HOy^ Y^OH 2a. Secoisolariciresinol 2b. Enterodiol*
4. ENDOGENOUS ESTROGEN
5. SYNTHETIC MOLECULES
3. Coumestrol oh oh
Fig. 1. A comparison of the chemical structures of 1) isoflavones: a) formononetin, b) daidzein, c) equol*, d) biochanin A, e) genistein; 2) lignans: a) secoisolariciresinol, b) enterodiol*, c) matairesinol, d) enterolactone*; 3) coumestan: coumestrol; 4) endogenous estrogen: 170-estradiol; 5) synthetic molecules: a) synthetic estrogen diethylstilbestrol, b) synthetic anti-estrogen tamoxifen, c) synthetic analogue ipriflavone. (*Mammalian phytoestrogens found in tissues and biological fluids which were derived from plant phytoestrogen precursors)
tion prevalence between men (43%) and women (27%). Daily excretion of daidzein, genistein and O-Dma was similar between equol excreters and non-excreters and between men and women .
The factors that contribute to the use of a particular isoflavone metabolizing pathway (e. g., the capacity to produce equol) are unknown, but several explanations have been proposed. Setchell et al.  hypothesized that the composition of the intestinal microflora, intestinal transit time and variability in the redox potential of the colon might contribute to variation in equol production in humans. The intestinal bacteria vary widely between individuals, and only selected strains are capable of hydrolyzing plant ^-glucosides . In addition, the relative stability of the composition of human intestinal microfloral population may account for the consistent patterns of isoflavone excretion. However, external and internal conditions influence microbial populations and their activities and contribute to interindividual differences in bacterial populations. Dietary intake is one of these factors which can mediate its influence directly through a change in substrate availability or indirectly through its effect on host metabolic functions . Adlercreutz et al.  reported that equol excretion was positively associated with intake of fat and meat in a Japanese population and suggested that individuals consuming more fat and meat have an intestinal microfloral population capable of producing equol from daidzein. Kelly et al.  had observed that the patterns of isoflavone excretion remain consistent over several years and postulated that some inherent factor probably plays a more significant role than diet in determining isoflavone metabolism. Concentrations of the different phytoestrogen metabolites vary widely between individuals even when a controlled quantity of an isoflavone or lignan supplement is administered. Since dietary phyto-estrogen metabolism is predominantly determined by gastrointestinal flora, a variety of factors can modify metabolism such as antibiotic use, bowel disease, and gender [25,29-31].
Phytoestrogens can be measured in urine, plasma, feces, prostatic fluid, semen, bile, saliva, and breast milk [18,32-38]. Both unconjugated (free) and conjugated forms of isoflavones circulate in the human body. Many investigators have measured dietary estrogens in body fluids in correlation with dietary intake of phytoestrogens. For example, urinary and blood levels of isoflavones have been correlated with an increase with supplements of soybean. Blood levels of isoflavones increase within 30 min of consumption of a soybean supplement, and begin to decline after 5 h ingestion, although elevated levels remain at 24 h . However, only 7-30% of the ingested amount may be recovered in the urine [22, 23, 39]. Postmenopausal Australian women consuming a traditional diet supplemented with soy flour or clover sprouts had the respective concentrations of equol, daidzein and genistein reach 43,312, and 148 ng-ml-1, respectively . However, not all subjects were able to produce equol from daidzein. In a study conducted by Seow et al. , the distributions of dietary soy isoflavonoids (daidzein, genistein, and glycitein), urinary soy isoflavonoids and their metabolites (daidzein, genistein, glycitein, equol and O-Dma) among 147 Singapore Chinese of age 45 - 74 years were assessed. Among study subjects, there were statistically significant, dose-dependent associations between fre quency of overall soy intake and levels of urinary daidzein and the sum of urinary daidzein, genistein and glycitein. In contrast, there were no associations between frequency of overall soy intake and levels of the two daidzein metabolites equol and O-Dma in urine. This study suggests that within the range of exposures experienced by Singapore Chinese, the urinary levels of daidzein or the sum of daidzein, genistein and glycitein obtained from a spot sample can serve as a biomarker of current soy consumption in epidemiological studies of diet-disease associations. Several studies have shown that Japanese men and women who consume a traditional diet have high levels of isoflavonoids in both urine and plasma . Japanese women were found to excrete 10 times more daidzein and 20-30 times more equol and O-Dma than women in Boston and Helsinki . In Western populations, urinary excretion of isoflavonoids appears highest among macrobiotic women, who consume mainly cereals, grains, legumes, and vegetables, followed by vegetarians, who in turn have higher levels than omnivores [32, 43]. Using a 3-day food diary, Adlercreutz et al.  demonstrated a significant correlation between urinary excretion of daidzein, equol and O-Dma with intakes of beans and pulses, soy products, and boiled soybeans in 19 Japanese men and women. Subjects were fed a soy beverage for a period of 2 weeks (~42 mg of genistein and ~27 mg of daidzein per day) and the measured plasma levels of genistein and daidzein ranged from 0.55-0.86 |M, mostly as glucuronide and sulfate conjugates. Horn-Ross et al.  examined the racial/ethnic differences in urinary phytoestrogen levels in 50 Japanese, Caucasian, African American and Latina young women (ages 20-40 years) residing in the San Francisco Bay Area (SFBA). There was substantial variation in phytoestrogen levels along with racial/ethnic differences. The highest levels of coumestrol and lignans were observed in white women and the lowest levels in Latina and African American women. Genistein levels, however, were highest in Latina women. Other isoflavones levels did not differ significantly by race/ethnicity. Latina SFBA women, who were observed to have the highest urinary excretion of genistein and daidzein, reported consuming more beans and chili with beans. Although different varieties of beans contain different amounts and types of phytoestrogens , the urinary excretion of genistein and daidzein among these women is similar to that among persons consuming an experimental diet including garbanzo beans , which contains biochanin A, a precursor of genistein.
The lignans enterolactone and enterodiol form another class of phenolic compounds. Enterodiol and enterolactone are derived from the colonic microbial fermentation of seco isolariciresinol and matairesinol, respectively (Fig. 1) [15,47]. These lignan precursors occur in the aleuronic layer of the grain close to the fiber layer. Seco isolariciresinol and matairesinol have glucose residues attached to the hydroxy (OH) groups. During fermentation, the colonic bacterial flora remove both glucose and methoxy groups to form the diphenols, en-terodiol and enterolactone, which are structurally similar to E2 (Fig. 1). After absorption, these mammalian lignans also are excreted in urine. Urinary levels of lignans increase substantially when supplements of linseed are fed to human subjects [48,49]. There is also a moderate increase in urinary excretion following vegetable supplementation . Macrobiotic and other vegetarians, such as
Seventh Day Adventists, have the highest excretion values of lignans . Postmenopausal Australian women consuming a traditional diet supplemented with linseed had combined levels of enterolactone and enterodiol which reached 500 ng-ml-1 .
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