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.