Efficiency of utilization of soy protein isolate in Japanese young men.

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Abstract
The nutritional quality of soy protein isolate (SPI) was evaluated in young men by the N balance method using fish protein as a standard. Twenty-one male university students were given SPI, fish protein (cod fish) or a 50:50 mixture of the 2 proteins as the sole source of protein. SPI and fish were prepared as paste products (kamaboko). Four experimental periods were used in which each subject received 0.35, 0.45, 0.55 and 0.65 g protein/kg per day, respectively, given in random order. Each period consisted of 1 day on protein-free diet and 10 days on the experimental diet, with an interval of 3 days on a free-choice (ad lib) diet between periods. Energy intake was constant for each individual to maintain their body weight (44.6 .+-. 2.4 kcal/kg per day). The linear regression equations obtained between N intake (X: mg N/kg per day) and the apparent N balance (Y:mg N/kg per day) were as follows: SPI, Y = 0.298X =35.2; fish protein, Y = 0.365X-31.8; mixed protein, Y =0.423X-38.3. The N requirement for maintenance of N equilibrium determined from the regression equation was 118.1 .+-. 15.4 mg N/kg per day for SPI, 87.1 .+-. 17.2 mg N/kg per day for fish protein and 90.5 .+-. 17.1 mg N/kg per day for mixed protein. The NPU [net protein utilization] calculated at the respective maintenance nitrogen intakes were 38.9, 52.8 and 50.8 for SPI, fish protein and mixed protein, respectively. There was no significant difference between the nutritive values of mixed protein and fish protein. The nutritive value of SPI relative to fish protein was estimated as 82, 74 and 74% by the slope-ratio method, and values for the N requirement and NPU, respectively.