A Diet High in Wheat Fiber Decreases the Bioavailability of Soybean Isoflavones in a Single Meal Fed to Women

Abstract

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 fiber-supplemented diet containing 40 g dietary fiber, both fed with a single dose of 0.9 mg isoflavones/kg body weight from tofu or texturized vegetable protein (TVP). The fiber-rich diet produced 55% lower plasma genistein at 24 h after soy dosing (P < 0.05) and reduced total urinary genistein by 20% (P < 0.03). Urinary daidzein was not significantly related to fiber intake. Highly insoluble, dietary wheat fiber reduced the absorption of genistein probably by its bulking effect and hydrophobic 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 product intake. The higher urinary genistein after tofu consumption compared with TVP was apparently due to differences in amount of genistein between these soy foods, not the different forms of genistein present in these two soy food products.

Urinary isoflavonoid excretion is inversely associated with the ratio of protein to dietary fibre intake in young women

“…The mean recovery of total isoflavonoids, expressed as a percentage of supplement dose excreted during 24 h, was 2279%, which is similar to values of 19 and 30% reported in previous studies using supplements derived from red clover (Hodgson et al, 1998;Nestel et al, 1999). Greater recovery was observed with daidzein and related compounds (55723%) compared to those of genistein (7.876.0%), and is consistent with previous findings with isoflavones derived from soy (Kelly al, 1993;Hutchins et al, 1995;Xu et al, 1995;Lu et al, 1996;Tew et al, 1996).…”

“…Interventions with a soy protein beverage or textured soy protein reported that equol excretion was associated with significantly more carbohydrate and dietary fibre , or less fat and more carbohydrate (Rowland et al, 2000), consumed in the habitual diet. Conversely, isoflavone excretion after a soy-rich meal was reduced by 13% when accompanied by a wheat fibre supplement (Tew et al, 1996). However, two other studies found no changes in isoflavonoid excretion after treatment for 1 month with soy combined with a wheat fibre supplement (Lampe et al, 2001) or an increased intake of fat and protein .…”

“…However, two other studies found no changes in isoflavonoid excretion after treatment for 1 month with soy combined with a wheat fibre supplement (Lampe et al, 2001) or an increased intake of fat and protein . Discrepancies could arise from differences in design: two studies Rowland et al, 2000) focused on equol and did not evaluate total isoflavonoids, while in two other trials (Tew et al, 1996;Xu et al, 2000), the experimental period of 1day may have been inadequate and data on the isoflavonoid metabolites were not reported. The present findings are consistent with a unifying hypothesis that isoflavonoid excretion is associated with the relative intakes of protein and dietary fibre; this could explain the apparent contradictions in previous studies in which possible interaction between these macronutrients was not explored.…”

“…A dose-dependent relationship has been reported between intake and mean excretion of isoflavonoids after 9-day treatments with four levels of soy protein (Karr et al, 1997). However, intervention studies with soy products have reported surprisingly wide, largely unexplained variation in the excretion of isoflavonoids between individuals ingesting the same dose (Kelly et al, 1993;Hutchins et al, 1995;Tew et al, 1996;Lampe et al, 1998;Slavin et al, 1998;Rowland et al, 2000). The degree of metabolic conversion of the original plant isoflavones could vary according to differences in the population of the intestinal bacteria involved in the hydrolysis, demethoxylation and reduction of isoflavones.…”

Effects of soy foods on ovarian function in premenopausal women

“…Race/ethnicity may be associated with genetic variability in bacteria flora and/or differences in dietary habits (e.g. fat and fibre intake) which may influence the concentration of microfloral enzymes (Adlercreutz et al, 1992;Goldin and Gorbach, 1994;Xu et al, 1994;Tew et al, 1996;Zhang et al, 1999) and consequently the metabolic fate of these compounds. For example, faecal bacterial β-glucuronidase activity has been found to increase in association with high dietary fat intake and to decrease in association with high dietary fibre intake (Goldin and Gorbach, 1994).…”