Recommended Dietary Allowances for Protein

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In 1989, the Food and Nutrition Board subcommittee of the U.S. National Research Council updated their recommended dietary allowances (RDAs) for protein and amino acids (157). The RDAs are largely based upon the 1985 FAO/WHO/UNU committee report (156). The RDA values for protein shown in T.ab!e...2,13 were based largely upon N balance data (rather than factorial method data) from studies using a high-quality, highly digestible source of protein. The protein intake values that produced zero N balance were then increased by two standard deviations to encompass 97.5% of the population to get the RDA for the reference protein. For example, from studies of young adult men, the value of 0.6 g/kg/day was increased to 0.75 g/kg/day.

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Table 2.13 Recommended Intakes of High-Quality Reference Protein for Normal Humans

The types of protein consumed in the United States were reviewed to determine both essential amino acid composition (see below) and digestibility. The RDA for the reference protein was then adjusted for imbalances in essential acids and protein digestibility where deemed important to produce the values shown in T.§bIe 2,13. (The reader is referred to ref. 157 for details.)

Special cases are cited in TaMe.^J.^ in which growth and accretion of tissue must be accounted for in the RDAs: during pregnancy, during lactation, and in infants and children. In pregnancy, total protein deposited was estimated to be 925 g on the basis of maternal weight gain and an average birth weight at term. The rates of protein accretion were then divided by trimesters, with adjustments for variation in birth weight (+15%) and an assumed efficiency of conversion of dietary protein to fetal, placental, and maternal tissues (+70%) to produce increments in reference protein intake of +1.3, +6.1, and +10.7 g protein/day for the 1st, 2nd, and 3rd trimesters, respectively. These values were rounded to 10 g/day of additional protein for all trimesters to compensate for uncertainties about rates of tissue deposition and maintenance of those increases (I§ble...2J.,.3) (157).

A factor to account for the additional protein intake required by women who are lactating was also added to values in I§..bIe 2,1.3. This addition was based upon the composition of human milk, the volume of milk produced, an adjustment for the estimated 70% conversion efficiency of dietary protein into newly synthesized milk protein, and a 25% increase to account for a two standard deviation variance among women. This value was also adjusted for duration of lactation.

Protein requirements for newborn infants include additional intake to account for the approximate 3.3 g protein/day accretion of growth of infants fed human milk (which includes a 25% increase to account for a two standard deviation variance). The RDA for older infants and children is based upon the 1985 WHO report ( 156.), which used a modified factorial method. Rates of protein accretion due to growth were calculated by age group, increased by 50% to account for variability among children, and adjusted by a 70% conversion efficiency factor of dietary protein to body protein synthesis. This estimate was then increased by 25% (to account for a two standard deviation variance) to give the RDA.

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