Use of isoflavone supplements is associated with reduced postmenopausal breast cancer risk

Phytoestrogens and Health Effects

Phytoestrogens are literally estrogenic substances of plant origin. Although these substances are useful for plants in many aspects, their estrogenic properties are essentially relevant to their predators. As such, phytoestrogens can be considered to be substances potentially dedicated to plant-predator interaction. Therefore, it is not surprising to note that the word phytoestrogen comes from the early discovery of estrogenic effects in grazing animals and humans. Here, several compounds whose activities have been discovered at nutritional concentrations in animals and humans are examined. The substances analyzed belong to several chemical families, i.e., the flavanones, the coumestans, the resorcylic acid lactones, the isoflavones, and the enterolignans. Following their definition and the evocation of their role in plants, their metabolic transformations and bioavailabilities are discussed. A point is then made regarding their health effects, which can either be beneficial or adverse depending on the subject studied, the sex, the age, and the physiological status. Toxicological information is given based on official data. The effects are first presented in humans. Animal models are evoked when no data are available in humans. The effects are presented with a constant reference to doses and plausible exposure.

Soybean Processing Wastes: Novel Insights on Their Production, Extraction of Isoflavones, and Their Therapeutic Properties

Soybean processing waste (SPW) has potential as a sustainable source of phytochemicals and functional foods. A variety of phytochemicals, nutrients, and minerals have been characterized from SPW using various analytical methods. SPW utilization strategies may provide a new way to increase production of bioactive compounds, nutritional supplements, and cosmetic ingredients. SPW has the potential for value-added processing, to improve commercial use, and to lower environmental pollution through proper use. Okara, a byproduct generated during soybean processing of tofu and soy milk, is rich in dietary fiber, isoflavones, and saponins. Isoflavones, an important class of biologically active compounds owing to their multifunctional and therapeutic effects, are extracted from SPW. Further, studies have shown that okara has potential prebiotic and therapeutic value in lowering the risk of noncommunicable diseases. Therefore, in this review, we focus on several extraction methods and pharmacotherapeutic effects of different SPWs. Their effective uses in functional foods, nutraceuticals, and health applications, as biocatalysts, and as value-added resources have been discussed.

Isoflavones derived from plant raw materials: bioavailability, anti-cancer, anti-aging potentials, and microbiome modulation

Isoflavones are secondary metabolites that represent the most abundant category of plant polyphenols. Dietary soy, kudzu, and red clover contain primarily genistein, daidzein, glycitein, puerarin, formononetin, and biochanin A. The structural similarity of these compounds to β-estradiol has demonstrated protection against age-related and hormone-dependent diseases in both genders. Demonstrative shreds of evidence confirmed the fundamental health benefits of the consumption of these isoflavones. These relevant activities are complex and largely driven by the source, active ingredients, dose, and administration period of the bioactive compounds. However, the preclinical and clinical studies of these compounds are greatly variable, controversial, and still with no consensus due to the non-standardized research protocols. In addition, absorption, distribution, metabolism, and excretion studies, and the safety profile of isoflavones have been far limited. This highlights a major gap in understanding the potentially critical role of these isoflavones as prospective replacement therapy. Our general review exclusively focuses attention on the crucial role of isoflavones derived from these plant materials and critically highlights their bioavailability, possible anticancer, antiaging potentials, and microbiome modulation. Despite their fundamental health benefits, plant isoflavones reveal prospective therapeutic effects that worth further standardized analysis.

Isolation and identification of a human intestinal bacterium capable of daidzein conversion

Equol, which produced from daidzein (one of the principal isoflavones), is recognized to be the most resultful in stimulating an estrogenic and antioxidant response. The daidzein transformation was studied during fermentation of five growth media inoculated with feces from a healthy human, and a daidzein conversion strain was isolated. To enrich the bacterial population involved in daidzein metabolism in a complex mixture, fecal samples were treated with antibiotics. The improved propidium monoazide combined with the quantitative polymerase chain reaction (PMAxx-qPCR) assay showed that the ampicillin treatment of samples did result in a reduction of the total visible bacteria counts by 52.2% compared to the treatment without antibiotics. On this basis, the newly isolated rod-shaped, Gram-positive anaerobic bacterium, named strain Y11 (MN560033), was able to metabolize daidzein to equol under anaerobic conditions, with a conversion ratio (equol ratio: the amount of equol produced/amount of supplemented daizein) of 0.56 over 120 h. The 16S rRNA partial sequence of the strain Y11 exhibited 99.8% identity to that of Slackia equolifaciens strain DZE (NR116295). This study will provide new insights into the biotransformation of equol from daidzein by intestinal microbiota from the strain-level and explore the possibility of probiotic interventions.

Soy Isoflavones Inhibit Endothelial Cell Dysfunction and Prevent Cardiovascular Disease

Soybeans are among the most popular foods worldwide, and intake of soy-containing foods has been associated with many health benefits in part because of it structure similar to estrogen. Epidemiologic studies have demonstrated that soy consumption improves serum profiles of hypercholesterolemic patients. Several studies have also indicated an inverse relationship between the consumption of soy isoflavones and the incidence of cardiovascular diseases (CVD). Soy is a rich dietary source of isoflavones. The main soy isoflavones are daidzein and genistein; equol, another isoflavone and a major intestinal bacterial metabolite of daidzein, is generated by enterobacterial effects. Many isoflavones have antioxidative effects and anti-inflammatory actions, as well as induce nitric oxide production to maintain a healthy endothelium and prevent endothelial cell dysfunction. These effects may limit the development of atherosclerosis and CVD and restore healthy endothelial function in altered endothelia. Although the evidence supporting the benefits of soy isoflavones in CVD prevention continues to increase, the association between soy isoflavones and disease is not fully understood. This review summarized recent progress in identifying the preventive mechanisms of action of dietary soybean isoflavones on vascular endothelial cells. Furthermore, it describes the beneficial roles that these isoflavones may have on endothelial dysfunction-related atherosclerosis.

New Evaluation of Isoflavone Exposure in the French Population

The study relates the present evaluation of exposure to estrogenic isoflavones of French consumers through two approaches: (1) identification of the isoflavone sources in the French food offering, (2) a consumption-survey on premenopausal women. For the foodstuff approach 150 food-items were analysed for genistein and daidzein. Additionally, 12,707 labels of processed-foods from French supermarket websites and a restaurant-supplier website were screened, and 1616 foodstuffs of interest were retained. The sources of phytoestrogens considered were soy, pea, broad bean and lupine. A price analysis was performed. A total of 270 premenopausal women from the French metropolitan territory were interviewed for their global diet habits and soy consumption and perception. In supermarkets, there were significantly less selected foodstuffs containing soy than in restaurant (11.76% vs. 25.71%, p < 0.01). There was significantly more soy in low price-foodstuff in supermarket (p < 0.01). Isoflavone levels ranged from 81 to 123,871 µg per portion of the analyzed soy containing foodstuff. Among the women inquired 46.3% claimed to have soy regularly. Isoflavone intake >45 mg/day is associated to vegan-diet (p < 0.01). In total, 11.9% of soy-consumers had a calculated isoflavone intake >50 mg/day. This dose can lengthen the menstrual cycles. The actual exposure to phytoestrogen is likely to have an effect in a part of the French population.

Use of dietary supplements containing soy isoflavones and breast cancer risk among women aged >50 y: a prospective study

BACKGROUND

Soy-based dietary supplements have been promoted as natural alternatives to menopausal hormone therapy, but their potential effect on breast cancer development is controversial.

OBJECTIVES

We examined the relation between the consumption of soy supplements and the risk of breast cancer, overall and by tumor hormone receptor status, among women aged >50 y.

METHODS

In total, 76,442 women from the Etude Epidemiologique aupres de Femmes de la Mutuelle Generale de l'Education Nationale (E3N) cohort, born between 1925 and 1950, were followed from 2000 to 2011 (11.2 y on average, starting at a mean age of 59.5 y; 3608 incident breast cancers), with soy supplement use assessed every 2-3 y. HRs of breast cancer were estimated with the use of multivariable Cox models.

RESULTS

Compared with never using soy supplements, the HRs associated with current use of soy supplements were 0.92 (95% CI: 0.76, 1.11) for all, 0.78 (95% CI: 0.60, 0.99) for estrogen receptor (ER)-positive, and 2.01 (95% CI: 1.41, 2.86) for ER-negative breast cancers. There was no association between past use of soy supplements and breast cancer. HRs for current use were 1.36 (95% CI: 0.95, 1.93) and 0.82 (95% CI: 0.65, 1.02) among women with and without a family history of breast cancer, respectively (P-interaction = 0.03) and 1.06 (95% CI: 0.87, 1.30) ≥5 y after menopause compared with 0.50 (95% CI: 0.31, 0.81) in premenopause or ≤5 y postmenopause (P-interaction = 0.04).

CONCLUSIONS

In this cohort of women aged >50 y, we report opposing associations of soy supplements with ER-positive and ER-negative breast cancer risk. Our results also caution against the use of these supplements in women with a family history of breast cancer. Whether the risk profile of soy supplements could be more favorable among premenopausal or recently postmenopausal women deserves further investigation.

Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation

Isoflavones are secondary plant constituents of certain foods and feeds such as soy, linseeds, and red clover. Furthermore, isoflavone-containing preparations are marketed as food supplements and so-called dietary food for special medical purposes to alleviate health complaints of peri- and postmenopausal women. Based on the bioactivity of isoflavones, especially their hormonal properties, there is an ongoing discussion regarding their potential adverse effects on human health. This review evaluates and summarises the evidence from interventional and observational studies addressing potential unintended effects of isoflavones on the female breast in healthy women as well as in breast cancer patients and on the thyroid hormone system. In addition, evidence from animal and in vitro studies considered relevant in this context was taken into account along with their strengths and limitations. Key factors influencing the biological effects of isoflavones, e.g., bioavailability, plasma and tissue concentrations, metabolism, temporality (pre- vs. postmenopausal women), and duration of isoflavone exposure, were also addressed. Final conclusions on the safety of isoflavones are guided by the aim of precautionary consumer protection.

Postdiagnosis Isoflavone and Lignan Intake in Newly Diagnosed Breast Cancer Patients: Cross-Sectional Survey Shows Considerable Intake from Previously Unassessed High-Lignan Foods

BACKGROUND

Isoflavones and lignans (phytoestrogens) are dietary components with potential anticarcinogenic effects. Although the intake of isoflavones and lignans may affect breast cancer treatment and prognosis-and associations may differ by menopausal status-postdiagnosis intake data are limited.

OBJECTIVE

We aimed to describe postdiagnosis isoflavone and lignan intake in newly diagnosed breast cancer patients, examine differences by menopausal status and phytoestrogen type, and inform the assessment of diet and survival in future prognostic studies.

METHODS

Our cross-sectional study included 278 women aged 25-74 y, diagnosed with pathologically confirmed breast cancer in April-May 2010 and identified using the Ontario Cancer Registry. Intake in the previous 2 mo was assessed using questionnaires listing 17 soy and 3 high-lignan foods (flaxseed, flaxseed bread, sesame seeds), completed 71 d after breast cancer diagnosis, on average. Food consumption by menopausal status was examined. Geometric mean and median phytoestrogen intakes were estimated among all patients and in consumers only; differences by menopausal status and phytoestrogen type were assessed.

RESULTS

Among all patients, foods were similarly consumed by menopausal status and isoflavone intakes were low (median: 56 µg/d). Consumers (n = 219) had higher intakes (median isoflavones: 1808 µg/d); 7% of isoflavone and 21% of lignan consumers had intakes ≥10 mg/d. Intakes were higher in premenopausal than in postmenopausal consumers, particularly for lignans, but were not significantly different (median lignans: 4375 compared with 1863 µg/d; P = 0.07). Lignans were significantly higher than isoflavones among most consumers (postmenopausal means: 746 compared with 100 µg/d; P < 0.0001).

CONCLUSIONS

Postdiagnosis lignan intakes from 3 high-content foods may be considerable among newly diagnosed breast cancer patients, yet they have been unassessed in previous prognostic studies. The inclusion of these foods in dietary assessment methods may improve future intake estimates and the distributions on which breast cancer survival analyses are based.

Red Clover Aryl Hydrocarbon Receptor (AhR) and Estrogen Receptor (ER) Agonists Enhance Genotoxic Estrogen Metabolism

Many women consider botanical dietary supplements (BDSs) as safe alternatives to hormone therapy for menopausal symptoms. However, the effect of BDSs on breast cancer risk is largely unknown. In the estrogen chemical carcinogenesis pathway, P450 1B1 metabolizes estrogens to 4-hydroxylated catechols, which are oxidized to genotoxic quinones that initiate and promote breast cancer. In contrast, P450 1A1 catalyzed 2-hydroxylation represents a detoxification pathway. The current study evaluated the effects of red clover, a popular BDS used for women’s health, and its isoflavones, biochanin A (BA), formononetin (FN), genistein (GN), and daidzein (DZ), on estrogen metabolism. The methoxy estrogen metabolites (2-MeOE₁, 4-MeOE₁) were measured by LC-MS/MS, and CYP1A1 and CYP1B1 gene expression was analyzed by qPCR. Nonmalignant ER-negative breast epithelial cells (MCF-10A) and ER-positive breast cancer cells (MCF-7) were derived from normal breast epithelial tissue and ER+ breast cancer tissue. Red clover extract (RCE, 10 μg/mL) and isoflavones had no effect on estrogen metabolism in MCF-10A cells. However, in MCF-7 cells, RCE treatments downregulated CYP1A1 expression and enhanced genotoxic metabolism (4-MeOE₁/CYP1B1 > 2-MeOE₁/CYP1A1). Experiments with the isoflavones showed that the AhR agonists (BA, FN) preferentially induced CYP1B1 expression as well as 4-MeOE₁. In contrast, the ER agonists (GN, DZ) downregulated CYP1A1 expression likely through an epigenetic mechanism. Finally, the ER antagonist ICI 182,780 potentiated isoflavone-induced XRE-luciferase reporter activity and reversed GN and DZ induced downregulation of CYP1A1 expression. Overall, these studies show that red clover and its isoflavones have differential effects on estrogen metabolism in “normal” vs breast cancer cells. In breast cancer cells, the AhR agonists stimulate genotoxic metabolism, and the ER agonists downregulate the detoxification pathway. These data may suggest that especially breast cancer patients should avoid red clover and isoflavone based BDSs when making choices for menopausal symptom relief.

The potential health effects of dietary phytoestrogens

Phytoestrogens are plant-derived dietary compounds with structural similarity to 17-β-oestradiol (E2), the primary female sex hormone. This structural similarity to E2 enables phytoestrogens to cause (anti)oestrogenic effects by binding to the oestrogen receptors. The aim of the present review is to present a state-of-the-art overview of the potential health effects of dietary phytoestrogens. Various beneficial health effects have been ascribed to phytoestrogens, such as a lowered risk of menopausal symptoms like hot flushes and osteoporosis, lowered risks of cardiovascular disease, obesity, metabolic syndrome and type 2 diabetes, brain function disorders, breast cancer, prostate cancer, bowel cancer and other cancers. In contrast to these beneficial health claims, the (anti)oestrogenic properties of phytoestrogens have also raised concerns since they might act as endocrine disruptors, indicating a potential to cause adverse health effects. The literature overview presented in this paper illustrates that several potential health benefits of phytoestrogens have been reported but that, given the data on potential adverse health effects, the current evidence on these beneficial health effects is not so obvious that they clearly outweigh the possible health risks. Furthermore, the data currently available are not sufficient to support a more refined (semi) quantitative risk-benefit analysis. This implies that a definite conclusion on possible beneficial health effects of phytoestrogens cannot be made.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Isoflavones: Anti-Inflammatory Benefit and Possible Caveats

Inflammation, a biological response of body tissues to harmful stimuli, is also known to be involved in a host of diseases, such as obesity, atherosclerosis, rheumatoid arthritis, and even cancer. Isoflavones are a class of flavonoids that exhibit antioxidant, anticancer, antimicrobial, and anti-inflammatory properties. Increasing evidence has highlighted the potential for isoflavones to prevent the chronic diseases in which inflammation plays a key role, though the underlying mechanisms remain unclear. Recently, some studies have raised concerns about isoflavones induced negative effects like carcinogenesis, thymic involution, and immunosuppression. Therefore, this review aims to summarize the anti-inflammatory effects of isoflavones, unravel the underlying mechanisms, and present the potential health risks.

Consensus: soy isoflavones as a first-line approach to the treatment of menopausal vasomotor complaints

The association between an increased uptake of isoflavones and a reduced frequency of menopausal hot flushes was first described in 1992, based on a lower incidence of hot flushes in countries with a high consumption of soy. Since then, numerous clinical trials with various sources of isoflavones including soy and red clover have been presented, with practically all of the studies with adequate design delivering an outcome in favour of isoflavone supplementation. An in-depth risk assessment (EFSA 2015) concludes that the amply available human data does not indicate any suspected harmful effects from a potential interaction of isoflavones with hormone-sensitive tissues in the mammary gland, the uterus and the thyroid gland. Safety was ascertained with long-term intake of up to 150 mg isoflavones per day ingested for the duration of at least 3 years. Moreover, high isoflavone intake was found to have preventive effects with respect to breast cancer. Clinical findings indicate potential benefits of isoflavone exposure even during breast cancer treatment with tamoxifen or anastrozole.

European medicinal and edible plants associated with subacute and chronic toxicity part I: Plants with carcinogenic, teratogenic and endocrine-disrupting effects

In recent decades, the use of herbal medicines and food products has been widely embraced in many developed countries. These products are generally highly accepted by consumers who often believe that "natural" equals "safe". This is, however, an oversimplification because several botanicals have been found to contain toxic compounds in concentrations harmful to human health. Acutely toxic plants are in most cases already recognised as dangerous as a result of their traditional use, but plants with subacute and chronic toxicity are difficult or even impossible to detect by traditional use or by clinical research studies. In this review, we systematically address major issues including the carcinogenicity, teratogenicity and endocrine-disrupting effects associated with the use of herbal preparations with a strong focus on plant species that either grow natively or are cultivated in Europe. The basic information regarding the molecular mechanisms of the individual subtypes of plant-induced non-acute toxicity is given, which is followed by a discussion of the pathophysiological and clinical characteristics. We describe the genotoxic and carcinogenic effects of alkenylbenzenes, pyrrolizidine alkaloids and bracken fern ptaquiloside, the teratogenicity issues regarding anthraquinone glycosides and specific alkaloids, and discuss the human health concerns regarding the phytoestrogens and licorice consumption in detail.

Improving the estimation of flavonoid intake for study of health outcomes

Imprecision in estimating intakes of non-nutrient bioactive compounds such as flavonoids is a challenge in epidemiologic studies of health outcomes. The sources of this imprecision, using flavonoids as an example, include the variability of bioactive compounds in foods due to differences in growing conditions and processing, the challenges in laboratory quantification of flavonoids in foods, the incompleteness of flavonoid food composition tables, and the lack of adequate dietary assessment instruments. Steps to improve databases of bioactive compounds and to increase the accuracy and precision of the estimation of bioactive compound intakes in studies of health benefits and outcomes are suggested.

Polyphenols and the human brain: plant “secondary metabolite” ecologic roles and endogenous signaling functions drive benefits

Flavonoids and other polyphenols are ubiquitous plant chemicals that fulfill a range of ecologic roles for their home plant, including protection from a range of biotic and abiotic stressors and a pivotal role in the management of pathogenic and symbiotic soil bacteria and fungi. They form a natural part of the human diet, and evidence suggests that their consumption is associated with the beneficial modulation of a number of health-related variables, including those related to cardiovascular and brain function. Over recent years, the consensus as to the mechanisms responsible for these effects in humans has shifted away from polyphenols having direct antioxidant effects and toward their modulation of cellular signal transduction pathways. To date, little consideration has been given to the question of why, rather than how, these plant-derived chemicals might exert these effects. Therefore, this review summarizes the evidence suggesting that polyphenols beneficially affect human brain function and describes the current mechanistic hypotheses explaining these effects. It then goes on to describe the ecologic roles and potential endogenous signaling functions that these ubiquitous phytochemicals play within their home plant and discusses whether these functions drive their beneficial effects in humans via a process of “cross-kingdom” signaling predicated on the many conserved similarities in plant, microbial, and human cellular signal transduction pathways.

Genistein-induced fluid accumulation in ovariectomised rats' uteri is associated with increased cystic fibrosis transmembrane regulator expression

OBJECTIVE

High genistein doses have been reported to induce fluid accumulation in the uteri of ovariectomised rats, although the mechanism underlying this effect remains unknown. Because genistein binds to the oestrogen receptor and the cystic fibrosis transmembrane regulator mediates uterine fluid secretion, we hypothesised that this genistein effect involves both the oestrogen receptor and cystic fibrosis transmembrane regulator.

METHODS

Ovariectomised adult female Sprague-Dawley rats were treated with 25, 50, or 100 mg/kg/day genistein for three consecutive days with and without the ER antagonist ICI 182780. One day after the final drug injection, the animals were humanely sacrificed, and the uteri were removed for histology and cystic fibrosis transmembrane regulator mRNA and protein expression analysis using real-time polymerase chain reaction and Western blotting, respectively. The cystic fibrosis transmembrane regulator protein distribution was analysed visually by immunohistochemistry.

RESULTS

The histological analysis revealed an increase in the circumference of the uterine lumen with increasing doses of genistein, which was suggestive of fluid accumulation. Moreover, genistein stimulated a dose-dependent increase in the expression of cystic fibrosis transmembrane regulator protein and mRNA, and high-intensity cystic fibrosis transmembrane regulator immunostaining was observed at the apical membrane of the luminal epithelium following 50 and 100 mg/kg/day genistein treatment. The genistein-induced increase in uterine luminal circumference and cystic fibrosis transmembrane regulator expression was antagonised by treatment with ICI 182780.

CONCLUSION

Genistein-induced luminal fluid accumulation in ovariectomised rats' uteri involves the oestrogen receptor and up-regulation of cystic fibrosis transmembrane regulator expression, and these findings reveal the mechanism underlying the effect of this compound on changes in fluid volume in the uterus after menopause.

Soy, red clover, and isoflavones and breast cancer: a systematic review

BACKGROUND

Soy and red clover isoflavones are controversial due to purported estrogenic activity and possible effects on breast cancer. We conducted a systematic review of soy and red clover for efficacy in improving menopausal symptoms in women with breast cancer, and for potential impact on risk of breast cancer incidence or recurrence.

METHODS

We searched MEDLINE, Embase, the Cochrane Library, and AMED from inception to March 2013 for human interventional or observational data pertaining to the safety and efficacy of soy and red clover isoflavones in patients with or at risk of breast cancer.

RESULTS

Of 4179 records, we included a total of 131 articles: 40 RCTs, 11 uncontrolled trials, and 80 observational studies. Five RCTs reported on the efficacy of soy for hot flashes, showing no significant reductions in hot flashes compared to placebo. There is lack of evidence showing harm from use of soy with respect to risk of breast cancer or recurrence, based on long term observational data. Soy intake consistent with that of a traditional Japanese diet (2-3 servings daily, containing 25-50mg isoflavones) may be protective against breast cancer and recurrence. Human trials show that soy does not increase circulating estradiol or affect estrogen-responsive target tissues. Prospective data of soy use in women taking tamoxifen does not indicate increased risk of recurrence. Evidence on red clover is limited, however existing studies suggest that it may not possess breast cancer-promoting effects.

CONCLUSION

Soy consumption may be associated with reduced risk of breast cancer incidence, recurrence, and mortality. Soy does not have estrogenic effects in humans. Soy intake consistent with a traditional Japanese diet appears safe for breast cancer survivors. While there is no clear evidence of harm, better evidence confirming safety is required before use of high dose (≥ 100 mg) isoflavones can be recommended for breast cancer patients.

Soy isoflavone: The multipurpose phytochemical (Review)

Soy isoflavones are compounds found in soybean and soybean products. They have been reported to possess numerous physiological properties, such as antitumor, anti-menopausal (female) osteoporosis and anti-aging. They have also been reported to improve learning and memory skills in menopausal women and aid in the prevention and treatment of heart disease, diabetes and Kawasaki disease (KD). In this review, the effects of soy isoflavones on various diseases were analyzed. Based on the analysis, it was hypothesized that the function of soybean isoflavones in the prevention and treatment of various diseases results from their phytoestrogen and antioxidant properties. However, due to their phytoestrogen properties, it is recommended that the risks of soy isoflavone intake as food and/or medical treatment be further evaluated.

Soy isoflavones and breast cancer

The soybean and its products have been a staple in the Asian diet for centuries. Although intake of soy remains low in most Western populations, the use of soy isoflavone supplements has become commonplace, and an increasing number of food products contain soy ingredients. This review will present an updated summary of the observational results on soy isoflavones and risk of breast cancer development and outcome in patients with breast cancer. Results from soy intervention studies that have specifically examined the effects of soy on breast cell proliferation in breast tissues will be discussed. We will conclude by highlighting gaps in our knowledge on soy and breast cancer and issues that need to be addressed in future studies.