Effect of fortifiers on the osmolality of preterm human milk

Background: The nutritional content of preterm human milk (HM) can be increased by adding human milk fortifier (HMF). Premature formula (PF) has been used as an alternative to HMF due to the high cost of HMF in some countries. However, the osmolality of HM after fortification remains a matter of concern. Aim: To evaluate the osmolality of fortified preterm HM. Methods: This was a cross-sectional study. HM was collected from 28 mothers of infants with a gestational age of <32 weeks or birthweight <1500 g. Expressed HM was divided into (i) pure HM; (ii) HM+PF to 24 kcal/oz; (iii) HM+PF to 28 kcal/oz; and (iv) HM+HMF to 24 kcal/oz + protein powder 0.5 g/100 ml. Results: Twenty-eight samples of preterm HM were analysed. The mean (SD) osmolality of baseline HM was 297.6 (9.7) mOsm/kg. Mean osmolality of preterm HM after fortification with PF to 24 and 28 kcal/oz was 357.2 (11.1) and 419.9 (18.8) mOsm/kg, respectively. The mean osmolality after fortification with HMF plus protein powder was 464.1 (18.8) mOsm/kg. Repeated-measures ANOVA was used to compare osmolality between pure HM and HM fortified with different fortifiers. All pairwise comparisons by the Bonferroni method were statistically significant (p < 0.001). Conclusions: The osmolality of preterm HM fortified with PF up to 28 kcal/oz does not exceed the American Academy of Paediatrics recommendation of 450 mOsm/kg. The addition of extra protein to preterm HM fortified with commercial HMF must be cautiously considered due to the risk of excessively high osmolality.