A DIET HIGH IN WHEAT FIBER DECREASES THE BIOAVAILABILITY OF SOYBEAN ISOFLAVONES IN A SINGLE MEAL FED TO WOMEN

The absorption of some dietary components may be inhibited by dietary fiber. To study the effect of dietary fiber on the bioavailability of isoflavones, seven healthy women were randomly assigned in a crossover design to a control diet containing 15 g dietary fiber or a wheat fib er-supplemented diet containing 40 g dietary fiber, both fed with a si ngle dose of 0.9 mg isoflavones/kg body weight from tofu or texturized vegetable protein (TVP). The fiber-rich diet produced 55% lower plasm a genistein at 24 h after soy dosing (P < 0.05) and reduced total urin ary genistein by 20% (P < 0.03). Urinary daidzein was not significantl y related to fiber intake. Highly insoluble, dietary wheat fiber reduc ed the absorption of genistein probably by its bulking effect and hydr ophobic binding to this compound. Urinary genistein was greater by 23% after tofu than after TVP consumption (P < 0.02), but the percentage of ingested genistein recovered in urine was not affected by soy produ ct intake. The higher urinary genistein after tofu consumption compare d with TVP was apparently due to differences in amount of genistein be tween these soy foods, not the different forms of genistein present in these two soy food products.