The purpose of this study was to evaluate the dose-dependent effect of human milk fortifier (HMF) on the osmolality of various preterm human milks (PHMs) at different gestational and lactation stages, and with different storage and treatment conditions that are routinely used in neonatal intensive care units (NICUs).

Twenty-four mothers who had given birth to their baby before 28 or between 29 and 31 weeks of pregnancy participated in the study after 1–2 weeks or 3–4 weeks breastfeeding after delivery. The study was a prospective, multicenter, comparative, and noninterventional study. Osmolality of fresh or pasteurized human milk stored at 4°C was measured (cryoscopy) at baseline, and 24hours after adding Suppletine^® Human Milk fortifier (SHMF) at 3%, 4%, 4.5%, and 5% (w/v).

PHM without supplementation had an osmolality (mean±SD) of 301±8 mOsm/kgH₂O (n=40; 95% CI: [298; 303]). Adding 3–5% SHMF induced a linear increase of osmolality (P<0.001; r²=0.975). With 4% SHMF, the osmolality measure was 443±13mOsm/kg H₂O (95% CI: [439–447]). Neither a 24-hour storage at 4°C nor pasteurization induced a modification of osmolality compared to the fresh samples.

Whatever the origin and quality of milk as well as hospital practices, adding up to 4% (w/v) SHMF to PHM increases its nutritional quality and osmolality without exceeding 450mOsm/kgH₂O, which is generally recognized as safe.