Protein requirements of man: comparative nitrogen balance response within the submaintenance-to-maintenance range of intakes of wheat and beef proteins.

Sixteen young male students participated in two studies designed to determine the nitrogen balance response to beef or stone-ground, whole wheat protein over the submaintenance-to-maintenance range of protein intake. The objective of the studies was to evaluate the relative capacities of these proteins to meet the minimum protein needs in young adult males. A modified Latin-square design was used to allocate subjects to the four 15-day metabolic balance diet periods in each study. The last 10 days were used for fecal nitrogen determination and the last 5 days for evaluation of urinary nitrogen excretion. The diet periods were separated by a 4-day break period and began with 1 day on a "protein-free" diet. The levels of protien (N times 6.25) intake tested were 0.2, 0.3, 0.4, and 0.5 g/kg body weight/day in the beef study. The results have been compared with those obtained in a previous similar study with egg protein. The regression of estimated "true" nitrogen balance (Y) (mg N/kg/day) on nitrogen intake (X) (mg N/kg/day) was: Y equals 0.51(plus or minus 0.08)X minus 41.9(plus or minus 4.6) for beef and Y equals 0.27(plus or minus 0.06)X minus 33.6(plus or minus 5.0) for whole wheat protein. The amounts of beef and wheat proteins estimated to be requried to support body nitrogen balance in 97.5% of the population supplied 96 and 178 mg N/kg/day, respectively. The relative protein value of beef and wheat proteins, in comparison with egg protein, was 78 plus or minus 12 and 41 plus or minus 10, respectively. It is concluded that the variations in dietary protein quality should be taken into account in assessing the protein adequacy of diets for individuals and population groups.