Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Food Science and Human Nutrition



First Advisor

Suzanne Hendrich


Soybean isoflavones are one of major phytoestrogens in the human food supply. The proposed health effects of soy consumption include preventing hormone-dependent cancers and cardiovascular disease and lowering the risk of osteoporosis. But there also exist health concerns such as the potential hormonal effects in infants fed soy-based infant formula, and anti-thyroid effects in humans. Bioavailability of isoflavones must be better understood in order to better assess their potential health and adverse effects. Isoflavone degradation phenotype and gut transit time (GTT) may have great influence on their bioavailability. To characterize what forms have optimal bioavailability, isoflavone aglucons and glucosides were compared between high and low daidzein degradation phenotypes with various GTT in women. Daily urinary isoflavone excretion was similar for aglucons and glucosides (51.5 +/- 6.1% and 50.8 +/- 5.6% of ingested amount, respectively). Apparent absorption of isoflavones was significantly greater in the low daidzein degradation phenotype coupled with shorter GTT than in the high phenotype with longer GTT as reflected in total urinary isoflavone excretion.;To study the influence of dose on isoflavones bioavailability, high and low doses of isoflavone were fed crossover for 7 days in women of both high and low daidzein degradation phenotype with varying GTT. As reflected in urinary isoflavone excretion, dose-dependent response was significant and expected (high vs. low dose: 197 +/- 11 v. 72 +/- 11 mumol/d, P < 0.05); phenotypic difference coupled with GTT difference greatly influenced isoflavone bioavailability (low vs. high daidzein degraders: 189.0 +/- 16.3 v. 119.0 +/- 5.0 mumol/d, P < 0.05). Repeated dosing decreased urinary isoflavone excretion by 20~30%.;Higher intakes of dietary fiber may accelerate GTT, therefore may increase apparent isoflavone absorption. High (45g/d) or moderate fiber (15g/d) diets were given to women of the low daidzein degraders with longer GTT (>84 h) for 7 days as well as isoflavones in a randomized crossover feeding trial. Since GTT were decreased after both high fiber and moderate fiber diets, the apparent absorptions of isoflavones did not differed between the two fiber treatments. Repeated isoflavone feeding showed lower total urinary isoflavone excretion (week 1 vs. 2, 9.0 +/- 0.9 vs. 4.8 +/- 0.9 mumol/kg, P = 0.02) and plasma isoflavone (week 1 vs. 2, 4.8 +/- 0.7 vs. 1.8 +/- 0.7 mumol/L, P = 0.005). These data suggested that isoflavone degradation phenotype coupled with GTT is crucial in determining isoflavone bioavailability. Repeated dosing lowered isoflavone bioavailability.



Digital Repository @ Iowa State University,

Copyright Owner

Yan Zheng



Proquest ID


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File Size

167 pages