Protein digestion begins in the stomach with the action of pepsin on the intact protein (20). Pepsin is activated by acid hydrolysis of its precursor molecule, pepsinogen. The newborn is capable of creating an acidic stomach environment by 1 week of age (21); thus, pepsin activation is thought to be intact. Dietary protein is then acted upon by pancreatic peptidases released into the duodenum. These enzymes include trypsin, chymotrypsin, carboxypeptidases A and B, and elastase, and are amino acid selective with respect to cleavage sites, resulting in peptides of relatively small length. The peptides are subsequently cleaved once more by peptidases located in the intestinal mucosal cells, absorbed as amino acids or dipeptides, and transported to the liver. Protein digestion and absorption in adults is very efficient; up to 95% of a protein load can be fully digested. Although term and preterm infants have relatively low concentrations of chymotrypsin, the carboxypeptidases, and elastase (22), they nevertheless achieve more than 80% protein digestion.

The efficiency of fat digestion in the neonate has been a controversial topic. Data from the 1970s and 1980s suggested that fat is the most poorly digested macronutrient in the neonate (23). Whereas adults will absorb close to 95% of a fat meal and term infants absorb 85% to 90%, early studies indicated that preterm infants absorb as little as 50%, depending on the type of fat presented to them (24). The perceived functional immaturity of fat digestion in the preterm infant led to modification of fat blends in the formulas used in preterm infants.

Fat digestion in the neonate begins in the stomach with the action of a lipase secreted in the mouth (lingual lipase) or by the gastric mucosa (gastric lipase) (25). The two lipases are identical, function ideally at acid pH, work primarily on medium-chain triglycerides (MCT), and do not require bile salts. Hamosh has estimated that this enzyme may be responsible for up to 50% of fat digestion in the newborn (26). Infants fed human milk have the additional benefit of a lipase secreted into the milk by the mother (27). This lipase is found in all carnivores (but not herbivores) and functions more like pancreatic or intestinal
lipases found in adults. It works primarily on long-chain triglycerides at a neutral pH, as is found in the intestine, and requires bile salts. This lipase may be responsible for the digestion of up to 20% of dietary fat (28). These two lipases are referred to as the “compensatory lipases” of the newborn and function in place of pancreatic and intestinal lipases seen in more mature humans (29).

Long-chain fatty acids are dependent on bile salts for proper micellization and uptake into the intestinal lymphatics. From there, the micelles are carried to the venous system via the thoracic duct, ultimately destined for the liver. Medium-chain fatty acids do not require micellization and can be directly absorbed into the blood stream. The bile acid, and hence bile salt, pools of the preterm newborn are low, thus restricting the fat-absorption capacity of the infant. Prenatal administration of glucocorticoids to the mother can mature the fetal bile salt pool in the preterm infant less than 34 weeks gestation to the level of the term infant (30). Without such priming, however, the preterm infant has significant impairment of fat absorption (including fat-soluble vitamins) prior to 34 weeks gestation. The fat blend in preterm infant formulas designed for infants less than 34 weeks gestation has been significantly modified to optimize fat absorption. These formulas contain a higher percentage of MCT and higher vitamin A, D, and E levels than formulas manufactured for term infants.

Like fats, carbohydrates can present a significant digestive challenge. The neonate has a limited ability to digest complex carbohydrates because of relatively small amounts of pancreatic amylase (31). Thus, beikost in the form of cereal rarely makes up a significant portion of the infant’s diet until after 4 months of age. The term and preterm newborn readily uses glucose, which can be delivered either parenterally or enterally. Intestinal glucose uptake is seen as early as ten weeks gestation, long before the fetus is viable (32). However, provision of all carbohydrate calories as glucose would result in the neonatal gut being exposed to a hyperosmolar solution with a high potential for mucosal damage.

The primary carbohydrate found in mammalian milk is the disaccharide lactose. Like other disaccharides (sucrose, maltose, isomaltose), enzymatic cleavage by a disaccharidase must occur before the monosaccharides can be absorbed. In the case of lactose, glucose and galactose are produced by the action of lactase. The disaccharidases sucrase and maltase appear very early in gestation and appear to be inducible enzymes (33). In contrast, lactase begins to appear at 24 weeks gestation and rises in concentration very slowly until term. It does not appear to be a particularly inducible enzyme (34). The preterm infant is thus functionally somewhat lactose intolerant and will have typical symptoms of gas formation, diarrhea, and acidic stools characteristic of lactose malabsorption when fed high doses of lactose. Positive hydrogen breath tests have been documented in preterm infants following lactose challenges (35).

Preterm infant formulas have lower lactose contents than term formulas for this reason. Up to 60% of carbohydrate calories in preterm infant formulas are derived from linear glucose polymers, which produce a lower osmolar load than the equivalent number of individual glucose molecules. The enzyme required to digest glucose polymers (glucoamylase) is present from 24 weeks gestation (36). The lower lactose content is also present in the premature discharge formulas, although it is likely that the preterm infant is fully mature with respect to lactose absorption at the time of discharge (37).

Healthy breastfed term infants show adequate growth on as little as 85 to 100 kcal/kg body weight per day during the first four months of life (53). Formula-fed infants have higher energy requirements (100-110 kcal/kg), most likely as a result of a lower efficiency of digestion and absorption of fat (54). The presence of a lipase in human milk increases the digestibility of its fats.

Preterm infants have higher energy requirements than term infants because of a higher resting energy expenditure and greater stool losses as a result of immature absorptive capacities (55,56) (Appendix J-1). Whereas the resting energy expenditure of the term infant is 45 to 50 kcal/kg/day, the preterm infant less than 34 weeks gestation consumes 50 to 60 kcal/kg/day (55,56). Stool losses vary between 10% and 40% of intake, depending on the diet. For example, a diet in which the carbohydrate is 100% lactose and the fats are predominantly long-chain triglycerides will promote more stool losses because of the low levels of lactase and the small bile salt pools in the premature infant. Replacement of up to 50% of the lactose with glucose polymers and between 10% and 40% of the fat with medium-chain triglycerides (MCT) appears to reduce malabsorption to approximately 10% (57). The preterm infant will need an additional 50 to 60 kcal/kg/day beyond the daily energy expenditure and the loss of energy in the stool to maintain growth along the intrauterine growth curve (10 to 15 g/kg/day). Thus, barring any excess needs from diseases that increase oxygen consumption or from malabsorption, the preterm infant will gain weight adequately on approximately 120 kcal/kg/day. Assuming that 2.5 kcal are needed to achieve 1 gram of weight gain, this upward adjustment of expected weight gain would require feeding an additional 10 to 15 kcal/kg body weight per day to the preterm infant. Thus, it is likely that energy intake on the order of 130 to 135 kcal/kg/day is a more reasonable target for the premature infant than the previous recommended intake of 120 kcal/kg/day (57).

Newborn infants are highly dependent on a source of glucose for normal brain metabolism (58). The primary source of glucose in the term infant is lactose in human milk and cow-milk formulas. Soy-based formulas provide glucose from the metabolism of dietary sucrose or glucose polymers. Preterm infants also receive glucose, initially as dextrose in parenteral solutions, but subsequently enterally from lactose or glucose polymers. Galactose is also important to the newborn, as it is needed for glycogen storage (59). The newborn infant typically utilizes between 4 and 8 mg/kg/minute of glucose (60). This figure is commonly used as the glucose infusion rate for parenteral nutrition. Because of their low glycogen stores and poorer gluconeogenic capacities, preterm infants are more prone to hypoglycemia than term infants (61). Higher rates of
glucose delivery (up to 15 mg/kg/minute) may be required in growth-retarded infants and in infants of diabetic mothers to maintain normal glucose concentrations.

Intravenous dextrose infusion rates up to 12.5 mg/kg/ minute are commonly used in preterm infants to promote catch-up weight gain. Beyond this rate, a cost/benefit analysis must be made. Although faster rates of weight gain can be achieved on higher glucose infusion rates (especially if the serum glucose is controlled with exogenous insulin infusion) (62), a higher metabolic rate and a shift in the respiratory quotient will also occur. Thus, a higher oxygen consumption rate coupled with proportionately more carbon dioxide generated by the cells may significantly affect serum carbon dioxide and ventilatory requirements. In one study, infants who received glucose and insulin remained on the respirator an average of 13 days longer than their counterparts given lower glucose infusion rates (63). Moreover, the increased “growth rates” demonstrated with high glucose infusion rates are as a result of fatty weight gain, without any increase in linear or brain growth (62). The overall metabolic cost of glucose infusion rates > 12.5 mg/kg/minute must be weighed against the benefit of increased rates of nonlean weight gain.

Lipids constitute the other major energy source for neonates. Certain fatty acids, such as linoleic (omega-6, 18:2) and linolenic (omega-3, 18:3), are essential in the diet, and their absence will produce deficiency syndromes characterized by growth failure and skin rash (64). Although the full syndrome is rare, lower essential fatty acid concentrations are seen within one week of discontinuing lipid intake. Infants receiving parenteral nutrition or on a fat-restricted enteral diet require 0.5 mg/kg/day of an intravenous fat blend containing these fatty acids at least three times per week to prevent deficiency. The American Academy of Pediatrics (AAP) has recommended that 3% of total energy intake in infants should be in the form of linoleic acid (65).

Daily fat intake varies greatly based on the method of delivery (enteral vs. parenteral) and the dietary source (human milk vs. formula). Enterally fed term infants consume approximately 5 to 6 g/kg/day of fat, whereas parenterally fed infants rarely receive greater than 4 g/kg/day, largely because of concerns about toxicity. Infants receiving human milk (especially human milk expressed by mothers who have delivered preterm) may receive up to 7 g/kg/day.

Infants fed human milk receive a unique blend of fats that has not been precisely replicated in infant formulas. Cow-milk fat is generally not well tolerated by newborn infants, forcing formula manufacturers to use vegetable oils as substitutes. The spectrum of fatty acids found in palm, palm-olein, corn, and coconut oils are distinctly different from human-milk fats.

The role of omega fatty acids such as docosahexaenoic acid (DHA) and arachidonic acid (ARA) in the infant diet continues to be a subject of intense research (57,66). These fatty acids are products of an elongation pathway from linoleic acid and are important in cell membrane structure, in cell-signaling cascades and in myelination (67). The synthetic pathways may be immature in preterm infants, and for some undetermined period of time after birth in term infants (68). Sources of these fatty acids include the placenta and human milk. In contrast, cow-milk fat and vegetable oil do not contain these compounds. A number of studies addressed whether these particular fatty acids are essential in the preterm and term infant (69,70,71,72). Because the preterm infant may be less capable of synthesizing the compounds and would have received them in utero, the European Society for Pediatric Gastroenterology and Nutrition has recommended that a source of these fatty acids be added to preterm infant formula (73). Evidence of their efficacy includes studies that demonstrate better visual acuity, more mature electroretinograms, and short-term gains in general neurodevelopment (74,75,76). Studies of the longer-term growth and developmental outcomes of infants who have been supplemented with these fatty acids are in their early stages but suggest continued positive effects on the visual system at one year of age (77). It remains unclear whether potential early neurodevelopmental gains are sustained beyond the first year (78). A review assessing the role of long-chain polyunsaturated fatty acid (LC-PUFA) supplementation on neurodevelopment concluded that the studies remain too underpowered to support a positive long-term effect (79). The lack of a consistent effect may be as a result of suboptimal dosing of the compounds. Additionally, any time a single component of human milk can be isolated and added to cow-milk-based infant formula, an important consideration is whether the component exerts its nutritional effect individually or in consort with other compounds (80).

The Food and Drug Administration (FDA) in the United States has recently approved a fungal source of DHA and ARA and its incorporation into infant formula under a provision termed “Generally Recognized as Safe” (GRAS). This designation confirms that the FDA has no questions of the manufacturers regarding the safety of the source and stability of these compounds when used in the intended matrix (e.g., infant formula). The GRAS designation is used to indicate that the ingredient in question has been in the food chain of humans and, based on prior usage or experimental evidence reviewed by an expert scientific panel, poses no safety concern. The GRAS determination does not assess efficacy claims; indeed, claims of efficacy for a new ingredient added to infant formula would be subjected to a more thorough examination process by the FDA, not unlike that required of new drugs. The major formula manufacturers in the United States have now added DHA and ARA to their term, preterm hospital, and preterm discharge formulas without making efficacy claims. The formulas appear to be as safe as formulas without added DHA and ARA. It is likely that formulas without the added fats will be phased out over time.

Carnitine is another compound involved in fat metabolism in the neonate. Although carnitine deficiency is rare in
the enterally fed infant because of high levels in human milk and supplementation of formulas, it remains a concern in infants receiving long-term exclusive parenteral nutrition in which carnitine has not been supplemented (81). Carnitine supplements can be added to total parenteral nutrition (TPN), and this is recommended in infants who receive TPN for greater than 3 weeks (82).

The full-term breastfed infant grows adequately and maintains normal serum and somatic i.e., muscle protein status on as little as 1.5 g/kg/day of protein. Although the protein content is low (1.1%), the quality of human milk protein is excellent because the spectrum of amino acids provides a unique “match” for the amino acid needs of the newborn. The protein content is predominantly lactalbumin, as opposed to casein, which makes for smaller curds and easier digestibility. Additionally, human milk is replete with nondietary nitrogen sources including nucleotides, which may enhance the immune system (87); immunoglobulins and other antimicrobial factors, which help protect the gut epithelium (88,89); growth factors, which stimulate intestinal growth (90); and enzymes (e.g., lipases), which aid digestion.

The term infant fed a cow-milk or soy-based infant formula requires a greater protein delivery rate, most likely to compensate for the less-than-ideal protein quality. Thus, the infant on cow-milk formula typically requires 2.14 g/ kg/day and the infant on soy formula up to 2.7 g/kg/day of protein (47). Cow-milk protein is predominantly casein, although a number of cow-milk-based formulas are modified to be whey predominant. The soy formulas also promote adequate growth of lean body mass. However, these formulas contain a smaller percentage of available nitrogen as essential or semi-essential amino acids (91).

Protein can also be delivered to the term infant by way of protein hydrolysate or individual amino acid formulas. These formulas are specifically designed to decrease the exposure of the infant to potentially antigenic cow- or soy-milk proteins. By hydrolyzing the cow-milk-based protein such that greater than 90% of the proteins have a molecular weight of 1,250 Daltons or less, allergic disease as a result of cow-milk allergy can be treated or potentially prophylaxed. These formulas provide approximately 2.8 g/kg/day of protein at an energy delivery of 100 kcal/kg/day.

Recommendations for protein intake in preterm infants follow many of the same parameters as in term infants. However, the needs of the preterm infant appear to be greater than the term infant. Early studies suggested that the most rapid weight gain and most efficient energy utilization was achieved with protein intakes of 3 to 5 g/kg/day (92,93). Kashyap and associates refined these goals when they demonstrated that weight gain and nitrogen retention were greatest in healthy 32-week-gestational-age infants fed 3.9 g/kg/day of protein (94). They also attempted to define the optimal energy-to-protein ratio which promoted growth, finding that approximately 30 kcal were necessary for each gram of protein delivered. In a later study by the same group, Schulze and associates proposed that preterm infants tolerate 3.6 g/kg/day of protein with an energy intake of at least 120 kcal/kg/day (95). Heird has emphasized that increasing protein delivery requires increased energy delivery, and vice versa (96). His conclusions include:

Denne has used stable isotopes of nitrogen to study the protein requirements and distribution (synthesis versus breakdown) in extremely-low-birth-weight (ELBW) infants (98). His calculations indicate that an average daily intake of 3.2 g/kg of protein is necessary to counter the negative nitrogen balance of neonatal illness and to match the expected in utero protein accretion rate. Ziegler’s data in stable, growing premature infants also confirms that enteral protein delivery less than 3.5 g/kg/day results in suboptimal growth (99).

Besides total protein delivery, recent studies have considered which amino acids may limit protein accretion in the preterm infant. The terms “essential” and “nonessential” amino acids have been replaced in the neonatal lexicon by “indispensable” or “limiting” and “dispensable” (100) because they are more descriptive of the effects of amino acids on protein metabolism. Threonine and lysine are clearly indispensable because they cannot be synthesized de novo from products of carbon intermediary metabolism. Heird suggests that these two amino acids may currently be the limiting amino acids in TPN solutions.

Finally, illnesses or medications that increase protein turnover or muscle breakdown will have an influence on protein delivery. Van Goudoever and associates demonstrated that the steroids used for the treatment of BPD cause negative nitrogen balance by increasing the rate of protein breakdown but have little effect on protein synthesis (101).

Many preterm infants receive protein initially as part of a regimen of parenteral nutrition. Intravenous amino acid solutions have advanced to the point of being specifically formulated for preterm infants. These amino acid solutions are designed to normalize the plasma amino acid profile of the healthy infant, promoting levels similar to those of a one-month-old breastfed infant (102). Anderson and associates demonstrated that dextrose solutions with amino acids given early in life promote better nutritional status than dextrose solutions without protein (97). Denne et al. have shown that newborn preterm infants respond to parenteral nutrition with an acute increase in protein synthesis and a decrease in proteolysis (103). Thus, it appears that amino acid delivery in the first days of life is critical.

The recommended amount of amino acids to be delivered and the rate of advancement remain to be determined, and are influenced by the infant’s condition. Based on the estimates of protein loss by Heird and associates (96), it is prudent to begin amino acids on day one. The BUN and acid-base status of the infant must be followed. Older studies in sick neonates suggest that 1.5 g/kg/day of amino acids on day two would achieve neutral nitrogen balance (51). However, more recent data from Thureen and associates indicate that preterm infants can be safely started on 3.0 g/kg/day on day one (86). Ultimately, the goal is to deliver between 3.0 and 4.0 g/kg/day of parenteral amino acids. Some newborn intensive care units carry stock solutions of dextrose with added protein to be started upon admission to the nursery before parenteral nutrition from the pharmacy can be ordered and obtained. The amount of protein is designed to deliver 2 g/kg/day when the fluid solution is prescribed at 75 cc/kg/day.

Preterm infants who begin enteral feeds in the first days after birth will be relatively protein restricted because they are typically given human milk or low amounts of preterm infant formula. Although human milk from mothers delivering preterm is initially higher in protein (Table 22-1) compared to term human milk (104,105,106), the content is still relatively low and the infant will likely receive low volumes. Fortification with a human milk fortifier addresses this low protein issue. Preterm infant formula has a high concentration of protein (3.1 to 3.6 g/120 kcal), but it is unlikely that the infant will achieve that type of delivery in the first week. With the trend toward increased protein delivery to the preterm infant, Ziegler and others have stressed the importance of supplying adequate energy intake (99,107).

The typical term infant requires 1 to 3 mEq/kg/day of sodium (108). Breastfed infants have a lower sodium intake than formula-fed infants, although formula manufacturers have reduced the sodium content of cow-milk formula to levels comparable to human milk. Preterm infants have higher sodium requirements and may become hyponatremic on human milk (19,110). Baseline sodium requirements range from 2 to 4 mEq/kg day in the preterm infant. The higher sodium requirement with lower gestational age is due predominantly to the immaturity of the proximal renal tubule in the small preterm infant. Treatment of these infants with salt-wasting diuretics, e.g., furosemide, hydrochlorothiazide may increase the needs to 10 to 13 mEq/kg/day.

Potassium requirements in the term infant generally range between 1 and 2 mEq/kg/day. Because potassium is also reabsorbed at the proximal tubule, the preterm infant needs 2 to 4 mEq/kg/day (110). Diuretics such as furosemide, metolazone, and hydrochlorothiazide can increase requirements to 8 mEq/kg/day.

Sodium and potassium are provided in human milk and infant formulas to satisfy the daily requirement for the normal infant. However, excessive demands from prior or ongoing losses may require supplementation. Supplements are typically in the form of chloride salts (NaCl or KCl) and are dosed based on calculations of maintenance requirements plus deficits. Depending on the acid-base status of the infant receiving parenteral nutrition, sodium and potassium can be added as chloride or as acetate.

The term infant who has not experienced intrauterine growth retardation has well-mineralized bones. Analysis of the reference fetus demonstrated that the average fetus accretes 80% to 90% of its calcium during the last trimester (2). If there has been no interruption of the process, the average term infant will grow well and remain well mineralized on a diet that provides approximately 40 to 60 mg/kg/day of calcium and 20 to 30 mg/kg/day of phosphorus. Human milk is an excellent source for this rate of delivery. Infant formulas have higher concentrations of calcium than human milk, but there is little evidence that term infants who are formula fed are better mineralized.

LBW infants (intrauterine growth retarded or preterm) have significantly lower calcium and phosphorus content than term, appropriate-for-dates infants. Daily enteral calcium and phosphorus requirements in the preterm infant are estimated at 120 to 230 and 60 to 140 mg/kg/day, respectively (111). This rate of daily accretion can only be achieved enterally. The two minerals become insoluble in typical TPN solutions when added at concentrations that would provide more than 60 mg/kg/day of calcium and 30 mg/kg/day of phosphorus (at typical fluid delivery of 150 cc/kg/day). Human milk is relatively low in calcium and phosphorus and thus must be supplemented with a fortifier to achieve adequate mineralization in the less than 1,500 g infant (112). Preterm infant formula contains enough calcium and phosphorus to deliver the recommended daily amount and some excess to begin to replace previously acquired deficits. This is important because most preterm infants do not tolerate full enteral feedings immediately after birth. Preterm infants will invariably need intakes greater than the projected daily in utero requirement to avoid becoming osteopenic since parenteral nutrition results in negative calcium and phosphorus balance (4). Because serum calcium levels will always be maintained at the expense of the bone, following calcium levels as a way of monitoring status is not useful. Instead, indicators of bone activity (serum alkaline phosphatase) or urinary phosphorus excretion are used to monitor long-term calcium status.

As with calcium, the rates of magnesium accretion in utero are high during the third trimester. Thus, the magnesium requirement for infants born preterm are greater than those for term infants. Human milk and preterm infant formula appear to support normal magnesium levels. Magnesium delivery in parenteral nutrition is titrated based on serum magnesium levels, which should be monitored. The daily enteral magnesium intake should be 10 to 26 mg/kg/day (57) although the advisable parenteral intake should be 4.3 to 7.2 mg/kg/day (111).

The majority of total body iron found in the term infant is accreted during the third trimester. The fetus maintains a constant total body iron content of 75 mg/kg during the last trimester, increasing from 35 to 40 mg at 24 weeks gestation to 225 mg at term (113). Preterm delivery results in disruption of this process, and premature infants are therefore born with lower iron stores than term infants. Small-for-dates infants are frequently born with low iron stores, presumably because of decreased placental iron transport (114). Infants of diabetic mothers are born with low stores
because much of the fetal iron is in the expanded red cell mass (114). These infants also appear to be low in total body iron, most likely as a result of altered transport of iron by the diabetic placenta, such that the increased iron need of the infants of diabetic mothers exceeds the placental transport capacity (116,117).

Ten trace elements are nutritionally essential for the human: zinc, copper, selenium, chromium, manganese, molybdenum, cobalt, fluoride, iodine, and iron (144). Zlotkin and associates have written an excellent review of trace element requirements in newborns (144). Most trace elements are accreted during the last trimester. Thus, the term infant is fully replete and needs modest dietary intake of these elements. Both human milk and infant formula ensure adequate intakes. The preterm infant or the term infant on prolonged TPN would rapidly go into negative balance of any of these elements if not provided with an exogenous source. While on TPN, infants should receive neonatal trace elements (Table 22-2). Preterm infant formulas and preterm human milk appear to supply adequate amounts of trace elements to the enterally fed premature infant (145).

Selenium is a potent antioxidant. Preterm infants have lower selenium stores than term infants (146), and this has been proposed as an etiology for diseases such as BPD and ROP (147). Studies linking selenium insufficiency with these diseases have not been persuasive (148), but the general consensus is that selenium status should be supported in the preterm infant (149). Selenium is not found in commercially available products and must be added separately to TPN at the rate of 2 μg/kg/day (149). Iodine is not added to TPN, but adequate amounts of iodine are absorbed through the infant’s skin from iodine solutions applied topically (144). Nevertheless, a dose of 1 μg/kg/day has been recommended for infants who are on TPN for more than 6 weeks (149).

Vitamin requirements in newborn infants can be most easily conceptualized by considering water- and fat-soluble vitamins separately. An extensive review of all of the vitamins and their deficiencies is beyond the scope of this chapter and the reader is referred to sources dedicated to this subject (150). This section will deal primarily with vitamins that are of particular relevance to neonates and to those with a specific risk for deficiency.