Developing a comprehensive database of nitrogen-to-protein conversion factors: the NiCE factors database

Accurately calculating food protein content requires summing the proteinogenic amino acids or multiplying total nitrogen by the appropriate nitrogen-to-protein conversion factor (NCF). However, food composition databases still rely on the universal NCF of 6.25, introduced nearly 200 years ago. This study addresses this limitation by developing the first comprehensive NCF database - the NiCE (Nitrogen Conversion Estimate) Factors Database - established through a European collaboration and aligned with the FoodEx2 classification. Empirical NCFs were systematically compiled, quality-assessed, aggregated, and hierarchically propagated using a standardized computational framework. The resulting database provides harmonized values for the purified-protein factor (k_A), the food factor (k_P), and their mean (k) across FoodEx2 foods. At the highest FoodEx2 level, k-values range from 5.02 (vegetables) to 5.86 (milk and dairy products). By replacing the universal NCF of 6.25 with updated food-specific NCFs, this database establishes an evidence-based standard for accurate protein calculation in diverse fields.