The nutritional value of a soy protein concentrate (STAPRO-3200) for long-term protein nutritional maintenance in young men

Six healthy young male M.I.T. students participated in a metabolic study to assess the capacity of a soy protein concentrate (STAPRO-3200) to serve as the sole source of dietary protein for long-term maintenance of protein nutritional status. Following an initial 9-day period, during which the subjects received an egg-protein, formula diet supplying 1.5 g protein per kilogram per day, the soy protein was given at a level of 0.8 g protein (N X 6.25) per kilogram per day for 82 days. Throughout, nitrogen balances were measured, and at intervals of 3 to 4 weeks, blood chemistries were determined, and evaluations of physical performance were undertaken. In addition measurements were made of zinc absorption and balance and of iron absorption, with the aid of 70Zn and 58Fe. Mean nitrogen balances were slightly positive for all subjects and the protein source was judged to support adequately maintenance of protein nutritional status, confirmed by the absence of changes in relevant blood parameters and maintenance of performance in the exercise tests. Mean zinc absorption was 23% of intake and did not change during the 82-day soy period and crude zinc balances remained slightly positive throughout. Iron intake was solely from the soy concentrate, and each meal was supplemented with ascorbic acid. Iron absorption was highly variable between and within subjects, as determined by the stable isotope balance procedure. Associated with the withdrawal of blood for purposes of monitoring the subjects, serum ferritin declined indicating a reduction in iron stores. These blood changes were used to approximate iron absorption and it was estimated to be 16%. It is concluded that well-processed soy concentrate can serve as the sole source of nitrogen and essential amino acids for long-term maintenance in adult humans.