The promotive effect of soy sauce on iron absorption in human subjects

The effects on iron absorption of a traditionally fermented Japanese soy sauce added to soy and rice meals were assessed. The addition of soy sauce to a soy flour meal could not overcome the strong inhibition of iron absorption (geometric mean absorption 7.2 per cent with soy sauce vs. 8.7 per cent without, P = 0.5). However, soy sauce added to rice meal instead of soy flour significantly improved the geometric mean iron absorption (13.9 per cent with soy sauce vs. 5.2 per cent with soy flour, P = 0.002) and had a promotory effect on absorption from a rice meal alone (11.4 per cent with soy sauce vs. 3.5 per cent without, P = 0.0002). Although soy sauce contains appreciable amounts of organic acids, the addition of 340 mg lactic acid to rice did not enhance iron absorption (3.1 per cent with lactic acid vs. 2.2 per cent without, P = 0.11). The promotory effect of soy sauce on iron absorption appears to be due not only to its lack of soy protein content but may also be due to the presence of fermentation products other than organic acids.