Neither Background Diet Nor Type of Soy Food Affects Short-Term Isoflavone Bioavailability in Women

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Abstract
To characterize bioavailability of soybean isoflavones, proposed anticarcinogenic food components, eight women, ages 20–41 y, were fed 0.9 mg isoflavones/kg body wt from soymilk at 0730, 1230 and 1730 h for 1 d. Subjects consumed three background diets in random order: a diet prepared for them (basic foods diet) or a self-selected diet at the specified times, or a self-selected diet eaten ad libitum. In a second study, women were fed single isoflavone doses of 0.8–1.4 mg/kg in breakfast casseroles containing tofu, tempeh, cooked soybeans or texturized vegetable protein. Both studies were conducted in randomized, cross-over designs. Plasma, urine and fecal isoflavones were measured by reverse-phase HPLC. After consumption of background diets, 48-h urinary recovery of daidzein (D) was 26–27%, and of genistein (G), 18–20% of the dose given with each diet. At 24 h after consumption of different background diets, plasma D and G concentrations were similar (1.4 ± 0.7 mmol/L) and were not affected by diet selection. Urinary recoveries of D over 24 h from the various soy foods were 38–51%, and of G, 9–16% of the dose given. In both studies, urinary recovery of D was significantly greater than that of G. Only a few percentage of the total isoflavone dose was recovered in feces, probably due to bacterial breakdown of these compounds. Therefore, isoflavone bioavailability may not be affected by choice of background diet or food source of isoflavones.