At any age, the most important aspects in relation to diseases are proper diagnosis and treatment; for this reason, numerous drugs and surgical methods have been developed. In recent years, however, the importance of professional treatment beyond simple medical treatment is increasing, and proper nutritional support is essential [12]. Thus, interest in nutrition is increasing in modern medicine. In the management of complications of diseases or sequelae during and after treatment, the importance of nutritional therapy or artificial nutrition is increasing [345].

Pediatric patients, in particular, require specialized attention and have diverse demands for proper growth and development, not necessarily because of illness [678]. Unlike adults, ongoing care for proper growth is also needed, and the quality of life of parents and other families caring for their children should also be considered. In addition, infants, children, and young adolescents require different medical and psychological approaches because of diseases and prognosis depending on age and weight [678].

Therefore, for nutritional approach and artificial nutrition, gastrointestinal function such as digestion and absorption is important, as well as age and clinical condition. In addition, considerations should be given to the likelihood of oral intake, the degree of functioning, the possibility of intervention, the cost of nutritional management, diet, and compliance [26]. In this article, we will examine the basic concepts of nutritional therapy in children, particularly enteral nutrition (EN).

The initial steps to nutritional therapies and approaches are to determine nutritional status through nutritional counseling [12]. Whether an intervention such as artificial nutrition is necessary in consideration of disease state, economic power, and ethical issues is determined. Then, oral nutritional supplements are added, and EN or parenteral nutrition (PN) according to the indications are considered. Essentially, EN is the basic artificial nutrition performed by medical doctors [2].

Nutritional support is provided to pediatric patients under two possible conditions [67]. The first is to supply <60% to 80% of the nutritional requirements for >10 days, or >4 to 6 hours per day of total feeding time for children with disabilities. In the case of insufficient oral intake, nutritional support should be initiated within 5 days at the age of 1 year and within 3 days at the age of <1 year.

The second is wasting and stunting status as follows: 1) inadequate growth and >1 month of weight loss for <2 years; 2) weight loss of ≥3 months or no weight gain at the age of 2 years; 3) >2 stages of weight change in the growth charts; 4) triceps skinfolds consistently below the fifth percentile of age; 5) decreased height velocity by ≥0.3 SD per year; and 6) decreased height velocity by >2 cm per year during puberty.

The concept of EN is known to have originated from ancient Egypt and later Greece [91011]. Initially, the practice was to put food into the rectum. In the 16th century, a hollow tube with a bladder attached is inserted into the esophagus. In the 18th century, orogastric tubes were designed to supply jellies and eggs along with milk and water. By the 19th century, basic foods such as broths, eggs, milk, and even alcohol were fed through rudimentary tubes to the esophagus. In the 1930s, protein hydrolysate formulations were supplied to surgical patients, and in the 1940s, the first infant formula was developed. Since the 1980s, EN has been recognized as a safe, effective, and cost-effective way to provide adequate nutrition. Accordingly, various devices for nutritional supply have been developed, and formulas with various indications and purposes have been produced [1112].

In the past, EN has been traditionally defined as providing food to the stomach or small intestine beyond the esophagus through a tube. However, recently, the need for nutritional intervention has increased; thus, various oral nutritional supplements have been developed. EN is defined as the use of oral food for medical purposes [2613]. EN is easier and safer to supply than PN. As intravenous access is not needed, EN has no side effects such as catheter-related or metabolic complications, and is advantageous in preserving the gastrointestinal function [1415161718].

As pediatric patients differ in the type and course of nutrition depending on age, consideration should be given to the EN supply [619]. The current age, life expectancy, acute or chronic disease, adverse effect of treatment, or temporary deterioration due to disease should be determined. The method and duration of EN should be decided in accordance with whether the patient's condition is stable or reversible, and with the function of the gastrointestinal tract [2021].

The indications for EN in pediatric patients are not significantly different from those required in nutritional support (Table 1). EN is needed if the general dietary intake alone cannot meet the energy and nutritional needs of children with growth retardation, weight faltering, or weight deficit [11]. It can also be considered useful for the treatment of diseases such as Crohn's disease, food allergy, and intolerance [2223]. However, the most important condition is that the function of the gastrointestinal tract allows for EN, at least in part [6]. It may also be used for various diagnoses or illnesses, but its use should be determined by consideration of the cost of the patient, or whether it is feasible or not [2425]. In neonates, EN supply is required depending on the condition in situations such as premature or necrotizing enterocolitis [7].

Absolute contraindications to EN are problems with the gastrointestinal function, such as paralytic or mechanical ileus, and intestinal obstruction or perforation. Relative contraindications include intestinal dysmotility, necrotizing enterocolitis, toxic megacolon, diffuse peritonitis, gastrointestinal bleeding, and high-output enteric fistula. However, if possible, full fasting should be avoided and a minimum intestinal nutritional supply should be maintained [2627].

The decision on the location and route of EN administration is based on the patient's disease status, the structural and functional status of the gastrointestinal tract, the purpose and duration of EN, and the risk of aspiration. The mode of delivery of nutrients is preferably through the stomach because it is more physiological [611].

Gastric feeding has the risk of gastroesophageal reflux and pulmonary aspiration. However, it can play a bactericidal role through gastric hydrochloric acid and helps to absorb certain nutrients. Bolus feeding is also possible because the position is fixed and easy to administer, and the stomach serves as a reservoir [36]. This intermittent bolus feeding provides a cyclic surge of gastrointestinal hormones, so it has a trophic effect on the intestinal mucosa, allows the feeding patient to freely perform activities, and is more physiological [628]. It does not require a feeding pump and is cheap. However, it carries a risk of osmotic diarrhea and is disabled in jejunal feeding (Table 2).

Post-pyloric feeding is performed in situations where gastric feeding is difficult, such as tracheal aspiration, gastroparesis, gastric outlet dysfunction, or previous gastric surgery. Rapid infusion of nutrients is not possible, so intermittent or continuous infusion should be performed. However, this carries the risk of high-energy, hyperosmolar feeding. Therefore, post-pyloric feeding in preterm infants requires more attention and carries a higher risk of complications. Nevertheless, continuous feeding delivered through infusion at a constant rate provides constant mucosal stimulation to aid intestinal adaptation and enable optimal absorption. It also has a lower probability of emesis than intermittent feeding, and is more effective at enteral balance and weight gain [2930]. However, the development of taste or oral motor function may become problematic (Table 2).

To compensate for the advantages and disadvantages of these feeding methods, a method of preserving the oral feeding skill through the combination of bolus feeding during the day and continuous feeding during the night may be used depending on the patient's condition.

To determine the route of administration for EN delivery, the expected duration of EN and the underlying disease of the patient or the structure and functioning of the gastrointestinal tract should be considered [6731]. Technical experience or cost, psychological approaches, or patient activity may also be an additional consideration (Fig. 1).

If a short period of EN of <4 weeks is required, nutrition should be directly supplied to the stomach, duodenum, or jejunum via a tube. Gastric tube feeding, or inserting a feeding tube directly through the mouth or nose, is a relatively less invasive procedure and less difficult to locate. It is the most common and effective method of short-term EN supply, but the possibility of blockage is high because of the small luminal diameter. Nasoduodenal or nasojejunal tube feeding can be performed when gastric tube feeding is difficult owing to the risk of aspiration. Owing to the difficulty of positioning, various methods are used, such as endoscopy, fluoroscopy, or administration of prokinetic drugs [32333435].

Even if EN is required for >4 weeks, it is positioned according to the risk of aspiration. Nutrition is supplied endoscopically or through a surgical access established using gastrostomy or jejunostomy [31]. Chronic diseases associated with nutritional imbalance or neurological abnormalities such as cerebral palsy, neuromuscular disorder, and coma are indications for percutaneous endoscopic gastrostomy (PEG) or jejunostomy (PEJ). PEG(J) is also considered for feeding and decompression if malignant tumors of the head, neck, or esophagus or chronic intestinal pseudo-obstruction is present [36]. PEG(J) feeding has fewer complications and discomforts, such as irritation, ulceration, bleeding, displacement, and clogging, than nasogastric tube feeding [37]. However, it may be difficult to apply according to the abdominal wall or cooperative condition of the patient, and may be a contraindication even if life expectancy is limited or serious coagulation disorders are present [3138].

Endoscopic techniques are the most commonly used method for enteral access, but laparoscopic, sonographic, fluoroscopic, and surgical methods are also often used [31]. In the case of the PEG tube, it can be used for a long time in a fixed position, is easy to use and remove, and is associated with relatively fewer complications. The button- or balloon-type PEG tube is designed to be fixed in the stomach and considered more frequently for pediatric patients of relatively older age. PEJ is similar to PEG but forms a tract between the duodenum and the abdominal wall. After laboratory tests (hemoglobin, platelet, and coagulation tests) are performed before and after the PEG or PEJ procedure for checking for contraindications, the procedure is performed under aseptic conditions after prophylactic antibiotic treatment [31]. After the procedure, the patient can start diet 6 hours later and full feeding after 24 hours.

EN is the preferred method of nutritional supplementation if sufficient calories are not available in oral feeding. It is preferred to nourish the stomach from above it rather than supplying nutrients to the duodenum beyond the pylorus. Intermittent bolus feeding is also preferred because it is more physiological than continuous feeding in small amounts. Supply through gastrostomy or enterostomy should be considered when long-term nutrition of >4 weeks is expected. Complications should be minimized through careful attention and regular monitoring, as problems such as EN-related infections or metabolic abnormalities can occur.