Preterm infants often need fortified human milk to support their growth and development. However, fortification can significantly increase milk osmolality, which requires careful monitoring to ensure it remains within safe limits to prevent potential health risks.

What is Milk Osmolality?

Milk osmolality, the measure of solute concentration in human milk, plays a crucial role in providing infants, especially preterm babies, with the right balance of nutrients while supporting healthy digestion and hydration. The osmolality of human milk typically ranges between 270 to 310 mOsm/kg of solution (milliosmoles per kilogram of solution).

In simple terms, mOsm/kg of solution indicate the amount of dissolved particles, such as sugars, fats, and proteins, for a given quantity of the solution (in this case, the solution will be milk). The higher this number, the more concentrated the milk is. The solute concentration can vary depending on the stage of lactation, the mother’s diet, and the infant’s health needs (as detailed below). The largest contributors to milk osmolality include proteins, carbohydrates (especially lactose), and electrolytes. When milk is fortified to meet the higher nutritional demands of preterm infants, these additives can further elevate the osmolality, which in turn can increase the risk of feeding intolerance and digestive complications in vulnerable, preterm infants.

Factors Influencing Osmolality in Human Milk

Several factors influence the natural osmolality of human milk, including:

 

Osmolality in Fortified Milk and Preterm Infants

Preterm infants often require fortification through human milk fortifiers (HMFs) or formula powders to meet their increased nutritional needs. Recent research (Pineda et al., 2024) shows that fortifying donor milk can increase osmolality, sometimes exceeding the 450 mOsm/kg threshold set by the American Academy of Pediatrics. Elevated osmolality, especially above 450 mOsm/kg, can delay gastric emptying, increase feeding intolerance, and potentially harm the gut integrity of preterm infants. While some studies (Ellis et al., 2019) suggest that high osmolality is not always linked to gastrointestinal issues, caution is critical for preterm infants, who are at higher risk for complications such as necrotizing enterocolitis (NEC). The European Society for Pediatrics, Gastroenterology, Hepatology, and Nutrition (Embleton et al., 2022) also advises caution when fortified feeds exceed 450 mOsm/kg in osmolality.

Clinical Considerations

To mitigate the risks associated with high osmolality in fortified human milk, experts recommend the following best practices:

Here, advanced tools like Preemie (preemiesensor.com), through its NutriNTrack feature, provide real-time osmolality estimates of fortified milk. This technology helps healthcare providers ensure that osmolality remains within safe limits, reducing the risk of adverse effects on preterm infants.

Conclusion

Managing milk osmolality is critical in the care of preterm infants, whose immature digestive systems require a delicate balance. While fortification is necessary to meet their heightened nutritional needs, healthcare providers must remain vigilant about the risks associated with increased osmolality, particularly in relation to feeding intolerance and Necrotizing enterocolitis (NEC).

By adopting safe fortification practices and continuously monitoring osmolality, providers can better support the growth and development of preterm infants while minimizing potential risks. Innovative tools like Preemie’s NutriNTrack offer a valuable solution, enabling accurate, real-time osmolality estimation to keep feeds within safe limits.

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REFERENCES
Ellis ZM, Tan HSG, Embleton ND, et al. Milk feed osmolality and adverse events in newborn infants and animals: a systematic review. Arch Dis Child Fetal Neonatal Ed 2019;104:F333–40 

Embleton, N. D., Moltu, S. J., Lapillonne, A., Van Den Akker, C. H., Carnielli, V., Fusch, C., … and Domellöf, M. (2023). Enteral nutrition in preterm infants (2022): a position paper from the ESPGHAN committee on nutrition and invited experts. Journal of pediatric gastroenterology and nutrition, 76(2), 248-268.

Pineda, D., Bingham, R., Gates, A., Thompson, A. B., and Stansfield, B. K. (2024). Osmolality of fortified donor human milk: An experimental study. Journal of Parenteral and Enteral Nutrition, 48(1), 57-63.