Factors Affecting the Bioavailability of Soy Isoflavones in Humans after Ingestion of Physiologically Relevant Levels from Different Soy Foods

Abstract

The precise role that isoflavones play in the health-related effects of soy foods, and their potential for adverse effects are controversial. This may be due in part to a lack of basic knowledge regarding their bioavailability and metabolism, particularly as it relates to the soy source. To date, there is little information concerning possible differences in the bioavailability of isoflavones derived from natural soy foods consumed at physiologically relevant intakes and whether age- or gender-related differences influence that bioavailability. In the current study of healthy adults [premenopausal (n = 21) and postmenopausal (n = 17) women and a group of men (n = 21)], we examined the effect of age, gender, and the food matrix on the bioavailability of isoflavones for both the aglycon and glucoside forms that are naturally present in 3 different soy foods, soy milk, textured vegetable protein, and tempeh. The study was designed as a random crossover trial so that all individuals received each of the 3 foods. The dose of isoflavones administered to each individual as a single bolus dose was 0.44 mg/kg body weight. Pharmacokinetic parameters were normalized to mg of each isoflavone ingested per kilogram body weight to account for differences in daidzein and genistein content between the diets. Serum isoflavone concentrations in all individuals and groups increased rapidly after the ingestion of each soy food; as expected, genistein concentrations exceeded daidzein concentrations in serum. In this small study, gender differences in peak concentrations of daidzein were observed, with higher levels attained in women. Consumption of tempeh (mainly isoflavone aglycon) resulted in higher serum peak levels of both daidzein (P < 0.001) and genistein (P < 0.01) and the associated area under the curve (P < 0.001 and P < 0.03, respectively) compared with textured vegetable protein (predominantly isoflavone glucosides). However, soy milk was absorbed faster and peak levels of isoflavones were attained earlier than with the other soy foods. Only 30% of the subjects were equol producers and no differences in equol production with age or gender were observed.

Role of plasma lipoproteins in the transport of the soyabean isoflavones daidzein and daidzein-7-O-β-d-glucoside

“…In most subjects the plasma appearance and disappearance curves revealed a biphasic pattern with an early peak before the peak plasma concentration which has also been reported by other authors (5,6) . Possible reasons for this behaviour discussed so far are enterohepatic recycling and some small initial absorption in the upper gastrointestinal tract along with predominant absorption in the large intestine.…”

Estimating perchlorate exposure from food and tap water based on US biomonitoring and occurrence data

“…Equol, but not O-desmethylangolensin, has estrogenic activity. About 30% of adults can be characterized as equol producers and demonstrate higher serum equol levels after daidzein consumption (Cassidy et al, 2006;Setchell et al, 2003). This ability to produce equol may be related to an individual's intestinal microflora and influenced by dietary habits (Rowland et al, 2000).…”

The axon protective effects of syringic acid on ischemia/reperfusion injury in a rat sciatic nerve model

“…Although biflavonoids are known to display a variety of biological activities, such as anti-inflammatory activity, inhibition of cytochrome P450 enzymes, and antiviral activity, their neuroprotective roles have not been known (7,9,16 SOD activity may reduce (1,4,6,14,20). Similarly, we found that the SOD activity in the SA and MP group was significantly increased in comparison to that of the control group.…”