Bioavailability of isoflavones from soy products in equol producers and non-producers in Japanese women

Abstract

BackgroundThe estimated intake of soy isoflavones from a meal has been based on the content in a food, but the health effects of soy isoflavones are possibly affected by their bioavailability. In this study we have evaluated the isoflavone bioavailability after the intake of three kinds of soy foods and a commercial soy isoflavone supplement, and examined whether the isoflavone bioavailability is different between equol producers and non-producers. MethodsHealthy female subjects (n = 20; 9 equol producers, 11 equol non-producers) aged between 20 and 26 y participated in this study. To assess the bioavailability of isoflavones, equol producers and non-producers consumed three kinds of soy foods (tofu, fermented soybeans: natto in Japanese, soy milk) and a soy isoflavone supplement containing 75 mg isoflavones together with breakfast. Urine was collected for 24 h, and the amounts of urinary excretions of daidzein, genistein, and the metabolite equol were measured. The intra- and inter-individual variations in the bioavailability of isoflavones were also examined. ResultsThe bioavailability of daidzein following the consumption of tofu, natto, soy milk, and soy isoflavone supplement were 66.9%, 45.2%, 65.7%, and 57.9%, respectively, and the bioavailability of genistein following the consumption of these soy products were 33.7%, 24.4%, 31.2%, and 17.7%, respectively. The bioavailability of daidzein and genistein was significantly greater in equol non-producers than equol producers, especially following the consumption of soymilk. In the equol producers, the 24 h urinary excretion of equol was significantly greater after the intake of soy isoflavone supplement than after the intake of natto or soymilk. The analysis of relative reliability of intra- and inter-individual variations suggested that bioavailability of isoflavones is less variable following soymilk intake than soy isoflavone supplement. ConclusionsThe results in this study suggest that bioavailability of isoflavones are different between equol producers and non-producers, because the 24 h urinary excretion of equol in the equol producers were significantly lower than those in the equol non-producers.