Blood levels of the soy isoflavone genistein were higher after consuming soy food with an isoflavone content of 96 mg than for isoflavone-rich supplements, according to new data published in the Journal of Nutritional Biochemistry.
“The findings from this investigation suggest that bioavailability may be higher from food sources of isoflavones than from tablets, and that spreading the intake out in multiple doses over the course of the day will lead to more constant steady-state plasma concentrations,” wrote lead author Christopher Gardner.
Soy isoflavones are well known phytoestrogens - active substances derived from plants that have a weak estrogen-like action. They have been studied for their role in cancer prevention and slowing down the ageing process in peri-menopausal women, and have proved to be a popular alternative to hormone replacement therapy for those wishing to control menopause symptoms without resorting to drugs.
Despite numerous studies reporting the potential health benefits of soy, controversy and contradiction still exists, particularly in terms of how bioavailable the isoflavone forms (glycoside versus aglycone) are.
Some studies have reported no difference between bioavailability of the isoflavone type, whereas others have reported higher bioavailability for aglycones.
“This is relevant because many soy isoflavone supplements and food products contain primarily the glycoside forms versus pure isoflavones in their aglycone forms,” wrote the Stanford researchers.
“A greater understanding of isoflavone pharmacokinetics that may differ by source and dose could help to explain some of the heterogeneity of results among the large and growing number of trials testing the effects of soy isoflavones on a variety of potential health outcomes,” added Gardner.
The Stanford researchers recruited 12 generally healthy people to take part in the randomized, cross-over trial. All participants completed the three phases of the trial. These included six days on each of the following interventions: daily supplements containing a low dose (144 mg/day) or a high dose (288 mg/day) of isoflavones (Novasoy, ADM), or consumption of soy foods providing a daily isoflavone dose of 96 mg. All the doses are noted in aglycone equivalents. The supplements contained isoflavones predominantly in the glycoside form.
Gardner and his co-workers report that both the soy food and the high dose isoflavone supplement produced blood levels of isoflavone over four micromoles per liter.
Blood levels of genistein were “higher overall in the soy food versus both the lower and the higher dose supplement phases of the study”, wrote the researchers. “When comparing plasma concentrations for the two doses of tablets, saturation appeared more evident for genistein than for daidzein at the higher dose level,” they added.
“We observed important differences in the pharmacokinetics of genistein and daidzein contrasting the sources and doses of isoflavones when administered three times daily, including a possible advantage for increasing serum concentrations of isoflavones from consuming soy foods relative to isoflavone supplements,” wrote the researchers.
Gardner and his co-workers noted that future studies should match the isoflavone doses between food and supplements, as well as comparing intakes for a single bolus against those of multiple intakes during the course of the day.
The study was supported financially by the National Institutes of Health.
Source: Journal of Nutritional Biochemistry Volume 20, Pages 227-234"Effects of isoflavone supplements vs. soy foods on blood concentrations of genistein and daidzein in adults"Authors: C.D. Gardner, L. M. Chatterjee, A.A. Franke