Nitrogen sources in donor human milk: True protein, nonprotein nitrogen, and amino acid profile

Background: Devices measuring the macronutrient content of human milk are commonly used to assist with clinical decision-making. It is unknown if these devices accurately measure protein content in donor human milk (DHM). Our objective is to quantify the nitrogen sources and protein content in commercial DHM.

Methods: The total nitrogen content (Dumas method) and nonprotein nitrogen content (Kjeldahl method) was measured in triplicate from six commercial DHM samples with protein content noted on the labels. In addition, the amino acid content was measured in 15 commercial DHM samples and protein content in each sample was calculated. The calculated protein content for each DHM sample was compared for consistency.

Results: The nonprotein nitrogen content in DHM was consistently higher (0.33 ± 0.05 g/g) than previous reports, leading to overreporting of protein content on DHM labels by a median value of 0.15 g/dl (range 0.02-0.23 g/dl). Similarly, calculation of the protein content from the total nitrogen content with an assumption of 20% (grams per gram) nonprotein nitrogen consistently overrepresented the protein content as determined from the amino acid profile for DHM.

Conclusion: Common methods for assessing the macronutrient content of human milk may overestimate the protein content of DHM.