Comparison was made between three methods for calculating the protein content of 68 foods. The 3 methods of calculation were: (1) multiplication of Kjeldahl nitrogen by 6.25; (2) multiplication of Kjeldahl nitrogen by factors varying from 5.30 to 6.38 depending on food type; and (3) summation of amino acid content as determined by chemical analyses. New conversion factors were calculated based on amino acid analyses. Substantial differences (20 to 40%) were found in protein content for many foods depending on the calculation method. The protein content of six typical menus for mature American males was calculated using the three methods. The resulting protein content of menus did not vary substantially (less than 3%) as a function of calculation method since relatively large differences in various foods tended to cancel out. Conversion factors based on amino acid analyses are recommended whenever more accurate approximation of protein content of individual food is required.
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