Phytoestrogens are naturally occurring polycyclic phenols found in certain plants. These are chemicals that may have weak estrogenic effects when they are ingested and metabolized. Two important groups of phytoestrogens are isoflavones and lignans. The table shows the phytoestrogen classes, examples, and some human urinary metabolites.
The isoflavones considered here include formononetin, daidzein, biochanin A, genistein, O-desmethylangolensin, and equol. Plant sources of isoflavones include legumes, with the largest contribution coming from soy-based foods. Because soy flour and soy protein isolates may be added to processed meats, meat substitutes, breads, and protein food bars, these items can be a major source of isoflavones (Grace et al., 2004, Lampe et al., 1999). However, the isoflavone content of soy protein preparations can vary widely and is affected by production techniques (Erdman et al., 2004). Daidzein and genistein are the main soy isoflavones. Kudzu root, used in some dietary supplements, also contains appreciable amounts of daidzein. Naringenin, a precursor to genistein, is found in some citrus fruits. Formononetin and biochanin A are methylated isoflavones found in clovers, which may be used in red clover dietary supplements; these isoflavones are metabolized in the body to daidzein and genistein, respectively.
Ingested daidzein is further metabolized to O-desmethylangolensin and to equol by intestinal bacteria. 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 relevance of equol-producer status to potential health related effects is unclear (Vafeiadou et al., 2006).
Lignans include matairesinol and secoisolariciresinol, which are transformed by intestinal bacteria into the estrogenic compounds enterolactone and enterodiol, respectively (Cornwell et al., 2004; Rowland et al., 2003). Enterodiol may also interconvert with enterolactone. Lignans are found in flax seeds, whole wheat flour, tea, some fruits, and other cereal grains. Other phytoestrogens of interest are resveratrol and trans-resveratrol, found in grape skins, wine, and peanuts.
Diet is the source of human exposure to phytoestrogens. The absorption and metabolism of phytoestrogens demonstrate large interindividual variability, which may relate to differences in both human pharmacokinetics and metabolism by intestinal bacteria. Phytoestrogens are ingested in their naturally occurring beta-glycosidic forms. The beta-glycosidic forms are hydrolyzed to their aglycones in the intestine, absorbed, and then glucuronidated in the intestinal wall and liver (Doerge et al., 2000; Rowland et al., 2003). The glucuronidated metabolites of isoflavones predominate in blood and urine (Rozman et al., 2006a; Setchell et al., 2001).
The isoflavones are excreted from the body about 24 hours after ingestion, mainly in urine and, to a lesser extent, in feces. Urinary concentrations of daidzein and genistein did not correlate well with the ingested doses, possibly due to limited absorption of these isoflavones at higher doses (Setchell et al., 2003a). In contrast, plasma and urine lignan concentrations after flax seed consumption increased in a dose-dependent manner (Nesbitt et al., 1999). Equol excretion may depend on diet, the type of intestinal bacteria present, and individual genetic factors (Rowland et al., 2000; Setchell et al., 2002; Setchell and Cassidy, 1999).
After hydrolysis to the aglycone forms, phytoestrogens can weakly bind to estrogen-beta receptors (ER-beta) which are expressed in arteries and smooth muscle. Individual phytoestrogens may be either estrogen agonists or antagonists. Equol has more potent estrogenic activity than its precursor, daidzein. Equol also has been shown to have antiandrogenic activity in animals (Lund et al., 2004; Magee and Rowland, 2004). Genistein binds ER-beta with greater affinity than equol (Doerge and Sheehan, 2002). Although far less potent, phytoestrogens can be present in concentrations 100 to 1000 times greater than the endogenously produced estrogens. Soy-based infant formula can result in plasma concentrations of isoflavones in infants that are 13,000-22,000 times higher than endogenous estrogen concentrations in infants (Setchell et al., 1997). Phytoestrogens may also act through pathways other than the interaction with estrogen receptors. These actions include inhibiting the transformation of estrone to estradiol, inhibiting enzymes important for steroid biosynthesis and cell growth, and having antioxidant and anti-angiogenesis activities. (Adlercreutz et al., 1995a; Dixon and Ferreira, 2002; Sirtori et al., 2005). Numerous studies of either dietary soy or phytoestrogens and health outcomes have demonstrated inconsistent or inconclusive results. Consensus reviews of these studies suggest that no evidence clearly shows that dietary phytoestrogens significantly reduce cardiovascular disease risk, reduce postmenopausal vasomotor symptoms, improve bone mineral density, or reduce cancer risk (Cornwell et al., 2004; Messina et al., 2006; NAMS, 2000; Nedrow et al., 2006; Sacks et al., 2006; Sirtori et al., 2005).
Adverse effects on fertility have been observed in animals that graze on red clover. Results of chronic feeding studies in pregnant animals suggest that high doses of phytoestrogens alter the fetal hormonal environment (Cornwell et al., 2004). Studies of children who had been fed soy-based formula as infants and who were followed through adolescence (Klein, 1998) and young adulthood (Strom et al., 2001) found no adverse reproductive or endocrine effects. In vitro and animal studies suggest that soy isoflavones may have immunologic and thyroid effects (Doerge and Sheehan, 2002; Sirtori et al., 2005). The Center for the Evaluation of Risks to Human Reproduction (CERHR) of the National Toxicology Program reviewed developmental and reproductive toxicity of both soy formula and genistein and concluded that available data were inadequate to determine whether soy formula has developmental or reproductive toxicity (Rozman et al., 2006a). The expert review panel expressed negligible concern for adverse effects in the general population consuming dietary sources of genistein (Rozman et al., 2006b).
Phytoestrogens and Urinary Metabolites
|Phytoestrogen Class||Phytoestrogen orMetabolite (CAS number)|
The concentrations of urinary phytoestrogens observed in the NHANES 1999–2000, 2000–2001, and 2003–2004 subsamples generally reflect a diet consumed in the U.S. that is lower in isoflavones than in lignans. This is consistent with a Western diet in which whole grains and cereals, rather than soybean products, contribute the bulk of phytoestrogens (CDC, 2005). Enterolactone levels were highest, followed by daidzein, enterodiol, genistein, equol, and O-desmethylangolensin. Isoflavone levels at the higher percentiles may reflect dietary supplementation with soy products. The relationship between the dose and urinary excretion is linear for many phytoestrogens, except for equol (Karr et al., 1997; Slavin et al., 1998). Because excretory half-lives are reported to be in the range of 3-10 hours (Lu et al., 1995, Setchell et al., 2001), urinary concentrations reflect recent consumption.
Levels of lignans (enterolactone, enterodiol) in the NHANES 1999–2000, 2001–2002, and 2003–2004 subsamples appeared broadly similar to levels found in studies of postmenopausal women in the United Kingdom (Grace et al., 2004); men and women in the U. S. (Valentin-Blasini et al., 2003); men and women in Minnesota (Lampe et al., 1999); postmenopausal Dutch women (den Tonkelaar et al., 2001); young African-American, Latina, and Japanese women in the San Francisco Bay Area (Horn-Ross et al., 1997); Japanese men and women (Adlercreutz et al., 1991; Uehara et al., 2000a); premenopausal omnivorous women in Boston (Adlercreutz et al., 1986); and healthy postmenopausal Finnish women who were omnivores and vegetarians (Uehara et al., 2000a,b). Vegetarian women in Boston and Helsinki (Adlercreutz et al., 1986), men and women consuming an experimental cruciferous diet (Kirkman et al., 1995), and Boston women consuming a macrobiotic diet excreted significantly higher urinary levels of these lignans (Hutchins, 1995a). Urinary enterolactone and enterodiol levels have been reported to vary by age, gender, race/ethnicity, and income (Valentin-Blasini et al., 2003). Men were shown to have higher urinary mean levels of the isoflavones and higher levels of total phytoestrogens when compared with women (Lampe et al., 1999).
Levels of isoflavones (daidzein, genistein, equol, and O-desmethylangolensin) in the NHANES 1999–2004 subsamples appeared broadly similar to those seen in young Caucasian, African-American, Latino, and Japanese women in the San Francisco Bay area (CDC, 2005; Horn-Ross et al., 1997); men and women in the United States (Valentin-Blasini et al., 2003; Lampe et al., 1999); Caucasian and Filipino women living in Hawaii (Maskarinec et al., 1998); postmenopausal women from Holland (den Tonkelaar et al., 2001) and the United Kingdom (Grace et al., 2004); omnivorous and vegetarian Helsinki women (Uehara et al., 2000a,b); and premenopausal omnivorous Boston women (Hutchins, 1995a,b).
Isoflavone levels seen in the NHANES 1999–2004 subsamples were 4 to 50 times lower than levels observed in Japanese men and women (Adlercreutz et al., 1991; CDC, 2005; Uehara et al., 2000a);Japanese women (Arai et al., 2000); postmenopausal Chinese women (Zheng et al., 1999); Singaporean women (Chen et al., 1999; Seow et al., 1998); and Japanese women living in Hawaii (Maskarinec et al., 1998). Genistein and daidzein levels in NHANES 1999–2004 subsamples were twice as high as levels reported in people consuming a carotenoid diet, but lower than levels found in people consuming a cruciferous diet; O-desmethylangolensin levels were seven times lower (Kirkman et al., 1995). Levels of genistein, daidzein, and O-desmethylangolensin in urine of people consuming a soy diet were 6 to 100 times higher than levels found in NHANES 1999–2004 subsamples (CDC, 2005). Supplementing an omnivorous U.S. diet over a three month period with 60 grams of soy powder for female subjects increased isoflavone levels by more than thirteen-fold. (Albertazzi et al., 1999). Among U.S. adults, non-Hispanic whites were reported to have higher urinary isoflavone levels than non-Hispanic blacks or Hispanics (Valentin-Blasini et al., 2003).
Finding a measurable amount of one or more phytoestrogen metabolites in urine does not imply that the levels of the metabolites or the parent phytoestrogen cause an adverse health effect. Biomonitoring studies on the levels of phytoestrogen metabolites provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of phytoestrogens than those found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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