Unique to the pediatric patient is growth and development. The term newborn infant grows at a rate of 25 to 30 g per day over the first 6 months of life, leading to a doubling of the birth weight by 5 months of age (1). The average infant triples the birth weight by 12 months. By 3 years of age, the weight is four times the birth weight, and by completion of the first decade the weight increases by 20-fold. Body length increases by 50% by the end of the first year of life and increases three fold at the end of the first decade of life. The preterm infant’s growth pattern is quite distinct from term infants. Most nutrients are accumulated by the fetus in the third trimester of pregnancy. Thus, fat accounts for only 1% to 2% of body weight in a 1-kg infant compared with 16% in a term (3.5-kg) infant. A loss of 15% of a preterm infant’s birth weight is anticipated in the first 7 to 10 days of life, compared with a 10% weight loss for a term infant. After this initial period of weight loss, a preterm infant less than 27 weeks gestation gains weight at a slower rate, approximately 10 to 20 g per day, because he or she has not yet entered the accelerated weight gain of the third trimester (2).

The serum albumin level has been used as an indicator of chronic nutritional status. However, albumin turnover is relatively slow (t^1/2 = 18 days). Therefore, other proteins such as prealbumin (t^1/2 = 2 days) and retinol-binding protein (t^1/2 = 12 hours) are better indicators of current intake. There are no established norms for serum prealbumin levels in infants and young children. Yet, ideally a baseline level is obtained, then subsequent levels can be used to establish the effects of disease and/or nutritional supplementation.

A gross estimate of calorie needs can be obtained by using the recommended daily allowance for energy requirement, based on the child’s age and ideal body weight. Energy requirements vary, depending on age and physiologic status of the child (Table 7-1). Periods of active growth and extreme physical activity will increase energy requirements. The average distribution of kilocalories in a well-balanced diet is protein, 15%; fat, 35%; and carbohydrates, 50%. Another method of measuring energy expenditure is indirect calorimetry. In indirect calorimetry, CO₂ production and O₂ consumption are measured using a metabolic cart. The sample is most accurately obtained in intubated infants, yielding a calculated resting energy expenditure. The energy expenditure or metabolic rate, as measured in cubic centimeters of oxygen per minute, can be converted to calories per hour or per day, if the substrates are known. All measurements are approximations of caloric needs for which the surgeon must further adjust according to the clinical course of the patient. Such estimations offer an excellent way to monitor patients, particularly those who are in an intensive care unit setting.

The water content of infants is higher than that of adults (75% of body weight versus 65%). Fluids provide the principal source of water; however, some is provided via oxidation of food and body tissues. Water requirements are related to caloric consumption; therefore, infants must consume much larger amounts of water per unit of body weight than adults. In general, calorie requirements (kcal per kg per day) are matched to the amount of fluid needs (mL per kg per day). The daily consumption of fluid by healthy infants is equivalent to 10% to 15% of their body weight, in contrast to only 2% to 4% by adults. In addition, the natural food of infants and children is much higher in water content than that of adults; the fruits and vegetables consumed by infants and children contain about 90% water. Only 0.5% to 3% of total fluid intake is retained by infants and children. About 50% is excreted through the kidneys, 3% to 10% is lost through the gastrointestinal tract, and 40% to 50% is insensible loss.

The requirement for protein in infants is based on the combined needs of growth and maintenance (Table 7-1). Two percent of the infant’s body weight, compared with 3% of the adult’s body weight, consists of nitrogen. Most of the increase in body nitrogen occurs during the first year of life, which explains the major protein requirements of infancy. The nutritional value of protein is based not only on the amount of nitrogen available, but also on the amino acid composition of the protein (5). Protein provides 4 kcal per gram of energy, and should generally be included in estimates of caloric delivery. Twenty amino acids have been identified, of which nine are essential in infants (Table 7-2).
New tissue cannot be formed unless all essential amino acids are present in the diet simultaneously. The absence of only one essential amino acid will result in negative nitrogen and protein balance. Protein requirements in premature infants are higher than in term infants, ranging from 3 to 3.5 g per kg per day. In very low birth weight (VLBW) infants, this requirement may approach 3.8 g per kg per day (6). Such protein loads must be balanced against the immaturity of the renal system. The development of uremia should be monitored. Three amino acids are considered conditionally essential in the neonate: histamine, cysteine, and tyrosine (7). Deficiencies in these amino acids may be due to immaturity of certain enzyme systems; thus, premature infants are at particular risk. Specialized crystalline amino acid solutions should therefore be used in premature infants who require prolonged support (more than 2 weeks) with parenteral nutrition.

The greatest contribution to caloric needs is supplied by carbohydrates. Carbohydrates are stored chiefly as glycogen in the liver and muscle, but account for no more than 10% of body weight (8,9). Of major importance is that infant liver and muscle mass are proportionately smaller than that of an adult; therefore, an infant’s glycogen, or carbohydrate reserve (34 g), is significantly smaller than an adult’s. Glycogen is converted to glucose in the liver and is then metabolized throughout the body either anaerobically to lactic acid or aerobically to carbon dioxide and water. Aerobic metabolism results in much greater production of energy in the form of adenosine triphosphate. Delivery of carbohydrates in amounts more than can be used results in hyperglycemia and lipogenesis (see the “Complications of Parenteral Nutrition” section and Chapter 8).

Fats comprise the other major source of nonprotein calories for the body, yielding 9 kcal per g when metabolized. The most common variety is the triglycerides. Naturally occurring fats contain straight-chained fatty acids, both saturated and unsaturated, varying in length from 4 to 24 carbon atoms, with most containing 16 to 18 carbon atoms. Humans do not synthesize linoleic acid, an 18-carbon chain with two double bonds; therefore, it must be supplied in the diet and is considered an essential fatty acid. Linoleic and linolenic acids are essential fatty acids for children, and neonates. Due to their limited reserves, premature infants may develop biochemical evidence of essential fatty acid deficiency in as little as 3 days, if exogenous fat is not provided. Deficiency is generally not seen in older children until subjected to 2 to 3 weeks without exogenous replenishment (10). Monitoring of deficiency is performed by calculating a triene to tetraene plasma-level ratio of starvation, where trienes consist of 5,8,11-eicosatrienoic acid and tetraenes consist of linoleic and arachidonic acids, thus creating an eicosatrienoic/arachidonic ratio. A ratio greater than 0.4 is consistent with fatty acid deficiency. Clinical manifestation of essential fatty acid deficiency consists of a flaking erythematous, papular skin rash, generally limited to the legs, chest, and face. A low lipid administration (approximately 4% of dietary needs) is sufficient to prevent essential fatty acid deficiency (11). Linoleic acid is an omega-6 fatty acid, whereas linolenic acid is an omega-3 fatty acid. The metabolism of omega-6 fatty acids through the cyclooxygenase pathway results in the formation of prostaglandin E (PGE₂) and thromboxane A (TBXA₂), both of which have immunosuppressive properties along with procoagulant activities. Provision of a balanced combination of exogenous lipids results in lower levels of PGE₂ and TBXA₂ production, and possibly less host immunosuppression.

Rapidly growing infants need more minerals than do mature adults. This is especially true of phosphorus and calcium because of the exceptional rate of skeletal growth in infants and children.

The ash content, reflecting mineral composition, of the fetus is low and at birth constitutes only 3% of body weight. It increases continuously throughout childhood, both absolutely and relatively. The mineral content in adults is 40 times greater than in newborns, whereas the body weight is only 23 times greater (12). Due to the intrinsic difficulty formulating a solution with both calcium and phosphate, conventional parenteral nutrition in VLBW infants will fail to deliver adequate amounts of these minerals for bone growth.

Trace elements comprise less than 0.01% of the total body weight in humans. They typically function as metalloenzymes, maximizing enzymic reactions. They may also act as soluble ionic cofactors or nonprotein organic molecules. Vitamins are essential components or cofactors for various metabolic reactions. All commercial infant formulas contain adequate amounts of vitamins to meet normal daily requirements. Recommendations for normal daily requirements of vitamins have been recently revised by the U.S. Food and Drug Administration (13).

Infants in good health before surgery or trauma can sustain a 5- to 7-day period of little or no energy substrate without serious systemic consequences, provided that adequate nutritional support is initiated thereafter. Premature infants less than 34 weeks gestation do not generally have a mature, coordinated suck and swallow. Feedings must therefore be provided enterally, either by bolus every 2 to 3 hours or by continuous feedings. Enteral feedings are begun after the resolution of the postoperative ileus. There are many high-calorie formulas available to address specific needs (Tables 7-3 and 7-4). Children who have disease processes associated with malabsorption may benefit from specialized formulas.

Aside from oral intake, a number of modalities are available for enteral delivery. These include nasogastric and nasojejunal feedings. Patients requiring feedings for more than 4 to 8 weeks should be considered for a more permanent feeding access (e.g., gastrostomy tube). Use of jejunal tubes has a number of problems, including involuntary dislodgement of transpylorically placed tubes and catheter obstruction due to inspissation of feedings. Short-term complications of surgically placed J-tubes include intraabdominal abscess, perforation, bowel obstruction, and volvulus with bowel infarction. When using tubes passed distal to the pylorus, continuous drip feedings are recommended to prevent the development of diarrhea and other symptoms of dumping. Verification of the location of the tube is mandatory before beginning enteral tube feedings. This requires either aspiration of enteric contents or radiologic confirmation.

Standard premature infant formulas are milk-based formulas that provide 22 to 24 cal per oz. A portion of fat is provided as medium-chain triglycerides (MCT) to compensate for the limited bile salt pool in young infants. MCTs can be absorbed directly through the basolateral surface of the epithelial cell without the need for bile salts. MCTs, however, cannot be used to prevent essential fatty acid deficiency (long-chain triglycerides). The carbohydrate is composed of glucose polymers and lactose to optimize carbohydrate absorption in the presence of limited lactase activity. These formulas also provide increased amounts of vitamins, calcium, and phosphorus compared with infant formulas. This allows the increased nutrient needs to be met with a limited volume. In addition, the increased calcium and phosphorus help prevent osteopenia. Standard infant formulas are derived from either milk or soy protein at a concentration of 20 cal per oz. If a child is gaining weight poorly, the caloric density can be increased by adding relatively less fluid to either the concentrate or the powder. Caution should be used when concentrating these formulas, as rapid advancement in concentration may lead to feeding intolerance as well as occasional cases of enterocolitis.