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 protein (N × 6.25) intake tested were 0.2, 0.3, 0.4, and 0.5 g/kg body weight/day in the beef study, and 0.3, 0.4, 0.5, and 0.65 g/kg body weight/day in the wheat 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 = 0.51 (±0.08)X-41.9(±4.6) for beef and Y = 0.27(±0.06) X-33.6(±5.0) for whole wheat protein. The amounts of beef and wheat proteins estimated to be required 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 ± 12 and 41 ± 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.