristics of each formula and the patient’s condition to achieve optimal nutritional outcomes.
Methods and Routes of Enteral Nutrition
Enteral nutrition can be delivered via oral nutritional supplementation (ONS) or tube feeding.
Oral Nutritional Supplementation (ONS) involves increasing oral nutritional intake by adding nutritional liquids, semi-solids, or powders containing macronutrients and micronutrients to beverages or foods. For patients with normal or partially functional gastrointestinal systems who cannot meet caloric needs through a regular diet, oral nutritional supplementation is typically the first choice.
For those unable to meet caloric and protein targets through oral intake or for patients unable to eat orally, tube feeding is used to deliver enteral nutrition.
The delivery routes for enteral nutrition include oral intake, nasogastric/nasoduodenal tubes, nasojejunal tubes, gastrostomy, and jejunostomy. The specific route depends on the disease condition, duration of feeding, the patient’s mental status, and the gastrointestinal function.
Nasogastric/Nasoduodenal and Nasojejunal Feeding
These are among the most commonly used methods for tube feeding in clinical practice due to their simplicity. The advantage of nasogastric feeding lies in the stomach's large capacity, which makes it less sensitive to the osmolarity of the nutrition solution and suitable for various complete nutrition formulas. However, it carries risks of reflux and aspiration into the airway. Nasoduodenal and nasojejunal feeding involves positioning the feeding tube in the duodenum or jejunum, which reduces the risk of reflux. Nasogastric or nasoenteric feeding tube placement is suitable for short-term enteral nutrition support lasting less than two weeks. Prolonged use may lead to complications such as pharyngeal mucosal inflammation, discomfort, and an increased risk of respiratory complications.
Gastrostomy or Jejunostomy
For patients requiring prolonged enteral feeding, gastrostomy or jejunostomy is commonly used. These can be created surgically or through percutaneous endoscopic-assisted methods, the latter offering advantages such as simplicity and minimal invasiveness.
Methods of Enteral Nutrition Administration
Enteral nutrition can be administered through three methods: bolus feeding, intermittent gravity feeding, and continuous pump feeding.
Bolus Feeding
Prepared enteral nutrition solutions or commercially available formulas are slowly injected into the feeding tube using a syringe, with each administration amounting to approximately 200 ml and repeated 6–8 times per day. This method is commonly suitable for patients with gastrostomy who require long-term home enteral nutrition, due to the large gastric capacity and better tolerance to volume and osmolarity. It is straightforward and convenient to use.
Intermittent Gravity Feeding
Prepared enteral nutrition solutions are connected to the feeding tube via an infusion set, and gravity is used to allow the solution to drip slowly into the gastrointestinal tract. Each session typically delivers 250–400 ml, with 4–6 sessions per day. This method provides patients with more freedom to move and resembles normal eating habits.
Continuous Pump Feeding
This method involves the use of an infusion pump to deliver the enteral nutritional solution continuously and evenly over a 12–24 hour period. It is the preferred method for clinical enteral nutrition, as it is associated with fewer gastrointestinal side effects and delivers better nutritional outcomes.
The infusion of enteral nutrition solutions should follow a progressive approach. It begins with low concentration, low volume, and low infusion speed, which are gradually increased in terms of the formula's concentration, administered volume, and infusion rate. Typically, on the first day, one-fourth of the total required volume is given at a concentration diluted to twice its usual strength. If this is well-tolerated, the volume may be increased to half on the second day and to the full volume by the third or fourth day, allowing the gastrointestinal tract to adapt progressively to the enteral formula. The initial infusion rate is generally 25–50 ml/hour, with increments of 25 ml/hour every 12–24 hours, up to a maximum rate of 125–150 ml/hour. The temperature of the nutritional solution should be approximately 37°C, as cooler solutions may increase the risk of gastrointestinal complications.
Complications of Enteral Nutrition and Their Management
Common complications associated with enteral nutrition include mechanical complications, gastrointestinal complications, metabolic abnormalities, and infections.
Mechanical Complications
These include nasal, pharyngeal, or esophageal mucosal injury, feeding tube blockage, difficulties in removing the feeding tube, and complications related to the gastrostomy or other feeding access points.
Gastrointestinal Complications
Nausea, vomiting, diarrhea, bloating, and intestinal cramps are common gastrointestinal complications. Most of these can be prevented and managed effectively with appropriate techniques and timely interventions.
Metabolic Complications
These primarily involve abnormalities in water, electrolyte, and acid-base balance, disturbances in glucose metabolism, deficiencies in trace elements, vitamins, or fatty acids, and dysfunction of various organs.
Infectious Complications
These are mainly related to aspiration of nutritional solutions or contamination of the formula. Aspiration pneumonia is the most serious complication of enteral nutrition, especially in infants, elderly patients, and those with impaired consciousness. Preventing gastric content retention and reflux serves as a key measure to reduce the risk of aspiration pneumonia. Timely treatment is required in cases where aspiration is identified.