Background: Human milk is nutritionally better than formula milk for preterm infants. However, unfortified
human milk may fail to meet the theoretical requirements of very low birth weight (VLBW). Human milk
fortifier (HMF) increases the nutritional content of human milk. However, the important factor of concern in
feeding VLBW is the osmolality, the higher the osmolality, the greater the risk of necrotizing entero colitis
(NEC). Therefore, high osmolality in fortified human milk should be considered for this condition.
Objective: To evaluate the effect of fortification on the osmolality of human milk
Material and Method: Twenty samples of human milk were collected from mothers of gestational age less than
32 weeks, at about 1 week postpartum in Songklanagarind Hospital. The osmolality of each sample was
determined at baseline and after supplementation with HMF at 10 minutes, 1, 2, 4, and 6 hours at room
temperature and 24 hours at 4oC in a refrigerator.
Results: The mean osmolalities (SD) of human milk and HMF dissolved in sterile water were 285.3 (3.3)
mOsm/kg H2O and 64.6 (0.7) mOsm/kg H2O, respectively. Thus, the expected osmolality of human milk after
supplementation with HMF was 349 mOsm/kg H2O. Mean measured osmolalities (SD) of human milk after
supplementation with HMF at 10 minutes, and 1, 2, 4, 6 and 24 hours was 394.7(2.9), 399.5(2.8), 402.1(2.2),
401.0(2.7), 401.3(2.3) and 401.2(3.1) mOsm/kg H2O, respectively. The mean osmolality at 10 minutes, 1, 2, 4,
6 and 24 hours were significantly higher than human milk (p < 0.001) and the mean osmolality at 10 minutes
was significantly higher than expected osmolality (p < 0.001). There were no significant differences among
groups of osmolality after supplementation with HMF at 10 minutes, 1, 2, 4, 6, and 24 hours (p > 0.05)
Conclusion: The supplementation of human milk with HMF induced an increase in osmolality after mixing.
The osmolality, after mixing with HMF which was about 400 mOsm/kg H2O, creates a greater risk of NEC.
Therefore, HMF milk should be considered for feeding in only high risk preterm neonates.

Keywords: Human milk fortifier, Osmolality, Breast milk