Nitrogen to Protein Conversion Factors - An update and practical guidance for their use and for determining specific factors for novel protein sources

Protein content in food is measured indirectly (since the 1880’s) by determining nitrogen (N) and using a nitrogen to protein conversion factor (NPCF). NPCFs were initially based on proteins that were readily available at the time and found to contain about 16% N. Hence, a NPCF of 6.25 (100/16) was applied to all proteins, assuming all N was from amino acids (AAs). Recent technological advances revealed differences in AA composition and content of non-protein N between proteins indicating that an NPCF of 6.25 overestimates the protein content of most foods. However, 6.25 is still widely used. In 2017, use of specific NPCFs for proteins received attention from the Codex Committee on Nutrition and Foods for Special Dietary Uses (CCNFSDU) as they addressed standards for infant and follow-up formula. Should proposed specific NPCFs for soy protein of 5.71 (proposed in 1931 based on the AA composition of glycinin) and 6.38 for dairy protein (based on unique methods of determining NPCFs) be used? In July 2019, a joint FAO/WHO Expert Meeting on Nutrition (JEMNU) systematically analyzed all data and developed a recommendation. The JEMNU report concluded that there were shortcomings of methods used for protein estimation resulting in low to moderate confidence of the factors recommended: NPCFs of 6.1 and 5.7 for dairy and soy protein, respectively. A September 2021 CCNFSDU report concluded that the NPCF value for 6.25 should be retained based on the low confidence of NPCFs recommended by JEMNU and the significant ramifications of changing the NPCFs. This highlights the challenges faced with determining the protein content for novel proteins. There are published, unstandardized methods for determining NPCFs and if the limitations of these methods are recognized and reported, this can allow data-based comparisons of NPCFs between proteins. Ideally, a universal direct method of protein content determination is needed.