By William Hay, Jr., MD
Professor of Pediatrics, University of Colorado, Retired
Chief Medical Officer, Astarte Medical
Preterm infants are born weighing only a fraction of what a normal infant born at term weighs. Growing the preterm infant to term size requires considerable amounts of nutrients, of all kinds and in the right amounts, to promote healthy growth. Delivering and maintaining such nutrition to very preterm infants is essential to promote growth of all body organs and tissues, but particularly to ensure appropriate growth and development of the brain that leads to better long term neurodevelopmental outcomes. How best to do this is hotly debated and the focus of considerable research and clinical trials, but there is general consensus about the goals. According to the American Academy of Pediatrics, nutrition of preterm infants should support their rate of growth and body composition that matches those of a normally growing human fetus, as well as to maintain normal concentrations of blood and tissue nutrients.
Unfortunately, many preterm infants do not grow after birth for many days and their subsequent growth does not keep up with normal fetal growth rates; thus they end up relatively growth restricted by the time they reach term age. A principal reason for the failure to achieve desired postnatal growth in very preterm infants is inadequate nutrition, as shown by cumulative deficits in energy and protein with current nutritional practices that compound the effects of many stressful conditions that prevent growth or actually break down body tissues.
Nutrition of very preterm infants rests on several basic principles.
- Nutritional requirements do not stop with birth.
- Nutrient requirements of the very preterm infant are at least equal to those of the fetus of the same gestational age.
- The smaller the infant, the smaller the amount of nutrients stored in the infant’s body.
- Preterm infants can be fed intravenously (IV) or by milk and formula. But IV nutrition is always indicated when nutritional needs are not met by milk or formula feeding alone.
IV nutrition: IV nutrition should start right after birth to match the relatively continuous supply of nutrients that the fetus receives from the mother. Intravenous feeding uses solutions of amino acids and glucose and separate solutions of lipids (fatty acids, mostly) that are pumped through very small plastic tubes (catheters) that are placed into very small blood vessels in the infant’s legs or arms or even its scalp. Essential amino acids, those that must come from the diet because they cannot be produced within the body, are particularly important, because they are the structural components of all cells and tissues in the body. Unfortunately, they still are not provided in needed amounts in currently available commercial IV nutrition solutions. Because the preterm infant grows at rapid rates, IV amino acids are critically important for supporting protein balance and growth. Appropriate amounts of energy, primarily from IV glucose and fatty acids, also are necessary to support growth, but excessive energy intake produces excessive fat (even making the preterm infant relatively obese), not greater protein balance. IV glucose infusion rates must be adjusted frequently to maintain normal glucose concentrations, because blood glucose concentrations that are too low (hypoglycemia) or too high (hyperglycemia) cause tissue injury and brain damage. Essential fatty acids, particularly the omega 3 essential fatty acids, are fundamentally important for brain growth, much more so than just more lipids that contribute excess calories and lead to too much fat production (obesity). Growth factors, particularly insulin, should be produced by the infant in response to appropriate nutrient supplies, and should not be infused. Emphasizing greater amounts of IV amino acids relative to energy reduces the time to regain birth weight and promotes earlier discharge from the hospital.
Milk and formula Nutrition: Intravenous feeding may be essential, but it should not be a long term substitute for milk and formula feeding. Milk and formula are usually fed by a tube into the stomach, since very preterm infants cannot suck well enough for oral feeding. Such feeding into the stomach is known as enteral feeding. Enteral feeding should be started as soon as possible after birth, using mother’s milk as the first choice. Maternal milk feedings reduce the risks for infection and necrotizing enterocolitis. Necrotizing enterocolitis (NEC) is a dangerous condition that involves death of sections of the intestines and colon and spreading of infection throughout the body. Mother’s milk also specifically improves brain growth and neurodevelopmental outcomes in preterm infants. Even donor human milk, despite its reduced immune functions and protein concentrations, reduces the risk of NEC relative to formulas. Enteral feeding with milk improves gastrointestinal development and growth, which does not occur with IV feeding. Enteral feedings also promote gastrointestinal motility, particularly emptying of the stomach into the intestines, which improves how well the infant can advance to full enteral feeding. Improved GI function also decreases the need for phototherapy for jaundice, decreases the risk for liver failure, improves bone mineralization and bone growth, and increases gastrointestinal blood flow and gastrointestinal growth hormones that help the GI track take up nutrients.
Enteral feeding should begin with small, growth promoting amounts and advanced as rapidly as tolerated. Most studies now show that enteral feeding, particularly of mother’s milk, can start as early as the first day of life in relatively stable preterm infants and can be advanced to full enteral feeding by 7-10 days, even in extremely preterm infants.
The amount of protein in mature mother’s milk and donor milk, when fed at normal rates, is still not sufficient to support appropriate protein balance and growth in very preterm infants. For such infants, maternal and donor milk are commonly supplemented with milk fortifiers that increase the amounts of protein and energy in the milk. Simply providing larger volumes of unfortified milk works for preterm infants closer to term, but in the very preterm infant does not provide necessary amounts of protein and minerals for better growth.
How are we doing with nutrition of preterm infants? The good news is that modern nutritional practices are producing growth rates of preterm infants that are beginning to match those of the normally growing, healthy fetus. Preterm infants today are showing improved neurodevelopment with higher IQs and more normal body composition, with less risk of long term problems such as obesity. But, there still is much work yet to do to truly promote the best nutrition of preterm infants. Every infant is unique, too, so it also is important to develop techniques to measure responses to nutritional supplies in individual infants. In this way, each infant can be fed the right amounts of nutrients for its best growth and development.