Feeding the Premature Infant – the NICU and Beyond
The premature infant’s nutritional requirements are substantially different from those of the term infant, and meeting their unique needs can be challenging. Early care in the NICU is focused on vital organ development; nutrition may be introduced only gradually in the first weeks of life in recognition of the risk of intolerance due to immaturities of the infant’s digestive system and feeding abilities. On the other hand, optimizing nutrition early in life is essential to improve survival and promote growth and development. Avoiding early malnutrition can have both short- and long-term benefits for the infant.
Physical challenges to optimal nutrition
Providing adequate nutrition to the preterm infant is complicated by immature organ systems, particularly the GI system, and metabolic processes.
GI immaturities in the preterm infant include:
- inability to coordinate breathing, sucking and swallowing
- low esophageal sphincter pressure
- delayed gastric emptying
- slower upper and lower intestinal motility
- immature digestion and absorption of carbohydrates, protein and lipids.
In addition, prior to birth the GI tract is sterile and therefore immunologically immature. Normal gut colonization, acquired through contact with the mother and feeding, may be delayed or absent following birth due to isolation of the infant and residence in the NICU setting. Preterm infants may therefore be at risk of acquiring abnormal bacterial flora and developing nosocomial infections.1
Parenteral or enteral feeding?
Taken together, these immaturities pose particular demands on the composition and route of nutrition for the preterm infant.
The rapid advancement of feeding that is possible with enteral nutrition has been shown to contribute to development of necrotizing enterocolitis (NEC). Recognizing this risk, the development of complete parenteral nutrition led to the strategy of withholding enteral nutrition in preterm infants for the first several weeks of life. Practices in the NICU have varied over the years between emphasis on early enteral feeding and complete parenteral nutrition; both strategies have benefits and drawbacks.2,3
Parenteral nutrition: Preterm infants have low energy reserves and require support soon after birth to meet their needs for energy, as well as protein and lipids. The goal of parenteral nutrition is to meet the infant’s energy needs to prevent catabolism while providing sufficient protein and lipids to avoid early deficiencies. However, while early parenteral nutrition provides crucial nutritional elements while avoiding putting stress on an immature GI system, with prolonged parenteral nutrition these digestive processes are not receiving the very stimulation that will help to mature and initiate normal GI functioning.3,4
Minimal enteral nutrition: Growing recognition of the importance of enteral feedings in stimulating growth and development of the gastrointestinal tract has prompted a number of studies that have shown the benefits and safety of early minimal enteral nutrition as a supplement to parenteral nutrition.
Minimal enteral nutrition refers to enteral feeding of breastmilk, formula, or a combination of the two. Even at very low volumes, enteral nutrition is considered beneficial to the preterm infant in large part because of its role in stimulating GI function including maintaining mucosal mass and function, supporting fluid and electrolyte balance, nutrient absorption and immune defenses.2-4
A strategy of early parenteral nutrition, followed by combined parenteral and enteral nutrition, then phasing to complete enteral nutrition is currently common practice.3,4
Enteral nutrition: when, what, and how much?
Studies suggest that minimal enteral nutrition can be started within the first days of life in many preterm infants, particularly those who are clinically stable. Extremely small volumes may be initiated to “prime” the digestive system, increasing the volume as the infant becomes more stable and tolerance is confirmed.2,3
Breastmilk provides the same advantages to the preterm infant as to the full term infant. Early immune system development is particularly important for the preterm infant to help protect against infection, including NEC, and the contributions of breastmilk to immune development are well confirmed. The nutrient content of breastmilk may not be sufficient to meet the needs of the preterm infant, particularly for calcium, phosphorus and protein, and energy in the form of fat content, and nutrient fortifiers are often recommended. Monitoring of the infant’s nutritional status is important to ensure that breastmilk is meeting the infant’s needs.3
If breastmilk is not available or not in sufficient quantity, a preterm formula containing 24 calories/oz is recommended. Concerns about feeding tolerance with cow’s milk formula – characterized by vomiting, larger gastric residuals, gas and constipation – have prompted studies on the implications of protein source and type. In term formulas, improved digestibility has been observed with hydrolyzed formulas as compared to formulas with intact protein, and whey protein-based formula has been shown to promote a faster gastric emptying rate than casein.5,6
It is difficult to develop optimal recommendations for minimal effective volumes and quantity of volume increases considering that preterm infants differ widely with respect to developmental stage, particularly GI maturity. One study comparing minimal feeding volumes with advancing volumes found that 10% of infants receiving advancing feeding volumes developed NEC versus 1.4% receiving minimal volumes. Compared with later introduction of enteral feedings, early minimal enteral nutrition does not increase the incidence of NEC, shortens the time to full enteral feedings, improves weight gain, and produces lower rates of feeding intolerance while promoting maturation of GI functioning.2,3,7
Goals of nutrition
Feeding tolerance, digestibility, and progression to full feeds are key goals in nutrition of the preterm infant. The nutritional guideline for postnatal nutrition in preterm infants, established by the AAP in 1985, aimed to duplicate normal in utero growth rates. But while weight gain has been seen as a primary goal for optimal nutrition of the preterm infant, the emphasis has expanded to include enhancement of neurodevelopment, organ maturity and functioning, prevention of infection and development of immune function.2,4
Post-discharge nutrition: after the NICU
The preterm infant’s digestive and absorptive capabilities improve as they mature and with exposure to enteral nutrition.4 However, at discharge from hospital, most preterm infants have moderate to severe growth failure and many may be malnourished. Malnutrition contributes to early growth deficits in preterm infants and may have long-lasting effects on health and neurodevelopmental outcomes. Approximately 30% of preterm infants remain below the 10th percentile for weight at 18 months, and about 20% at 7 to 8 years of age.3,8,9
Management of potential nutritional deficiency following discharge is important to the infant’s long-term growth and development. The rate of catch-up growth varies according to a number of factors including birthweight, gestational age, clinical course, and nutrition. Most catch-up growth occurs within the first 2 to 3 years of life, although it can continue into adolescence and adulthood. With the critical period of brain development spanning the first 18 months of life, accelerated rates of catch-up growth are associated with better neurodevelopmental outcomes.9
For the breastfed infant, continuing supplementation/fortification post-discharge is commonly advised. For the infant receiving preterm formula in hospital, recommendations for post-charge formula choice vary. Some sources recommend that if the infant is gaining weight well, they may be maintained on a term formula, while nutrient-enriched formulas are often prescribed for VLBW preterm infants following discharge. Several studies have reported that preterm infants fed an enriched versus standard term infant formula have greater rates of catch-up growth during the first year of life, including greater increases in head circumference. The tolerability and digestibility of the formula are key criteria in formula choice. Formula based on partially hydrolyzed whey protein is associated with appropriate growth in premature infants. It may also support feeding tolerance, since whey has been shown to have a faster gastric emptying rate than casein in term infants. Whey also has a high biological value and protein efficiency ratio, so it compares favorably to other protein sources used in routine infant formulas.3,6,8,9
Overall, the goals for ongoing nutrition of the preterm infant are to continue to meet their increased nutritional needs to safely support catch-up growth and development while supporting GI function and helping to build their developing immune system.