One of the major target of renewing the INRA feed evaluation system for ruminants was to update their metabolizable protein (MP) requirements as well as their responses to changes in protein supply. These updates have been made from meta-analyses of large databases of INRA and of literature. For the requirements, the nitrogen losses observed in practical situations were quantified. The 3 main routes of losses, which result in non-productive protein requirements, are, in decreasing order of importance (1) metabolic fecal protein (MFP) which was calculated from non-digestible organic matter intake, (2) urinary endogenous nitrogen losses (NUE) which was the NU losses corresponding to digestible N intake = 0 and corrected for purine-N derivative from rumen microorganisms, (3) loss of protein from phaneres (PPH). After, the values of “non-productive” MP requirements were applied to 873 treatments of 296experiments performed on lactating cows or goats with the aim to study the milk response to MP supply. MP supply was calculated using the renewed INRA model (Sauvant and Nozière, 2013). The efficiency of available MP utilization (EffMP%) was determined with an iterative calculation, assuming the same common EffMP value not only for milk protein synthesis but also for MFP, for PPH as well as for the accretion or mobilization of body proteins which was related to the calculated energy balance. This leaded to the most accurate prediction of EffMP in response to the dietary concentration of MP (g/kg DM). EffMP presented the same decreasing function to the MP concentration for cows and goats. Moreover, for a given MP content, inter-experiments variations were significantly explained, positively by actual or potential milk protein yield (MPY, g/d) and negatively by the level of dry matter intake (DMI, kg/d). Therefore, for dairy cows, the following regression was calculated: EffMP = 68.3 – 0.54 (MP-100) + 0.029 (MPY-1000) – 1.04 (DMI – 21) [n = 873, R² = 0.83, RMSE = 2.9]. In conclusion, the major novelties of these updates were the fecal and urinary nitrogen losses and the common value of EffMP for all the functions of proteosynthesis.