Daidzein is a more bioavailable soymilk isoflavone than is genistein in adult women.

Soybean isoflavones are proposed to be anticarcinogenic, but their effective doses have not been established. To study the bioavailability of soybean isoflavones for humans, 12 young adult women received single doses of 0.7, 1.3 and 2.0 mg isoflavones/kg body wt in soybean milk as part of a liquid diet. Plasma, urine and fecal isoflavones were measured by reverse-phase HPLC. Average 24-h urinary recoveries of daidzein and genistein were approximately 21% and 9%, respectively, at all three doses. Urinary recovery of daidzein was significantly greater than that of genistein (P < 0.001). Total fecal excretion of isoflavones was only 1-2% of the ingested amount. Plasma total isoflavone concentration was significantly increased to 4.4 +/- 2.5 mumol/L at 6.5 h after a dose of 2.0 mg/kg. The plasma concentrations of daidzein and genisten were approximately equal. Twenty-four hours after dosing, both plasma and urine isoflavone concentrations were nearly nil. Although soybean milk isoflavones seem to be 85% degraded in the intestine, the bioavailability, especially of daidzein, may be sufficient to exert some health-protective effects.