Human Milk Fortifiers for Preterm Infants: Do We Offer the Best Amino Acid Mix?

For preterm and small-for-gestational age infants on enteral nutrition, the best solution is to add human milk fortifier (HMF) to human milk (HM) which is provided by the mother or a milk bank. HMF provides a means to add additional protein, energy, and micronutrients, while maintaining HM as the main source of nutrition. Because of their rapid increase of lean body mass, preterm infants have much higher protein requirements than term infants. Recommendations on protein requirements of preterm infants are available, but protein quality - i.e. the amino acid (AA) profile in HMFs has not been systematically assessed. Present guidelines for enteral nutrition recommend protein intakes around 4 g/kg body weight (BW) for preterm infants <1,500 g, an intake that is not achievable with unfortified HM intakes <200 mL/kg BW/day. It is generally assumed that the AA profile of HM is the best reference for the AA profile of HMF. We calculated advisable intakes of AAs for preterm infants between 400-2,500 g which are based on AA increments of the fetus. Corrections for absorption, inevitable losses, oxidation, and variation of AAs in HM were introduced. Our calculations indicate that extremely low birth weight (ELBW <1,000 g) and very low birth weight (VLBW <1,500 g) infants have substantially higher AA requirements than low birth weight (LBW) infants growing from 1,900 to 2,400 g. In ELBW infants, daily intakes of the different indispensable AAs (IAA) with 4 g of (term) HM protein/kg BW range between 59 and 125% of the respective advisable intakes. Intakes of 7 IAAs and 3 conditionally indispensable AAs (CIAA) are below advisable intakes. On the other hand, with 4 g HM protein per kg BW/day, the IAAs isoleucine and leucine and some dispensable AAs are already supplied in abundance. In VLBW infants, daily intakes of the IAA methionine and 3 CIAAs are still below the advisable intakes. In LBW infants (<2,000 g) receiving 3.5 g HM protein per kg BW daily intakes of 1 IAA and 3 CIAAs would be too low. Preterm infants should receive HMFs which provide adequate amounts of AAs which are needed for their rapid growth and development while avoiding excessive intakes. In particular, very high AA requirements of ELBW infants are a challenge. AA composition of present HMFs for preterm infants should be reconsidered: spiking HMF protein with the AAs which are presently undersupplied or providing targeted AA-based HMF are options to further improve the AA profile in fortifiers.