Non-protein nitrogen in milk and infant formula and its relevance in infant nutrition

Non-protein nitrogen (NPN) is a fraction in milk comprised of numerous nitrogenous non-proteinaceous components, including but not limited to urea, ammonia, free amino acids, milk-derived peptides, and polyamines. Existing literature indicates significant differences in NPN concentrations and components between human and bovine milk. This also translates to differences in NPN concentrations or profiles in infant formulas manufactured from bovine milk ingredients. Many of the existing data are in some instances, decades old and may not accurately represent the concentrations of NPN in current formulas due to differences in raw materials and advances in analytical methods and processing techniques. Additional investigations are required to better reflect the current concentrations of NPN and the materials that contribute to NPN in infant formulas. In recent years, there has also been an increase in the interest in the functional roles and health benefits associated with many of these NPN components. This structured narrative review synthesises current knowledge on NPN in the context of infant nutrition; the NPN components, their biological functions, analytical methods of detection, key areas of consideration for the topic of NPN and future perspectives for the utilization of NPN in infant formula. The review was conducted using a targeted literature search and thematic synthesis approach. Advancing the understanding on the NPN fraction in milk will enable the development of next-generation infant formulas to more closely mimic the nutritional profile of human breast milk. • A comprehensive overview of the NPN fraction in milk is provided. • The role of NPN components in infant nutrition is discussed. • A comparison of NPN component concentrations in various milk sources is provided. • Findings suggest potential implications for the formulation of infant formulas. • Future research should explore NPN in milk and its effects on infant health.