Estimated digestible amino acids (AA: corrected for endogenous secretions) obtained from the NRC and CNCPS models appear to be good predictors of the AA profile flowing into the portal vein. However, for absolute amounts, the NRC model more closely predicts the changes measured in net portal fluxes compared to the CNCPS. The slopes of the regression “AA net portal flux vs AA digested-NRC” are in agreement with AA metabolism across the gut: slope smaller that unity indicating loss for the branched-chain AA (oxidation) and for Thr (endogenous loss); slope equal to unity for Lys, indicating little loss. Slopes greater than unity for His, Met and Phe suggest either an underestimation of the digestible flow with the NRC model or an overestimation of the net portal fluxes for these AA.