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SAN DIEGO—Equol, a potent phytoestrogen that some individuals can produce after soy consumption, may be the single most important factor mediating the health benefits of soy foods or supplements.
Equol production is highly variable; some people are able to produce it while others cannot. The difference appears to be related to differences in gut flora, and could account for the widely divergent conclusions in clinical studies of soy or soy extracts.
“I am firmly of the belief that equol may be the crucial issue with regard to the clinical efficacy of soy. It is equol, not genistein. Equol has a lot of clinical significance,” said Kenneth D. R. Setchell, PhD, a nutrition scientist at the Children’s Hospital and Medical Center, Cleveland, OH.
Speaking at the 4th International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Dr. Setchell explained that equol was once a forgotten footnote in the annals of nutrition science. But it is rapidly becoming the isoflavone du jour, as leaders in soy research concede that the near-universal focus on the isoflavones genistein and daidzein as the “active” components in soy was simplistic and somewhat misguided.
Equol production is associated with reduced risk of breast cancer and other estrogen-mediated cancers, improved bone mineral density, and may have cardiovascular benefits as well.
Horses, Clover, and Gut Bacteria
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| “Headline bean” courtesy of Peter Golbitz, Soyatech (www.soyatech.com); all other photos courtesy of American Soybean Association, St. Louis, MO (www.soygrowers.com). |
Equol (C15H14O3) was first discovered in 1932, in the urine of horses. Its name reflects this original equine discovery, but many other mammals including goats, sheep, cattle, cats, dogs, rodents and primates also produce this compound.
Horses show a clear seasonal variation in the production of this phytoestrogen, with peaks in summer, followed by declines in autumn, and a near absence in winter. “That does suggest a dietary connection,” said Dr. Setchell. Early researchers postulated it had to do with variations in the plants horses ate over the course of the year.
In 1946, this was corroborated, when equol derived from clover was identified as the chief culprit behind “Clover disease,” a constellation of infertility, failed conception, sperm abnormalities and fetal deaths, that plagued Australian sheep. The sheep were eating a lot of a type of clover very rich in formononectin, which is metabolized to equol in quantities vast enough to cause estrogen-mediated reproductive anomalies in the sheep.
Dr. Setchell and his research group got interested in the subject when they were trying to determine whether soy is a good source of lignans. Using rodent models, they found that equol excretion in soy-fed rats went “sky high.” This set them wondering about equol production in humans that eat soy.
In-Equolity: The Natural Variance in Equol Production
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In people, you typically don’t see equol unless they are consuming soy; it is a metabolite of daidzein. But not everyone produces equol. It appears to be dependent on certain gut flora. Japanese researchers studied the feces of 10 healthy men, all of whom ate a single serving of 40 grams of tofu. There were 3 distinct bacterial species—a form of streptococcus, a type of bacteroides, and a ruminococcus species—present in feces of men who produced equol, but absent from non-producers.
Fecal inoculates from equol producing people can induce conversion of daidzein to equol under both anaerobic and aerobic conditions. This process can be attenuated or stopped altogether if the samples are incubated with certain antibiotics. Metronidazole, vancomycin, kanamycin, rifampicin and tetracycline have all been shown to obliterate equol production. “It can take 4–6 months for the gut flora to reestablish itself and begin producing equol after wiping it out with antibiotics,” Dr. Setchell said.
Though the right bacteria are necessary, other factors come into play. In women, it may be influenced by the hormonal flux of the menstrual cycle. A small study of 6 premenopausal equol-positive women eating 60 grams of texturized soy protein daily, showed that those with the longest menstrual cycles were the highest equol producers (Cassidy A, et al. Am J Clin Nutr 1995 Jul; 62(1): 151–53). High producers also tend to have lower-fat, higher-carbohydrate diets (Rowland IR, et al. Nutr Cancer 2000; 36(1): 27–32).
The ability to make equol from daidzein is not inborn. Dr. Setchell’s lab has shown that infants are unable to produce it for the first four months of life, though some acquire the ability—presumably by acquiring the appropriate gut flora—during the first year. It seems, though, that once someone is an equol-maker, he or she will be for life.
Within a given population, there appears to be an intrinsic but stable variance in equol production. Several studies suggest roughly one-third of the US population are good equol producers, while two-thirds do not produce it, or make it at very low levels. Again this may reflect variance in gut flora.
The Equol Advantage
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| Kenneth D. R. Setchell, PhD, Children’s Hospital and Medical Center, Cincinnati, OH. |
Is it good to be an equol producer? Generally speaking, the answer is yes, presuming you like soy. The issue is largely irrelevant for the soy-averse.
Dr. Setchell explained that chemically, equol is quite similar to estradiol, and equol can bind to estrogen receptors. Physiologically, however, they are different. Equol has a very high affinity for estrogen receptors and poor affinity for serum proteins—the exact opposite of estradiol, which is largely protein-bound. Compared with its precursor, daidzein, equol has a stronger affinity for the estrogen receptors. If it is an estrogenic effect that is sought, it is good to be able to convert daidzein to equol.
An equol-producing woman who consumes a moderate amount of soy (approximately 0.5 mg/kg/day of total isoflavones) will show urinary excretion of equol that is more than 4,000 times higher than excretion of endogenous estrogen. But humans, unless they are eating nothing but soy, are unlikely to produce equol at anywhere near the level of that in sheep. There is little risk of estrogen-mediated anomalies, as seen in sheep with clover disease.
Indeed, most available data indicate that equol’s effect is quite salutary.
Equol and Cancer Risk Reduction
There is some debate about how soy isoflavones influence reproductive hormones, and by extension, risk of reproductive system cancers. There are currently 11 studies, most of them small, addressing these questions.
According to Mindy S. Kurzer, PhD, of the department of food science and nutrition, University of Minnesota, the data suggest an overall trend toward longer menstrual cycle, reduced circulating estradiol, lower luteal phase progesterone, reduced mid-cycle gonadotropins, and decreased sex hormone binding globulin. With the exception of the latter, all of these changes are statistically associated with reduced cancer risk.
However, in most of the studies, the differences are not statistically significant, and from study to study and patient to patient, the hormonal response to isoflavones varies.
Dr. Kurzer and colleagues studied the effects of isoflavone supplementation at doses of 0.15, 1.0 or 2.0 mg/kg body weight per day, in 14 normally cycling reproductive-age women. In the group as a whole, the isoflavones produced only minimal changes in sex hormone profiles. But when they looked specifically at the 5 equol producers in the cohort, “we were rather struck.”
There were strong, statistically significant differences between baseline and post-supplementation hormone levels. Equol producers had reduced estrone, estrone sulfate, and androgen levels, along with an increased SHBG value. “This is consistent with reduced risk of breast cancer, or breast protective effects,” said Dr. Kurzer, adding that the same pattern of equol-associated hormone changes were seen with all three isoflavone doses (Duncan AM, et al. Cancer Epidemiol Biomarkers Prev 2000; 9(6): 581–86).
She also noted that a number of studies (Xu X, et al. Cancer Epidemiol Biomarkers Prev 1998; 7(12): 1101–08; Martini MC, et al. Nutr Cancer 1999; 34(2): 133–39); Lu LJ, et al. Cancer Res 2000; 60(15): 4112–21) show that in premenopausal women, soy isoflavone supplementation increased production of the anti-proliferative 2-hydroxyestrone metabolites of estrogen, and reduced the 16-hydroxy metabolites, changes that are thought to lower risk of estrogen sensitive cancers (see related story, page 8).
Equol and Breast Health
Julie Maubach, PhD, of the University of Ghent, Belgium, is assessing tissue distribution of soy isoflavones, with a particular emphasis on breast tissue. She obtained tissue samples from women undergoing breast reduction surgery. The study subjects were given daily supplements containing 100 mg genistein, 37 mg daidzein, 15 mg glycitein, or a placebo for five days prior to surgery.
Isoflavone levels in breast tissue were higher than in serum from the same women, though not as high as in urine. But the various isoflavones showed different profiles. “Genistein does not show up in breast tissue, but equol does. Daidzein is present in very small amounts,” said Dr. Maubach at the conference. “Equol may be the most important and relevant isoflavone in breast tissue.”
In contrast with prior literature, most but not all of Dr. Maubach’s subjects were equol producers. They generated enough equol to make this the most prevalent isoflavone in breast tissue for the study population as a whole. “Equol producers make a lot of equol, and that balances out non-producers in the cohort.”
Dr. Setchell said equol’s affinity for breast tissue is not surprising given its lipophilic and hydrophobic nature. “Daidzein is extensively tissue distributed; equol is not. Equol may be concentrating in certain tissues, and though the half-lives are similar, equol hangs around longer.”
Equol After Menopause
In terms of menopause-associated symptoms, equol production correlates with reduced symptom scores. Uchiyama et al. studied 59 Japanese dietitians, 37 of whom were perimenopausal. The women with the lowest total symptom scores for hot flashes and shoulder pain were all equol producers and had the highest mean urinary equol excretion. Those with the highest symptom scores were the weakest equol producers.
The compound also appears to be bone-friendly. Lydeking et al. in Denmark studied 106 post-menopausal women randomized to daily consumption of soymilk with either high or low total isoflavone concentrations. At the end of the 2 years, lumbar BMD was higher in those on the high isoflavone soymilk. They did not lose any BMD from baseline. In contrast, those on the low isoflavone soy showed a mean 4–5% drop in BMD.
Dr. Setchell noted that 45% of the women in this study were equol producers, and that compared with the equol-negative women, the equol-producers had higher mean BMD. “The responsiveness of bone concentration to soy intake may be related to equol production.”
In terms of cardiovascular health, equol promotes the production of Akt protein kinase in the myocardium. This serine-threonine kinase (also called protein kinase B) down-regulates programmed cell death in the myocardium, and appears to be cardioprotective. Low levels of Akt are associated with cardiomyopathy.
Pre-menopausal women generally have markedly increased levels of Akt in their myocardium compared with men of the same age. Post-menopause, the profile in women is similar to men.
Experimental knockout mice that cannot produce Akt develop a characteristic cardiomyopathy, which can be partially reversed by feeding them genistein, which is converted to equol. The same may be true in humans, though this has not yet been tested clinically.
Equol Rights: Can Non-Producers Be Converted?
The million-dollar question is, can a non-producer be “converted” or induced to make equol by some sort of therapeutic intervention? Given the microbial connection, it makes sense to consider a “probiotic” approach to seed?ing a non-producer’s colon with the requisite organisms to facilitate conversion of daidzein to equol.
At this point, the question is largely theoretical, though some researchers have begun to look into it. Johanna Lampe, PhD, and colleagues at the Fred Hutchinson Cancer Research Center, Seattle, have experimented with “tweaking” equol production, by combining increased dietary fiber with soy supplements, in the hope that the altered GI motility time and intestinal pH induced by the fiber will promote growth of equol-generating organisms. So far, this approach has not worked.
But she and her colleagues are engaged in a long-term family cohort study to determine how genetic or familial factors influence expression of the equol-positive phenotype. “We are trying to recruit 100 or more families with 3 or more equol-positive family members representing at least 2 generations,” said Dr. Lampe.
They have also developed an equol phenotyping kit that contains 3 servings of a soy product containing at least 20 mg daidzein. “You take one serving a day for three days, and then on the 4th day, collect urine from the first void and send it, pre-paid, to our processing lab.”
It is also possible, at least theoretically, to consider equol supplementation for non-producers, so they could obtain the benefits associated with equol. So far, however, there are no such supplements available.
Like many a scientific discovery, the recognition of equol as a key factor in soy raises many more questions than it answers. To what extent does it account for individual variations in response to soy foods? Does it reflect larger differences in the ability to digest soy? Are there intrinsic ethnic differences in prevalence of equol producers among peoples who traditionally eat a lot of soy versus those that don’t?
It also underscores the importance of an individualized approach to dietary changes. While soy is clearly a nutritious and beneficial food, people are not uniform in how they metabolize it. As nutrition science becomes more sophisticated, simplistic, one-size-fits-all nutritional recommendations make less and less sense. The equol story is but one example of this.
THE REDUX: Much research on soy over the last two decades has focused on the isoflavones genistein and daidzein. But equol, a little known metabolite of daidzein may be the most clinically important soy-derived compound. Equol is extremely estrogenic, much more so that its daidzein precursor. Clinical research indicates it is associated with skeletal, vasomotor, and cardiovascular benefits in peri- or postmenopausal women, and also with reduction of risk for certain cancers. What is perhaps most interesting about equol is that not everybody can produce it; roughly 30% of the population make this compound after consuming soy, and the difference appears to be related to variances in gut flora. Variation in equol production may account for the widely divergent outcomes of clinical studies of soy supplementation.
