The effect of L-methionine supplementation on the utilization of a soy protein isolate (SPI) was evaluated by short-term nitrogen balance studies in young women. Thirteen female students were given SPI in an initial period and SPI supplemented with 1% methionine in a second period immediately after menstruation as the sole source of protein. After one day on protein-free diet, each subject received conventional low-protein diet for three days, and then low protein, semisynthetic diet containing 0.5 g/kg/day (seven subjects) or 0.3 g/kg/day (six subjects) of SPI or SPI supplemented with methionine for seven days. The energy intake was approximately of a maintenance level of 36.5 +/- 3.8 kcal/kg/day. The mean N balances of the subjects at an intake level of 0.5 g/kg/day in the SPI period and methionine supplemented period were -6.2 +/- 12.6 mg N/kg and -9.8 +/- 9.8 mg N/kg, respectively, while their N balances at an intake level of 0.3 g/kg/day were -17.8 +/- 7.2 mg N/kg in the SPI period and -15.5 +/- 3.0 mg N/kg in the methionine supplemented period. There was no significant difference between the values in the SPI and methionine supplemented periods at both levels of protein intake. Blood analyses were carried out before and after the SPI period and after the period of methionine supplementation. The urinary creatinine and urea excretions during these periods were not markedly affected.
Adolescent Adult