Phytoestrogens in clinical practice: a review of the literature

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1 MODERN TRENDS Edward E. Wallach, M.D. Associate Editor Phytoestrogens in clinical practice: a review of the literature Clemens B. Tempfer, M.D., a Eva-Katrin Bentz, M.D., a Sepp Leodolter, M.D., a Georg Tscherne, M.D., b Ferdinand Reuss, M.D., b Heide S. Cross, Ph.D., c and Johannes C. Huber, M.D., Ph.D. a a Department of Obstetrics and Gynecology, Medical University of Vienna, Vienna; b Department of Obstetrics and Gynecology, Medical University of Graz, Graz; and c Department of Pathophysiology, Medical University of Vienna, Vienna, Austria Objective: To review clinical studies assessing the effect of phytoestrogen supplementation on the signs and symptoms of the climacteric syndrome and on the incidence of breast cancer, cardiovascular disease, and skeletal fractures. Design: Literature research using PubMed and the Cochrane controlled trials register. Setting: None. Patient(s): None. Intervention(s): None. Main Outcome Measure(s): None. Result(s): Six systematic reviews and meta-analyses of 25 randomized, controlled trials (RCTs) assessing the use of phytoestrogens for the treatment of the climacteric syndrome were identified. Systematic reviews of RCTs show contradictory results, and meta-analyses demonstrate no statistically significant reduction of vasomotor symptoms for phytoestrogens. Individual RCTs report significant reductions in vasomotor symptoms for red clover and soy phytoestrogens. In selected patient populations, such as in women with early natural postmenopause and mild to moderate vasomotor symptoms, a systematic review of five RCTs found a significant reduction of hot flashes in five out of five RCTs. Twenty-two case-control and cohort studies examined the incidence of breast cancer among women with and without a diet high in phytoestrogens. A meta-analysis of 21 studies found a significantly reduced incidence of breast cancer among past phytoestrogen users. RCTs document beneficial effects of phytoestrogens on surrogate parameters such as bone mineral density, vasodilation, platelet aggregation, insulin resistance, and serum concentrations of triglycerides, high-density lipoprotein, and low-density lipoprotein. None of the available RCTs documents a protective effect of phytoestrogens for the clinical end points of breast cancer, bone fracture, or cardiovascular events. Conclusion(s): Based on the available evidence, phytoestrogens should only be used in selected women, i.e., those presenting with mild to moderate vasomotor symptoms in early natural postmenopause. None of the compounds investigated so far have been proven to protect against breast cancer, bone fracture, or cardiovascular disease. (Fertil Steril Ò 2007;87: Ó2007 by American Society for Reproductive Medicine.) Key Words: Phytoestrogens, climacteric syndrome, hormone replacement, soy, red clover, alternative, review Phytoestrogens have been widely used in various cultures and time periods. Empirical data and epidemiologic research indicate that the incidence of hormone-dependent diseases is reduced in countries with a high dietary content of phytoestrogens (1, 2). Thus, modern clinical and molecular biological research has increasingly focused on plant-derived phytoestrogens such as isoflavones, lignans, dihydrochalcones, and coumestans. The most widely studied phytoestrogenic compounds are isoflavones found in red clover and Received January 4, 2006; revised and accepted January 11, Reprint requests: Clemens B. Tempfer, M.D., Department of Obstetrics and Gynecology, Medical University of Vienna, Waehringer Guertel 18-20, A-1090, Vienna, Austria (FAX: þ ; clemens.tempfer@meduniwien.ac.at). soy, such as genistein, formononetin, biochanin A, and daidzein (3). The widely publicized and discussed results of the Women s Health Initiative (WHI) and the One Million Women Study have considerably increased the interest in the use of phytoestrogens. Because phytoestrogens interact with the estrogen receptor, inducing agonistic as well as antagonistic effects, the primary preventive potential as well as the oncologic safety of a prolonged use of phytoestrogens are of considerable interest. We summarize the clinical and experimental data on the use of phytoestrogens for the alleviation of signs and symptoms of the climacteric syndrome. Numerous randomized, controlled trials (RCTs) have investigated /07/$32.00 Fertility and Sterility â Vol. 87, No. 6, June doi: /j.fertnstert Copyright ª2007 American Society for Reproductive Medicine, Published by Elsevier Inc.

2 the therapeutic efficacy of phytoestrogens, allowing for an evidence-based recommendation for clinical practice. In addition, we summarize clinical evidence investigating the effects of phytoestrogens on the skeletal system and the cardiovascular system, the incidence of breast cancer, and skin health. PHYTOESTROGENS AND THE CLIMACTERIC SYNDROME A number of controlled and uncontrolled clinical studies have indicated that a dietary increase in phytoestrogens in addition to a regular diet (4) and phytoestrogen supplementation (5 9) are associated with an increase in quality of life and a significant amelioration of the signs and symptoms of the climacteric syndrome, as summarized by the Kuppermann index (10). On the other hand, several studies could not demonstrate a clinical effect, especially with regard to hot flashes (11). Several reasons may account for these and other discrepancies between studies, among them differences in patient selection, phytoestrogen content, and symptom assessment, and variations in endogenous estrogen production, intestinal resorption, and activation of orally ingested isoflavones. Study Selection We searched PubMed and the Cochrane controlled trials register (January 06, 2006, for search terms: climacteric syndrome, hot flashes, phytoestrogens, isoflavones, treatment) to identify RCTs, systematic reviews, and meta-analyses of RCTs assessing the efficacy of phytoestrogens in women with the climacteric syndrome. Studies were included if they were published as complete reports in English. Studies were eligible if they were RCTs comparing placebo or no intervention with phytoestrogens in perimenopausal and/or postmenopausal women with hot flashes. Phytoestrogens were defined as substances with a defined amount of isoflavones, lignans, or coumestans. Studies investigating black cohosh were not included in this review because the mechanism of action of this botanical is unknown. Studies assessing menopausal symptoms other than hot flashes were excluded. Two reviewers assessed eligibility of the studies and extracted data. Missing information and additional trials were not sought from the investigators. To identify subgroups of women selectively benefiting from phytoestrogen treatment, we used criteria proposed by the U.S. Food and Drug Administration (FDA) for the conduct of studies evaluating hormone replacement therapies (12) and by Messina and Hughes (13), who proposed to exclude women with iatrogenic menopause after treatment for oncologic diseases in the assessment of the efficacy of phytoestrogens. Following criteria used in previous meta-analyses (11),we recorded the adequacy of treatment allocation concealment according to criteria published by Schulz et al. (14). Trials were considered to have adequate concealment if they described satisfactory procedures to conceal treatment allocation such as coded identical containers or centralized randomization. We also recorded blinding participants and outcome assessors to treatment, use of intention-to-treat analysis, and the number of participants who withdrew or were lost to follow-up evaluation. When possible, effect sizes were calculated to provide a measure of the magnitude of a treatment effect for each study. The effect sizes represent the difference in mean hot flash frequency outcomes divided by the standard deviation. Negative effect size values favor placebo, and positive effect sizes favor phytoestrogen. In the scale suggested by Cohen (15), cutoff values of 0.2, 0.5, and 0.8 reflect small, moderate, and large treatment effects, respectively. If possible, outcome data were pooled and analyzed using the Cochrane Collaboration Review Manager (RevMan 4.2; Update Software, Oxford, United Kingdom). Weighted mean differences, the difference between treatment and control pooled means at end point, along with 95% confidence intervals (CI), were calculated for continuous variables. Efficacy of Phytoestrogens We identified 25 RCTs (5 9, 16 35) and six systematic reviews and meta-analyses of RCTs (3, 11, 36 39). In a systematic review and meta-analysis of 25 RCTs, red clover products were not shown to be effective for the treatment of hot flashes (weighted mean difference 0.60; 95% CI, ). Of the five soy extract trials reporting hot flush frequency, three trials, including the two largest trials, were negative (11). In another systematic review of 29 RCTs of alternative therapies for hot flashes and other menopausal symptoms, the investigators identified 12 RCTs examining soy or soy extracts (3). Only three of eight studies with treatment phases >6 months showed a significant improvement in hot flashes. The effects were modest, and most benefits disappeared after 6 weeks. This review cites two RCTs documenting no statistically significant benefit for red clover products. Similarly, Huntley and Ernst (36) identified 10 RCTs assessing soy preparations for the treatment of perimenopausal symptoms and found four of 10 RCTs to be positive, suggesting soy preparations are beneficial for perimenopausal symptoms. Six RCTs were negative, with one of the six showing a positive trend. Another review of botanical compounds for the treatment of menopause-related symptoms identified RCTs investigating soy isoflavone extracts with varying compositions and dosages of soy supplements. Results from five RCTs assessing the efficacy of semipurified isoflavone red clover (Trifolium pratense L.) leaf extracts were contradictory. The largest study showed no benefit for reducing symptoms associated with menopause for two different red clover isoflavone products compared with placebo. No significant adverse events were reported in the investigated studies (37). A meta-analysis by Nelson et al. (38) investigated alternative treatments for menopausal symptoms. The following compounds were found to significantly reduce the number of daily hot flashes compared with placebo: seven RCTs of selective serotonin reuptake inhibitors (SSRIs) or serotonin 1244 Tempfer et al. Literature review Vol. 87, No. 6, June 2007

3 norepinephrine reuptake inhibitors (SNRIs) (mean difference, 1.13; 95% CI, 1.70 to 0.57), four RCTs of clonidine ( 0.95; 95% CI, 1.44 to 0.47), and two RCTs of gabapentin ( 2.05; 95% CI, 2.80 to 1.30). In contrast, the frequency was not reduced in a meta-analysis of 17 RCTs of red clover isoflavone extracts and soy isoflavone extracts (39). Kang et al. (39) identified seven RCTs, of which five RCTs reported no beneficial effect on hot flashes of soy protein. Two RCTs reported a significant reduction of hot flashes in the group treated with soy product over placebo. Hot flash frequency was reduced by 45% and 28%, respectively, in these two trials, but the placebo effect was also high, with reductions of 30% and 18%. These data indicate that phytoestrogens have modest if any effects, and that these effects may be limited to selected patient populations. Thus, in an effort to better define possible patient subgroups who may selectively benefit from phytoestrogens, we tried to identify studies examining only early postmenopausal women with natural menopause and mild to moderate vasomotor symptoms using previously published recommendations (12, 13). We identified 25 RCTs examining phytoestrogens for the treatment of hot flashes (5 9, 16 35). We excluded 20 studies from further evaluation (16 35), leaving five RCTs with 411 early postmenopausal women with natural menopause (5 9). Tables 1 and 2 show the study and participant characteristics of this highly selected set of studies. All of the trials included postmenopausal women and reported double-blinding. Two of the five trials (5, 7) provided enough information to confirm adequate concealment of treatment allocation. Three of the five trials (5 7) reported intention-to-treat analysis. The minimal dose of isoflavones used in these studies was 40 mg per day, the maximal dose was 100 mg per day. The phytoestrogens tested were soy in four trials (5 7, 9) and red clover extract in one trial (8). The number of women included in these studies varied between 30 and 122. Table 3 shows the outcomes of trials and the symptom score results. All of the five individual studies demonstrated a statistically significant reduction of vasomotor symptoms. Four of five trials reported continuous variables such as mean percentage change of hot flashes per day or Kupperman index (5, 7 9). Only one study (6) reported absolute numbers. Therefore, a pooled analysis for the identification of mean weighted differences was not performed. One study reported sufficient data for the calculation of effect sizes (6). This study favored placebo with a moderate effect size (0.53). In summary, this meta-analysis of a predefined, selected patient population indicates that the efficacy of isoflavones as a treatment for the signs and symptoms of the climacteric syndrome is reduced in women with idiopathic premature ovarian failure, premature ovarian failure after cancer treatment, after long-term hormone therapy, and when isoflavone treatment is initiated >3 years after menopause (12, 13). Consequently, phytoestrogens may be used most effectively in early postmenopausal women with a natural menopause and mild to moderate climacteric syndrome. Meta-analyses of intervention studies with isoflavones without specific selection criteria may thus underestimate the clinical efficacy of this therapy. This may serve as a possible explanation for contradictory results of RCTs in unselected patient populations (3, 11, 36 39). From a clinical perspective, phytoestrogens may be used most appropriately as first-line treatment for women presenting with mild to moderate vasomotor symptoms in early natural postmenopause. This is also in accordance with the recommendations of the North American Menopause Society (40). PHYTOESTROGENS AND BREAST CANCER In Vitro Data In vitro experiments using benign breast cell lines, skeletal muscle cell cells, and rodent models have found that phytoestrogens exert a proliferation-enhancing effect (41, 42). Although these studies principally document that phytoestrogens can lead to cell proliferation, other in vitro and in vivo studies have demonstrated a concentration-dependent biphasic antiproliferative effect of isoflavones (43). Phytoestrogens can, under certain conditions, function as antioxidants and protect against oxidant-induced DNA damage (44). In vivo studies in rodents also indicate that low-dose dietary isoflavones abrogate tamoxifen-associated mammary tumor prevention (45). Thus, the experimental data available so far are not conclusive. Epidemiologic data and prospective case-control studies, on the other hand, clearly demonstrate TABLE 1 Study characteristics and isoflavone dosages. Study (reference) Cases (n) Controls Isoflavone (n) Compound P value a dosage Albertazzi et al., 1998 (5) Isoflavone protein < mg/day Faure et al., 2002 (6) Isoflavone capsules < mg/day Han et al., 2002 (7) Isoflavone capsules < mg/day Jeri, 2002 (8) Red Clover extract < mg/day Upmalis et al., 2000 (9) Isoflavone tablets mg/day a Highest P value for improvement of symptoms. Tempfer. Literature review. Fertil Steril Fertility and Sterility â 1245

4 TABLE 2 Characteristics of studies and participants. Study (reference) Participants (dropouts) Inclusion criteria Participant characteristics Study duration Albertazzi et al., 1998 (5) 104 (25) Amenorrhea R6 months or oophorectomy, or FSH >50 IU/L and E 2 <35 pg/ml; R7 HF/day Faure et al., 2002 (6) 75 (20) Amenorrhea R6 months or FSH >40 IU/L and E 2 <35 pg/ml; R7 HF/day Han et al., 2002 (7) 82 (2) Age years and amenorrhea R12 months or FSH >25 IU/L and E 2 <35 pg/ml; HF present Jeri, 2002 (8) 30 (0) Age <60 years and amenorrhea >1 year or FSH >30 IU/L; R5 HF/day Upmalis et al., 2000 (9) 177 (55) Age >50 years and amenorrhea R6 months or FSH >40 IU/L and E 2 <25 pg/ml; R5 HF/day Italy; MA 53 years; MDM 3.9 years France; MA 53 years; Brazil; MA 53 years; MDM 2.4yrs Peru; MA 52 years; MDM - U.S.; MA 55 years; MDM 12 weeks 16 weeks 4 months 14 weeks 12 weeks Note: E 2, estradiol; FSH, follicle-stimulating hormone; HF, hot flashes; MA, mean age; MDM, median duration after menopause. Tempfer. Literature review. Fertil Steril that countries with a high dietary content of isoflavones have lower incidence rates of breast and prostate cancer compared with countries with a low dietary content of phytoestrogens, which argues against a principally cell-proliferative effect of phytoestrogens. Clinical evidence from uncontrolled trials is consistent with a protective effect of phytoestrogens against the development of breast cancer. For example, a meta-analysis of 14 case-control and 7 cohort studies found a pooled risk reduction of 25% (Relative Risk [RR] 0.75; 95% Confidence Interval [CI] 0.59 to 0.95) for past soyfood, tofu, miso, or isoflavone intake (46). A recent nested case-control study within a Dutch prospective cohort found high plasma levels of genistein to be associated with a 32% reduced odds of developing breast cancer (47). In contrast to this evidence, three prospective trials evaluated this issue (1, 48, 49). All of these prospective trials showed no statistically significant reduction in breast cancer incidence. There has been no prospective, randomized study evaluating phytoestrogens as a primary preventive therapy of breast cancer. It is of note that a protective effect of isoflavones could not be reproduced in European or U.S. studies such as the San Francisco Bay study (50). Some researchers have claimed that the effect of a phytoestrogen diet depends at least in part on the individual age of the woman when starting the diet. Asian women are exposed to phytoestrogens starting with early childhood, but the typical Western initiation of a phytoestrogen-rich diet or phytoestrogen supplementation in perimenopause or postmenopause may be too late to exert a protective effect on the breast. Phytoestrogens may induce differentiation of breast epithelium during early childhood and puberty, thus making the breast epithelium less sensitive to noxious agents such as chemical carcinogens. Breast epithelia may no longer be sensitive to phytoestrogens after pregnancy. This model of a lifetime-dependent effect of protective agents has been demonstrated by Russo and Russo (51). Putting these data into a clinical perspective, women planning to use phytoestrogens can be advised that phytoestrogens can be used safely on a long-term basis. There are no data indicating that prolonged use of a phytoestrogen-rich diet induces malignant growth of hormone-dependent tissue. On the other hand, none of the compounds investigated so far has been proven to protect against breast cancer. There are no data with respect to other malignancies, but there is no indication that phytoestrogens increase the risk of malignant disease. PHYTOESTROGENS AND THE SKELETAL SYSTEM Isoflavones have been demonstrated to increase the synthesis of vitamin D in a number of nonrenal cell types (52). Due to this mechanism, isoflavones are thought to contribute to bone mineral density. A prospective, randomized 1246 Tempfer et al. Literature review Vol. 87, No. 6, June 2007

5 TABLE 3 Clinical outcomes of trials and symptom score results. Study (reference) and intervention HF/day at baseline (mean SD or median range) HF/day at study end (mean SD) Decrease HF/day (P value) Symptom score Albertazzi et al., 1998 (5) Soy 11.4 ( ) 6.4 ( ) 5.0 None reported Placebo 10.9 ( ) 7.5 ( ) 3.4 P<.01 Faure et al., 2002 (6) Soy 10.1 (6.4) 3.9 (0.7) 6.4 None reported Placebo 9.4 (3.4) 7.0 (1.2) 2.2 P¼.01 Han et al., 2002 (7) Soy Kupperman index Placebo 19.7 (soy) vs þ1.3 (placebo) P<.01 Jeri, 2002 (8) Red clover 7.0 (0.5) 3.6 (0.3) 3.4 None reported Placebo 5.7 (0.4) 5.1 (0.3) 0.6 P¼.001 Upmalis et al., 2000 (9) Soy 8.8 (6.2) Mean % HF/d None reported Placebo 9.4 (6.0) 20 (placebo) vs 28 (soy) P<.05 Note: HF, hot flashes; SD, standard deviation. Tempfer. Literature review. Fertil Steril study of 66 women found that 88 mg of isoflavones per day have a protective effect on the skeletal system, increasing the bone mineral density in the lumbar spine (53). On the other hand, a large study of 202 postmenopausal women found no benefit after 12 months of daily 99-mg isoflavone supplementation (52 mg of genistein, 41 mg of daidzein, and 6 mg of glycitein) (54). This double-blind, randomized trial does not support the hypothesis that the use of a soy protein supplement containing isoflavones improves cognitive function, bone mineral density, or plasma lipids in healthy postmenopausal women when started at the age of 60 years or later. Based on these data, isoflavones cannot be clinically recommended as a valuable intervention to increase bone health. No data are available to assess the efficacy of isoflavones as a means to treat women with osteoporosis and should thus not be recommended as the sole treatment in this indication. No RCTs are available, documenting the efficacy of phytoestrogens regarding the prevention of osteoporosis or bone fracture. PHYTOESTROGENS AND THE CARDIOVASCULAR SYSTEM Endothelium-derived nitric oxide (NO) is a potent vasodilator and mediates the effects of antihypertensive drugs such as nitroglycerin. Isoflavones have been shown to stimulate the activity of the endothelial NO synthase (NOS3), thus inducing vasodilation via NO (55). Isoflavones also have antithrombotic and antiatherogenic effects. For example, genistein and daidzein decrease monocyte chemoattractant protein-1 and collagen-induced platelet aggregation in a dose-dependent manner (56). In addition, 54 mg/day of genistein are sufficient to significantly decrease fasting glucose, fasting insulin, and insulin resistance (57). Clinical studies have assessed the effect of isoflavones on surrogate end points of cardiovascular health. A meta-analysis of 38 controlled human studies of soy consumption provides evidence for its positive effect on lipid profiles, including reduction in levels of low-density lipoproteins (LDL) and triglycerides and increases in high-density lipoprotein (HDL) (58). In a population-based study of 301 postmenopausal women, however, intake of between 0.2 mg and 11.4 mg of isoflavones did not alter systolic or diastolic blood pressure, or the prevalence of hypertension (59). In accordance, a 12-month, double-blind, randomized trial compared the effects of soy protein containing 99 mg of isoflavones/day (aglycone weights) with those of milk protein (placebo) and found no effect on blood pressure and Fertility and Sterility â 1247

6 endothelial function in 202 postmenopausal women aged 60 to 75 years (60). No RCTs regarding clinical end points of cardiovascular health (i.e., myocardial infarction, angina pectoris) are available to date. Therefore, phytoestrogens cannot be recommended as a primary preventive intervention to reduce the risk of cardiovascular disease. Women using phytoestrogens may be advised, however, that phytoestrogens used for the treatment of the climacteric syndrome have additional benefits on the cardiovascular system by reducing established risk factors of atherosclerosis such as hyperlipidemia and hypertension. Whether this translates into a clinical benefit regarding the reduction of clinical end points such as myocardial infraction is unknown. PHYTOESTROGENS AND SKIN HEALTH Isoflavones may have a protective effect on skin health. It has been hypothesized that skin elasticity is improved by increased local blood flow. In addition, the carcinogenic effect of exogenous noxious agents such as ionizing radiation and chemical compounds may be alleviated by the antioxidant and anti-inflammatory properties of phytoestrogens (61). Also, red clover protects from inflammation and immune suppression induced by ultraviolet radiation (62). No data with respect to clinical end points of skin health have been published. STANDARDIZATION OF PHYTOESTROGENS Because phytoestrogens are listed as food supplements, quality standards regarding manufacturing and isoflavone content are important. Phytoestrogen compounds used to treat women with climacteric syndrome should contain 40 to 100 mg of isoflavones consisting of variable combinations of different aglycans (i.e., genistein, daidzein, glycitein, formononetin, and biochanin A). A daily dose of mg of isoflavones is toxicologically safe and is consistent with the dietary content of isoflavones in Asian countries (63). 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