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The Nutritional Value of a Soy Protein Concentrate (STAPRO-3200) for Long-Term Protein Nutritional Maintenance in Young Men¹

Laboratory of Human Nutrition, Department of Nutrition and Food Science, Clinical Research Center, and Nuclear Reactor Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

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 eggprotein, 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 ⁵⁵Fe. 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.

KEY WORDS: • soy protein concentrate • N balance • long term • metabolic study • young adult men • stable isotopes • absorption • zinc • iron • body composition

¹ Supported by the A. E. Staley Manufacturing Company, Decatur, IL.

² Present address: American University Medical Center, Department of Internal Medicine, Beirut, Lebanon.

³ Present address: USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111.

Manuscript received 14 March 1983.

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