Review of the Factors Affecting Bioavailability of Soy Isoflavones in Humans

Abstract Soy isoflavones have anticarcinogenic, antioxidant, and antiatherosclerotic activities. They also interact with the estrogen receptor, which makes them weak or moderate phytoestrogens. Because of their bioactivity, isoflavone bioavailability has been extensively studied in humans. This review summarizes data from intervention studies in humans, focusing on the factors that affect bioavailability. Summarizing data from 16 studies shows that the maximum concentration in plasma normalized to a constant dose of genistin is ∼1.6 times that of genistein, and daidzin is ∼1.8-fold higher than daidzein, whereas the half-life is not significantly different for aglycone and glucoside. There is a wide variation in the reported percentage urinary excretion that is not dependent on dose. Bioavailability is increased by a rapid gut transit time and by low fecal digestion rates and is decreased by a fiber-rich diet. There is no difference in bioavailability between pre- and postmenopausal women. The daily ingest...