Preterm infants are born before 37 weeks gestation and thus before they are fully developed. This has big implications on their nutritional needs, as the last trimester is a time of rapid growth and a time when many nutritional stores are laid down.
Nutrition during early life is now recognised as a determinant not only of immediate growth and neonatal survival but also an important factor for long term health. Thus the aim of preterm nutrition is to provide nutrients to achieve growth similar to foetal growth as well as achieve satisfactory functional development1,2.
Experts provide recommendations on nutrient intakes for the preterm infant, the basis of which is used to develop the most appropriate products for preterm infants, which can be used in the hospital1,2. Due to its immune protective function, breast milk is the feeding choice for preterm infants. However, breast milk does not provide adequate amounts of most nutrients for premature infants and so should be supplemented with human milk fortifier3.
But if breast milk is not available and a formula is to be used in some way, then it should be a formula designed for preterm infants1. This is because there are nutritional differences between preterm and term formulae to meet the specific nutritional needs of preterm infants, some of which are highlighted here.
Protein is essential for growth and therefore the long term outcome4. The turnover rate of proteins in neonates is three times higher than in adults, resulting in a higher energy need for infants per kilogram body weight than for adults4. Protein intake also needs to compensate for accumulated protein losses that occur in almost all small preterm infants1.
Term hydrolysed formulae have been used in some neonatal units to feed preterm infants post surgery or who have a feeding intolerance to standard preterm formulae. There is limited evidence for the use of hydrolysed formula in preterm infants, but they are sometimes used as it is felt they are better tolerated than whole protein formula in post surgical preterm infants. However hydrolysed term formula will not meet the nutritional needs of the preterm infant and so a hydrolysed protein preterm formula should be considered first.
Lipids or fats are an important source of energy for the preterm infant as lipids provide high amounts of energy in a small volume. Lipids are laid down during the last trimester of pregnancy and so the lipid stores of preterm infants can be very limited. Thus the quantity and quality of the dietary fat is very important.
Long chain polyunsaturated fatty acids (LCPs) are very important to the structure and function of cell membranes, especially the brain, retina and nerve tissue. Rapid accretion of LCPs in the brain occurs during the last trimester of pregnancy, and so preterm infants generally miss out. Preterm infants also have a limited capacity to carry out the metabolic conversion of essential fatty acids to LCPs, as their enzyme systems are relatively immature. Thus preterm infants need a preformed source in their diet1.
Calcium and Phosphorus
These are major mineral components of bone. 80% of these minerals are laid down during the last trimester5, and is why preterm infants need more of these minerals than term infants.
The preterm infant needs iron for building red blood cells, brain development, muscle and heart function. Preterm infants are at a high risk of developing iron deficiency anaemia due to their increased growth rate, but also due to losses caused by frequent blood sampling. Unlike most other nutrients, however, there is no mechanism for the excretion of iron from the body and high intakes may have adverse effects e.g. increased risk of infections6.
Many preterm infants are born with poor vitamin A stores7. Vitamin A deficiency is often considered a contributory factor in the development of chronic lung disease, especially in extremely preterm infants1. Vitamin A concentrations in human milk vary according to the maternal diet8 and human milk does not contain enough vitamin A to meet the needs of preterm infants1.
Vitamin D is important for supporting a large number of physiological processes such as neuromuscular function and bone mineralisation1. A vitamin D intake of 800 to 1000 IU/day (i.e. not per kg) during the first months of life is recommended. This means that a formula should provide the basic needs to which a supplement must be given (e.g. 100–350 IU/100 kcal). An intake of 800 to 1000 IU/day would improve serum 25(OH)D concentrations and the plasma levels of calcitrol and subsequently the calcium absorption rate, allowing reduction of the high calcium content of some formulae.
The ESPGHAN guidelines recommend that a reasonable range of energy intake for healthy growing
preterm infants with adequate protein intake is 110 to 135 kcal/kg per day. Increasing energy intake may not be appropriate for infants whose growth appears inadequate (without evidence of fat malabsorption) because it is more likely that other nutrients (e.g., protein) are rate limiting. Synthesis of new tissue is energy intensive and strongly affected by the intake of protein and other nutrients; this means that achieving an adequate energy to protein ratio is as important as providing adequate energy intake1.
Comparison of some nutrients provided in preterm and term formula (per 100ml)
|Units||Preterm formula||Term formula|
|of which LCPs||mg||40||16|
|Vitamin A||μg RE||361||55|
|Vitamin D||μg RE||3||1.2|
Preterm infants have different nutritional needs compared to term infants and thus need to receive appropriate products to meet their specific nutritional needs to support appropriate growth and satisfactory functional development.