When a person is placed on a protein-free diet, rates of amino acid oxidation and urea production decrease over several days as the body tries to conserve its resources, but amino acid oxidation and urea production do not drop to zero ( Fig. ^.Z). There is always some obligatory oxidation of amino acids and urea formation and miscellaneous losses of N (Tabj§...2.,10). The factorial method assesses all routes of loss possible for adult humans on an N-free diet. The minimum daily requirement of protein is assumed to be the amount that matches the sum of the various obligatory N losses.
Whereas nonurinary nonfecal N losses are often ignored in N balance studies of adequate protein intake, they are of critical importance in the assessment of protein requirements by the factorial method. A variety of studies have been performed to assess these losses, and the results are tabulated in a World Health Organization/Food and Agriculture Organization/ United Nations University (WHO/FAO/UNU) panel publication ( 156) and summarized again in a U.S. National Research Council report (157). The obligatory N losses include (a) urinary N, estimated to be 38 mg/kg/day of N; (b) fecal loss of enzymes and desquamated intestinal cells that cannot be fully reabsorbed, estimated to be 12 mg/kg/day; and (c) loss of N through sweat, hair, skin, and nail turnover and growth, menstrual flow, seminal fluid, ammonia in breath, nasal secretions, etc., estimated to be 2 to 3 mg/kg/day on a protein-free diet and 5 to 8 mg/kg/day with a normal protein intake ( 48). Summation of these directly measurable losses gives a total obligatory minimum daily loss of N of 54 mg/kg/day (range, 41-69 mg/kg/day), corresponding to a protein intake of 0.34 g/kg/day (where 1 g N = 6.25 g protein) (157).
This value of 54 mg/kg/day of N is an "average value," which must be raised if it is to indicate a requirement that will apply to most adults in the population. The 1973 WHO/FAO report (158) suggested a coefficient of variability among individuals of 15%. Adding twice this amount (+2 standard deviations) gives a protein requirement that includes 97.5% of the adult population; thus, the 0.34 g/kg/day protein becomes 0.44 g/kg/day after rounding off ( 45). For adults, the dietary protein requirement is considered to be this amount plus an adjustment for the inefficiency of use of dietary protein and for the quality (amino acid composition and digestibility) of the protein source consumed. For children and pregnant or lactating women, an additional (theoretically determined) amount of protein is added to this recommendation to account for growth and milk formation (157). Clearly this approach is based upon extrapolation of N losses from protein-starvation conditions and may reflect an adaptation to N deprivation, which may not reflect normal metabolism and N requirements of healthy humans near the actual requirement level. Hence, another method is needed to produce an alternative assessment of protein requirements.
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