Isoflavones
1. Identity: Botanical and Chemical Classification
Isoflavones are a class of molecules with a chemical structure based on the 3-phenyl chromen-4-one backbone. More broadly, isoflavonoids constitute a diverse flavonoid subclass based on the 3-arylchromone parent skeleton, distinct from most other flavonoids based on a 2-arylchromone structure. Isoflavones are plant bioactive nonsteroidal polyphenolic metabolites with antioxidant properties. They have a very close structure with 17β-estradiol and possess estrogenic/antiestrogenic effects.
Isoflavones are phytoestrogens found mainly in the plants belonging to family Fabaceae (i.e., Leguminosae). Different legumes belonging to this family, such as soybeans, barley, fava beans, kudzu, lupine, broccoli, peanut, and cauliflower, are major sources of natural isoflavones. The major distribution of isoflavonoids is in the Papilionoideae subfamily of Fabaceae, and significant ecophysiological functions of isoflavonoids in these plants as defensive agents (as phytoalexins, insecticidal chemicals, and allelochemicals) and symbiotic signals to microorganisms have been noted.
Principal Isoflavones and Their Chemical Forms
The main source of isoflavones are legumes from the family Fabaceae, namely soybean (Glycine max) as a source of daidzein, genistein, and glycitein, and red clover (Trifolium pratense) as a source of formononetin and biochanin A. Genistein, daidzein, and glycitein (and their different chemical forms) comprise approximately 50%, 40%, and 10%, respectively, of total isoflavone content in soybeans, although there is considerable variation in this ratio among soybean varieties and foods produced from soybeans.
In soybeans and unfermented soy foods, isoflavones occur almost entirely as glycosides. Because the weight of the non-isoflavone part of glycosides (the sugar molecule) accounts for approximately 40% of the total weight, a value of 0.6 is typically used to convert glycoside into aglycone values, the biologically relevant amount. In plants, isoflavones can be found mainly as non-active glycosides, which are converted after ingestion into the corresponding aglycones (e.g., genistein, daidzein) that have pharmacological activity.
Genistin, daidzin, and glycitin are the main isoflavones present in glycosylated form. Red clover is another enriched natural source of isoflavones, which contains the same ones as soybeans, but predominantly presents the free and conjugated forms of formononetin and biochanin A. The main isoflavones are genistein, daidzein, glycitein, formononetin, biochanin A, and puerarin.
Natural Sources and Dietary Content
Most legumes investigated have been found to contain at least detectable levels of one or more of the five estrogenic isoflavones, but the richest sources are soybeans, lentils, chickpeas, fenugreek, clovers, alfalfa, and various varieties of beans. The richest sources of these compounds are the clovers (Trifolium pratense, Trifolium subterraneum) and soya (either whole soya or defatted soya or any materials ensuing as products of soya processing, including soya grits, soya hypocotyls, and soy molasses). Small amounts of isoflavones are also found in many fruits, vegetables, nuts, and cereals.
There are approximately 3–4 mg isoflavones per gram of protein in traditional Asian soy foods such as tofu, soymilk made from whole soybeans, and edamame. Thus, one serving, such as one cup of soymilk or 3–4 oz of tofu, provides approximately 25 mg isoflavones.
Common Preparations and Supplement Forms
Food supplements targeted at peri- and postmenopausal women typically provide a daily dose of isoflavones in the range of 35–150 mg/day. Capsules and tablets are the most common pharmaceutical dosage forms; the main challenge with isoflavones such as genistein, daidzein, and glycitein is that they have poor water solubility along with high membrane permeability (classified as Class II drugs in the Biopharmaceutics Classification System). Aglycone forms of isoflavones have a great capacity to be absorbed both in the gastrointestinal tract and skin, and have substantial therapeutic potential when compared to the conjugated forms. Other preparations include standardized soy protein isolates, soy germ extracts, and fermented soy-derived foods such as miso and tempeh.
2. Traditional and Historical Use
The rich history of soy began approximately 5,000 years ago on the windy plains of Eastern Asia. Legumes play an important role in the traditional diets of many regions of the world, and Glycine max (Leguminosae), commonly known as soybean or soya bean, is unique among the legumes. According to Chinese tradition, the soybean was one of the five sacred crops named by the Chinese emperor Sheng-Nung, who reigned 5,000 years ago. Historians maintain that Sheng-Nung mentioned the soybean in his "Ben Tsao Gang Mu," written in the year 2838 B.C.
Asian populations whose isoflavone exposure begins earlier in life and continues throughout life have been the source of epidemiologic data demonstrating a relationship between soy consumption and health benefits. Individuals from Western cultures consume approximately 1 to 2 mg of isoflavones daily, whereas total mean intake of isoflavones among Asian populations ranges from 25 to 50 mg daily.
However, the nature of traditional isoflavone exposure deserves careful qualification. Contrary to ancient times, when soybeans were boiled, modern commercial soy foods can contain up to 150 mg/100g of estrogenic isoflavones. Current estimations of isoflavone intake in the literature do not distinguish between the origins of soy food—whether it is homemade or commercial—and as a result, the isoflavone exposure in Asian countries may well be overestimated. Research implementing traditional recipes of tempeh and miso found that when compared to commercial foods, the isoflavone content was found to be 20, 2.6, 4.5, and 9.8 times lower in "homemade" soy juice, tofu, tempeh, and miso, respectively. This suggests that the ancient soy consumption, traditionally in Asia, only provided small amounts of estrogenic isoflavones.
The existence of isoflavones in Pueraria has long been known, with the roots of Pueraria containing several isoflavone compounds such as daidzin and puerarin. Kudzu root preparations have thus formed part of traditional East Asian herbal medicine. Historically, the consumption of soy products in Asian cultures from a very young age has not had any apparent negative effects related to hormone imbalances.
3. Key Constituents and Active Compounds
Primary Isoflavones
- Genistein — The most abundant isoflavone in soy by proportion (~50% of total), and the one most studied for biological activity. It is present in plants mainly as the glycoside genistin.
- Daidzein — The second most prevalent soy isoflavone (~40% of total), present in plants as the glycoside daidzin. A proportion of daidzein is metabolized by intestinal bacteria to equol.
- Glycitein — A minor soy isoflavone (~10% of total). Present as glycitin in plant material.
- Formononetin and Biochanin A — The principal isoflavones of red clover (Trifolium pratense). Biochanin A is a methylated precursor of genistein, and formononetin is a methylated precursor of daidzein.
- Puerarin — The primary isoflavone found in kudzu root (Pueraria lobata).
The Equol Metabolite
Equol (7-hydroxy-3(4′-hydroxyphenyl)-chroman) represents the major metabolite of the phytoestrogen daidzein. Equol, however, is not a phytoestrogen, because it is not a natural constituent of plants. Equol does not occur naturally in any plant-based products. The capacity to form the metabolite equol from daidzein is suggested as an important modulator of response to isoflavones; this capacity depends on gut colonization with appropriate bacteria. Daidzein is then further metabolized to equol in "equol-producing" individuals. Equol thereafter circulates in the bloodstream at relatively high concentrations. Equol is not normally present in the urine of most healthy adults unless soy is consumed.
4. Mechanisms of Action
Estrogen Receptor Binding
As isoflavones are structurally similar to estradiol-17β and have a molecular structure similar to animal estrogens, they also function as phytoestrogens. Some phytoestrogens exhibit affinity for estrogen receptors, producing estrogen or anti-estrogen effects. The phenolic ring structures of isoflavones enable these compounds to bind estrogen receptors (ER) and mimic estrogen. Although genistein and daidzein bind to ER, it is with a lower affinity when compared to estradiol, and with a greater affinity for ERβ than to ERα.
Additionally, phytoestrogens have been reported to act like natural selective estrogen receptor modulators (SERMs) at various tissue sites throughout the body. In some tissues, there is evidence that phytoestrogens act as estrogen agonists, whereas in others, they display antagonistic characteristics comparable to that of tamoxifen or raloxifene, where SERM activity appears to be sex-hormone and gender dependent.
Metabolic Activation via Intestinal Microbiota
Isoflavone glycoside conjugates are metabolized in the GI tract by intestinal bacteria, which hydrolyze the carbohydrate moiety to the biologically active phytoestrogen genistein. The same metabolic step occurs for the aglycone daidzein, which is converted from the glycosidic form daidzin. This step is a critical determinant of bioavailability, as only the aglycone forms are pharmacologically active. Individual variation in gut microbiota composition explains why some people are "equol producers" and others are not.
Non-Estrogenic Mechanisms
Isoflavones exert a range of biological effects beyond ER binding. The antioxidant property of soy isoflavones, namely genistein and daidzein, is well established in different experimental models and also in clinical studies. The compounds have been found effective in the management of diabetes by acting on peroxisome proliferator-activated receptors. Soy isoflavones also have the potential in the treatment of osteoporosis by acting on osteoclasts and inhibiting tyrosine kinase. Genistein in particular is a known inhibitor of protein tyrosine kinases, enzymes involved in cell proliferation and growth signaling, which underlies its investigation as an anticancer agent. Genistein modulates the expression of a wide variety of genes in cultured prostate cancer cells, including those involved in cell growth, apoptosis, angiogenesis, and metastasis.
5. Scientific Evidence by Area of Use
5a. Menopausal Symptoms (Vasomotor Symptoms / Hot Flashes)
The most extensively investigated clinical application of isoflavones is the alleviation of menopausal vasomotor symptoms. A pivotal systematic review and meta-analysis published in Menopause (2012) and conducted by Taku et al. searched PubMed and the Cochrane Controlled Clinical Trials Register for double-blinded randomized controlled trials. From 277 potentially relevant publications, 19 trials (reported in 20 articles) were included in the systematic review. Meta-analysis revealed that isoflavones significantly reduced hot flash severity by 26.2% (95% CI: −42.23 to −10.15, P = 0.001) compared with placebo, though with substantial heterogeneity (I² = 86%; random effects model).
A separate systematic review identified 17 trials of isoflavones and their effects on hot flashes and co-occurring symptoms. In five trials of soy isoflavone preparations, two (6 g soy germ extract and 25 g soy protein in soy nuts) significantly decreased hot flashes, but no other symptoms. None of the 5 well-designed red clover trials demonstrated a significant or clinically meaningful benefit for the relief of hot flashes. Further, the review supported the idea that higher concentrations of the isoflavone genistein may be more effective in the relief of hot flashes.
Dosing frequency may also be important: a pilot randomized trial suggested that a twice-daily to thrice-daily dosing frequency may improve the benefit of isoflavones for vasomotor symptom relief, particularly in equol producers and for nighttime symptoms, though larger studies are needed to confirm these findings.
Evidence strength: Moderate. Multiple RCTs and meta-analyses confirm a statistically significant, though modest, reduction in hot flash frequency and severity. High heterogeneity across trials, variations in products used, dose, and equol-producer status in study populations limit definitive conclusions.
5b. Bone Health / Osteoporosis Prevention
Epidemiological studies demonstrated a relationship between the lower incidence of osteoporosis in Asian women and a diet rich in soy foods, although results from intervention studies are still controversial. One of the potential reasons for these inconsistencies could be individual differences in isoflavone metabolism.
A systematic review and meta-analysis published in 2022 (Wietrzyk et al.) searched MEDLINE, EMBASE, and the Cochrane Library for articles published from 1995 to 2019. Eighteen randomized controlled trials were selected for meta-analysis. Different types of soy phytoestrogens, including genistein extracts, soy isoflavone extracts, soy protein isolate, and foods containing diverse amounts of isoflavones, were used. The analysis showed that daily intake of 106 mg (range, 40–300 mg) of isoflavones for 6–24 months moderately but statistically significantly positively affected bone mineral density (BMD) compared with controls: lumbar spine WMD = 1.63% (95% CI: 0.51 to 2.75%), p = 0.004.
An earlier meta-analysis that included data from 1,240 menopausal women found that daily ingestion of soy isoflavone extract supplements for 6 to 12 months increased spine bone mineral density by 2.38%, but no significant effects on femoral neck, hip total, and trochanter bone mineral density were found.
Clinical trials support the association with dose: isoflavone supplementation of 61.8 mg for four weeks showed potentially beneficial effects on bone metabolism and serum lipids in perimenopausal women in a randomized controlled trial; another trial demonstrated that continuous dietary intake of 37.3 mg/day for ten weeks may inhibit postmenopausal osteoporosis.
Evidence strength: There is scientific evidence showing a beneficial effect of isoflavones on bone health and thus in the prevention and treatment of osteoporosis in postmenopausal women, although the results do not seem entirely conclusive as there are discrepancies among the studies, probably related to their experimental designs. The results should be interpreted with caution, and more randomized clinical trials are required.
5c. Cardiovascular Health
Isoflavones reduce the risk of coronary heart disease by reducing the level of low-density lipoprotein and triglycerides, according to reported experimental findings. Epidemiological data from Japan have historically associated high soy consumption with lower cardiovascular disease rates.
However, clinical intervention data are less conclusive. It seems that soy isoflavones do not lead to a meaningful protective effect on cardiovascular risk, based on results gathered in systematic reviews. Several meta-analyses have reported beneficial but relatively weak effects of isoflavone consumption on improvement of cholesterol levels. In postmenopausal women, current meta-analytic evidence indicates that soy isoflavones improve oxidative stress, glycemia, and lipid abnormalities.
Evidence strength: Weak to moderate. Improvements in lipid profiles (particularly LDL cholesterol) have been noted in some RCTs, but the magnitude of effect is generally modest, and meta-analyses do not consistently support a clinically meaningful reduction in overall cardiovascular risk from isoflavone supplementation alone.
5d. Breast Cancer Risk and Outcomes
The relationship between isoflavones and breast cancer has been one of the most debated areas due to isoflavones' structural similarity to estrogen, which theoretically could be either protective or harmful. A recent systematic review and meta-analysis of 24 observational studies found that isoflavone intake was associated with a statistically significant decreased risk of developing breast cancer (OR 0.71; 95% CI: 0.72–0.81). The data showed that isoflavone reduced breast cancer risk (combined RR/OR of 0.68, 95% CI: 0.52–0.89) in Asian populations rather than in Western populations.
However, it has been proposed that the chemopreventive effects of isoflavones may stem primarily from intake early in life, a hypothesis first proposed in 1995, for which there is considerable epidemiologic support.
Regarding women diagnosed with breast cancer, concern has existed that isoflavone consumption could worsen prognosis or interfere with endocrine therapy. Their classification as phytoestrogens led to concern that isoflavones and soy food consumption could worsen the prognosis of women with breast cancer and interfere with the efficacy of endocrine therapy. Research in athymic ovariectomized mice shows isoflavones stimulate the growth of existing estrogen-sensitive mammary tumors. However, extensive clinical research indicates that neither soy foods nor isolated isoflavones affect markers of breast cancer risk, including mammographic density.
After a comprehensive, multi-year evaluation of the literature, the European Food Safety Authority (EFSA) concluded that in postmenopausal women, isoflavones do not adversely affect the three organs investigated: the breast, thyroid, and uterus. The North American Menopause Society also concluded that isoflavones do not increase risk of breast or endometrial cancer.
Evidence strength: Observational evidence suggests a protective association in Asian women. Clinical trial data do not demonstrate harm to breast tissue from isoflavone supplementation. The potential chemopreventive benefit remains biologically plausible but is not confirmed by prospective intervention trials. Animal models show conflicting results (protective vs. tumor-promoting), and in vitro concentrations required to inhibit tumor cells greatly exceed those achievable through diet.
5e. Prostate Cancer
Substantial evidence derived from in vitro studies suggests that genistein inhibits the growth and metastatic dissemination of diverse types of both hormone-dependent and hormone-independent prostate and breast tumors. Although a growing body of evidence has indicated that isoflavones may delay prostate cancer (PCa) progression, only a few studies have emphasized the relationship between isoflavone concentration in vitro and the serum of the human body. Isoflavone concentrations of 10 μmol/L or even as high as 50–100 μmol/L used in in vitro studies have never been achieved in the human body with a common diet.
In one Phase II clinical trial, twenty patients with rising PSA after prior local therapy were enrolled in an open-labeled, nonrandomized trial. Patients were treated with soy milk containing 47 mg of isoflavonoid per 8 oz serving, three times per day for 12 months. As most studies are of short duration, clinical trials with longer duration are necessary to confirm the effects of phytoestrogens on PCa. Progression from prostatic intraepithelial neoplasia to high-grade neoplasia and early latent cancer may take 10 or more years, which underscores the need for long-term research.
Evidence strength: Preliminary. Epidemiological associations are suggestive, and in vitro and animal data are promising, but human clinical evidence remains insufficient to draw firm conclusions about isoflavone supplementation for prostate cancer prevention or treatment.
5f. Glycemic Control and Metabolic Health
In postmenopausal women, current meta-analytic evidence indicates that soy isoflavones improve glycemia and lipid abnormalities. Genistein has been found to act on peroxisome proliferator-activated receptors (PPARs), which are central regulators of glucose and lipid metabolism. The compounds have been found effective in the management of diabetes by acting on peroxisome proliferator-activated receptors. However, most trials in this area are of short duration and conducted in postmenopausal women, limiting generalizability.
Evidence strength: Preliminary to moderate. Meta-analytic signals exist for improved glycemia and lipid profiles in specific populations, but evidence from well-powered, long-term trials in mixed populations is lacking.
5g. Thyroid Function
Isoflavones may interact with the synthesis of thyroid hormone. This has been an area of ongoing investigation and public concern. EFSA concluded that soy isoflavones were without effect on thyroid function. A subsequent meta-analysis showed no significant changes in free T3 or free T4 levels with soy supplementation, while a borderline elevation in TSH levels was observed (WMD: 0.248 mIU/L, 95% CI: 0.001 to 0.494, p = 0.049) with high heterogeneity (I² = 80.31%).
In 2015, EFSA concluded that "the administration of food supplements containing isoflavones is not associated with clinically relevant changes in thyroid function (hypo- or hyperthyroidism)" in peri- and postmenopausal women. Three years later, after extensively reviewing the literature, the German Senate Commission on Food Safety (SKLM) also concluded that isoflavone exposure does not adversely affect thyroid function in healthy women.
Some studies suggest that isoflavones may inhibit the function of the thyroid gland, though this inhibition may only be significant in individuals who are deficient in iodine. Therefore, people who consume large amounts of soy or isoflavones should ensure that their intake of iodine is adequate.
6. Body Systems Associated with Isoflavones
- Endocrine system: Estrogenic and anti-estrogenic effects via ER-α and ER-β; potential interaction with thyroid hormone synthesis; SERM-like activity.
- Skeletal system: Inhibition of osteoclast activity via tyrosine kinase inhibition; positive effects on bone mineral density, particularly at lumbar spine, in postmenopausal women.
- Cardiovascular system: Modest effects on LDL cholesterol, triglycerides, and oxidative stress markers; vasodilatory potential via endothelial effects.
- Reproductive system (female): Weak estrogenic activity at the uterus and breast; investigation in menopausal symptom management and cancer prevention.
- Reproductive system (male): Investigated in prostate cancer prevention and PSA modulation; no significant adverse effects on semen quality or reproductive hormones at dietary levels.
- Metabolic/endocrine: PPAR agonism relevant to glucose and lipid metabolism; antioxidant protection against lipid peroxidation.
- Immune system: Thymus cells, lymphocytes, and macrophages contain estrogen receptors, and binding of estrogen appears to suppress immune cell excessive activation and delay hypersensitivity.
- Integumentary system: Isoflavones show anti-skin oxidation properties by scavenging free radicals generated by lipid peroxidation and decreasing oxidative stress caused by UV exposure. Isoflavones, as phytoestrogens with the ability to increase hyaluronic acid synthesis, have a moisturizing effect and might be utilized in topical administration.
7. Dosage Forms and Doses Reported in Studies
Isoflavones are available in a variety of forms. Capsules and tablets are the most common pharmaceutical dosage forms. Other preparations include soy protein isolate powders, soy germ extracts, red clover extracts (standardized to total isoflavones), fermented soy-based preparations (e.g., tempeh-derived extracts), and functional foods. Food supplements targeted at peri- and postmenopausal women typically provide a daily dose of isoflavones in the range of 35–150 mg/day.
The following specific doses have been reported in published clinical studies:
- Daily intake of 106 mg (range, 40–300 mg) of isoflavones for 6–24 months was used in the trials included in a bone mineral density meta-analysis of 18 RCTs.
- Isoflavone supplementation of 61.8 mg for four weeks was studied for effects on bone metabolism and serum lipids; and 37.3 mg/day for ten weeks was used in a trial examining postmenopausal osteoporosis prevention.
- Red clover at 80 mg of isoflavones daily was used in one trial that significantly reduced mood symptoms.
- Soy milk containing 47 mg of isoflavonoids per 8 oz serving, given three times per day for 12 months, was the treatment regimen in a Phase II prostate cancer trial.
- A study administered a dietary supplement containing high-dose purified soy isoflavones (genistein 558 mg/day; daidzein 296 mg/day; and glycitein 44 mg/day) to 30 postmenopausal women for 84 days.
- In pharmacokinetic safety studies, thirty healthy men ingested single doses of purified isoflavone preparations; the delivered doses of genistein ranged from 1, 2, 4, 8, or 16 mg/kg body weight, which were higher than those previously administered to humans.
Pharmacokinetics
One consistent finding has been that peak blood concentrations of genistein and daidzein occur approximately 4–8 hours after ingestion of a single dose. It has also been consistently reported that genistein appears in higher concentrations than daidzein in serum or plasma, but urinary recovery of daidzein is consistently higher than genistein.
The mean elimination half-lives in single-dose studies were 3.2 hours for free genistein and 4.2 hours for free daidzein. The mean pseudo half-lives (reflecting enterohepatic recirculation and conjugate hydrolysis) were 9.2 hours for total genistein and 8.2 hours for total daidzein. Dietary supplements of purified unconjugated isoflavones administered to humans in single doses exceeding normal dietary intake manyfold resulted in minimal clinical toxicity. Genistein and daidzein (free and total) were rapidly cleared from plasma and excreted in urine.
8. Safety Considerations and Interactions
General Safety
With regard to isoflavone consumption safety, it seems that they are safe and that the most common adverse effect is mild and occurs at the gastrointestinal level. In terms of safety, no long-term human studies are available, and short-term evidence indicates they have an acceptable safety profile comparable to that of a placebo.
In single-dose pharmacokinetic safety studies at very high doses, no clinically significant behavioral or physical changes after treatment were observed. Elevations in lipoprotein lipase and hypophosphatemia were possibly related to the treatment but were associated with no clinical toxicity.
Breast and Uterine Safety in Postmenopausal Women
The human data did not support the hypothesis of an increased risk of breast cancer from observational studies, nor of an effect on mammographic density or on proliferation marker Ki-67 expression in interventional studies. No effect was found on endometrial thickness and histopathological changes in the uterus up to 30 months of supplementation with 150 mg/day of soy isoflavones. After 60 months, some non-malignant histopathological changes were reported.
Clinical data show that soy isoflavones, regardless of the source, and even when exposure greatly exceeds Japanese intake, do not exert harmful effects on breast tissue. These findings are consistent with the conclusion of EFSA, although their review focused on healthy postmenopausal women rather than breast cancer patients.
Thyroid Hormone Interaction
Thyroid hormone levels were not changed following intake of isoflavones from food supplements, according to the EFSA review. However, isoflavones have been documented to potentially reduce the absorption of synthetic thyroid medications (levothyroxine) if consumed concurrently. Some studies suggest that isoflavones may inhibit the function of the thyroid gland, though this inhibition may only be significant in individuals who are deficient in iodine. People who consume large amounts of soy or isoflavones should ensure that their intake of iodine is adequate.
Effects on Male Reproductive Hormones
Studies on men taking isoflavone supplements showed no effect on plasma hormones or semen quality. Neither soy nor isoflavone supplements have clinically relevant effects on reproductive hormones in women.
Endocrine Activity vs. Endocrine Disruption
While there is general recognition that isoflavones are endocrine-active substances, as discussed by the European Food Safety Authority (EFSA), endocrine-active substances are not necessarily endocrine-disrupting chemicals. It is clear that isoflavones should not be equated with the hormone estrogen. The literature is replete with clinical examples of differences between these two molecules.
Variation in Response: Equol Producer Status
Investigation of the proposed benefits of isoflavones has produced inconsistent data. The small sample size and short duration common to many intervention trials, combined with marked interindividual differences in isoflavone metabolism, likely contribute to the conflicting findings. Also, many different intervention products have been employed, which vary not only in the total amount but also in the relative proportion of the three soybean isoflavones and the form in which they are delivered (glycoside compared with aglycone).
Dietary Levels and Background Exposure
The background exposure from the diet in the general European population was estimated to be lower than 1 mg/day, whereas in consumers of soy-based foods it could be higher. In contrast, total mean intake of isoflavones among Asian populations ranges from 25 to 50 mg daily.
References
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