Effect of fortifiers and additional protein on the osmolarity of human milk: is it still safe for the premature infant?

OBJECTIVES

The present guidelines of the American Academy of Pediatrics recommend that osmolarity not exceed 450 mOsm/kg (or approximately an osmolarity of 400 mOsm/L) for breast milk or infant formulae, to minimize the risk factors for necrotizing enterocolitis. A commercial protein supplement has been developed to meet special protein requirements (4.0-4.5 g · kg(-1) · day(-1)) of infants with a birth weight <1000 g. Because its effect on osmolarity has not been systematically studied, we characterized the effects of fortification on the osmolarity of human milk (HM).

METHODS

Osmolarity of fresh and processed HM was measured at baseline, after fortification with a commercial HM fortifier and after further supplementation with additional protein increasing in 0.5-g steps up to 4.0 g. Measurements were performed immediately after adding fortifier and/or protein and after 24 hours. In addition, changes in osmolarity were determined after adding therapeutic additives such as iron, multivitamin supplement, and calcium-phosphorus capsules.

RESULTS

Native HM samples (n = 84) had 297 mOsm/L, (median; 95% confidence interval 295-299 mOsm/L). Adding HM fortifier increased osmolarity up to 436 mOsm/L (95% confidence interval 431-441 mOsm/L). Additional protein supplementation increased osmolarity by 23.5 mOsm/L per 0.5-g step, up to a maximum of 605 mOsm/L. Pasteurization decreased osmolarity by 20-30 mOsm/L (P < 0.001), and storage for 24 hours slightly increased osmolarity (by 11.5 mOsm/L P = 0.0002). Therapeutic additives increased osmolarity up to 868 mOsm/L.

CONCLUSIONS

Adding HM fortifier and additional protein to HM increased osmolarity to >400 mOsm/L and therefore above the recommended threshold. Because of the excessive increase in osmolarity combinations of HM + fortifier and additional protein should not be applied together with multivitamins or other additives.