Utilization of soy protein isolate mixed with rice protein in Japanese women

Utilization and requirement of soy protein isolate (SPI) and SPI-rice combination were examined in twenty-five female students. After 1 day on protein-free diet, each subject received a low-protein diet for 10 days. The protein sources were SPI for ten subjects and SPI-rice combination, in which the ratio of two proteins was 6:4, for fifteen subjects. The nitrogen intakes were about 45, 65 and 85 mg/kg in both the two series of experiments. Energy intake was at an approximate maintenance level of 36.1 +/- 3.0 kcal/kg. Apparent nitrogen balance improved with increase in nitrogen intake. The regression equations between nitrogen intake (X, mg/kg) and balance (Y, mg/kg) are shown in the following: SPI: Y = 0.411 X - 40.8 (n = 10, r = 0.812) SPI and rice protein: Y = 0.392 X -32.7 (n = 15, r = 0.739) From the above equations, the maintenance intakes of SPI and SPI-rice combination for an apparent nitrogen equilibrium were calculated to be 99 and 83 mg N/kg, respectively. Digestibilities were 98.2 +/- 5.0% for SPI and 93.1 +/- 6.1% for SPI-rice combination. The NPUs of SPI at intake levels of 40, 60 and 80 mg N/kg were 47 +/- 24 (n = 4), 49 (n = 2) and 44 +/- 3 (n = 4), respectively. The NPUs of SPI and rice mixed protein at intake levels of 45, 70 and 90 mg N/kg were 67 +/- 13 (n = 5), 51 +/- 7 (n = 5) and 54 +/- 12 (n = 5), respectively. It was concluded from the present study that both SPI and the SPI-rice combination had a high nutritive efficiency comparable with that of egg protein.