Osmolality of fortified human milk with contemporary human milk fortifiers

Human milk fortification is critical to support preterm infant growth. Human milk fortifed to caloric densities above manufacturers' recommendations are often necessary to achieve adequate growth. Fortified milk may be prepared 12-24 h before feeding, although practice variations exist. We investigated how human milk osmolality is affected by contemporary bovine and human milk-derived human milk fortifiers (HMFs) immediately and temporally for standard and higher-calorie recipes.

This study measured the osmolality of human milk fortified with four current HMFs across a range of caloric concentrations (22-30 kcal/oz) in fresh, previously frozen, and pasteurized human milk. Osmolality was tested at 0, 12, and 24 h after preparation, conducted in triplicate. Differences in initial osmolality were compared between fortifiers. Percentage change in osmolality at 12 vs 24 h was tested with mixed-effects linear regression to study the effect/interaction of time, fortifier, and caloric density.

Initial osmolality was significantly different between manufacturers (range, 315-620 mOsm/kg across caloric densities). There was a median 0.3% (IQR, -0.8 to 1.5) difference in change in osmolality at 24 vs 12 h (P = 0.02). In mixed-effects modeling, there appeared to be an interaction between time and fortifier, but after post hoc adjustments, no pairwise comparisons remained significant. At 24 h, there was an interaction between fortifier and milk type (P = 0.003), with up to 4% greater change in osmolality with fresh vs frozen milk.

Contemporary HMFs increase the osmolality of human milk to variable degrees, with some temporal effects observed, although clinical significance of these findings is unclear.