Evaluation of Proteins by the Nitrogen Balance Method

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Abstract
By employing suitably prepared dogs, the minimum amount of a protein required to maintain nitrogen balance (MPN value) was determined. The MPN values of casein, crude fibrin, and pharmaceutical grade fibrin were found to be, respectively, 140–160, 70 and 80–100 mg N/kg/day. Fortification of casein with cysteine reduced the MPN value to 80–100 mg N/kg/day. When both cysteine and methionine were added to casein in sufficient amounts, the MPN value fell to 60–70 mg N/kg/day. The MPN value for lactalbumin was not determined, but it maintained nitrogen balance when given at the rate of 100 mg of lactalbumin nitrogen/kg/day.