BIOAVAILABILITY OF SOYBEAN ISOFLAVONES DEPENDS UPON GUT MICROFLORA INWOMEN

Soybean isoflavones have been proposed to be anticarcinogenic, but the ir effective doses have not been established. To study their bioavaila bility, seven women consumed 3.4, 6.9, or 10.3 mu mol isoflavones/kg b ody wt in soymilk in each of three meals of a liquid diet on one of th ree feeding days that were separated by 2-wk washout periods. Subjects were randomly assigned to doses in a cross-over design. Plasma, urine and fecal isoflavones were measured by reverse phase HPLC. In two sub jects, fecal isoflavone recovery was 10-20 times that in the other fiv e subjects. Average 48-h urinary recoveries of ingested daidzein and g enistein were 16 +/- 4 and 10 +/- 4%, respectively, at all three doses among the five subjects excreting only small amounts of isoflavones i n feces, whereas urinary recoveries of daidzein and genistein in the t wo subjects who excreted large amounts of fecal isoflavones were 32 +/ - 5 and 37 +/- 6%, respectively. Urinary isoflavone excretion was near ly zero in all subjects at 48 h after dosing. Average plasma concentra tion of genistein at 24 h after the breakfast isoflavone dose in subje cts excreting large amounts of fecal isoflavones was significantly gre ater by 2.5-fold than in subjects who excreted small amounts of fecal isoflavones (P < 0.05). In vitro anaerobic incubation of isoflavones w ith human feces showed that intestinal half-life of daidzein and genis tein may be as little as 7.5 and 3.3 h, respectively. These data sugge st that human isoflavone bioavailability depends upon the relative abi lity of gut microflora to degrade these compounds.