Pharmacokinetics of the glucuronide and sulfate conjugates of genistein and daidzein in men and women after consumption of a soy beverage

Background: The soy isoflavones genistein and daidzein are found in blood and tissues as aglycones, glucuronides, and sulfates. Isoflavone conjugates may serve as sources of aglycones at specific target tissues and may have bioactivity. Yet, very little is known about the plasma pharmacokinetics of isoflavone conjugates after soy ingestion. Objective: The objective of this study was to determine the plasma pharmacokinetics of glucuronide and sulfate conjugates of genistein and daidzein in humans after the consumption of a drink made with soy-protein isolate. Design: Six men and 6 women (x̅ +/- SD age: 40.8 +/- 3 y) consumed a soy-protein-isolate drink. The pharmacokinetics of isoflavone glucuronide and sulfate conjugates were studied with the use of β-glucuronidase (EC 3.2.1.31) and sulfatase (EC 3.1.6.1) digestion and liquid chromatography–mass spectrometry. Results: Glucuronides of genistein and daidzein made up a significantly lower percentage (P < 0.05) of the total isoflavone concentration in plasma (48% and 33%, respectively) than in urine. The percentages of sulfates of genistein and daidzein in plasma (8% and 26%, respectively) were 2- to 6-fold those in urine (P < 0.05). Approximately 30% of the total genistein or daidzein was comprised of mixed conjugates (one glucuronide and one sulfate). For daidzein sulfate, genistein sulfate, daidzein glucuronide, and genistein glucuronide, the time to peak (tmax) was 4.5, 4.5, 4.5, and 6.0 h, respectively, and the apparent half-life (t1/2λz) was 3.1, 5.7, 3.2, and 8.4 h, respectively. Conclusions: These data suggest that there are significant differences in the pharmacokinetics of sulfate and glucuronide conjugates of isoflavones. This may have important implications for the meal frequency and maintenance of target tissue bioactivity required to elicit potential health benefits.