For Patients & Caregivers
Whether soy helps relieve menopausal symptoms is still unclear. It may reduce risk of some cancers.
Soy contains large amounts of substances known as isoflavones, also called phytoestrogens. These substances act in a similar way as estrogen in the body and may also be antioxidants. The effects of soy are weak in younger women because estrogen is abundant. At menopause, however, the effects of soy may increase due to the decrease in estrogen. Studies in animals suggest that soy can prevent or reduce bone loss by acting like estrogen and by reducing loss of calcium.
In laboratory studies, soy isoflavones slowed down the growth of several different types of cancer cells, including breast and prostate cancers. Animal studies showed that genistein, one of the isoflavones, may interfere with tamoxifen that is used for breast cancer. But a new study showed that soy foods can benefit women with breast cancer.
Patients should consult their physicians about use of soy supplements.
- To prevent cancer
Laboratory and some clinical studies suggest that women with high soy intake have a lower risk of developing breast cancer. High intake of soy isoflavones can also reduce risk of lung cancer.
- To prevent heart disease
Clinical trials show that soy protein (not soy pills) reduces LDL (bad) cholesterol levels, which may help prevent heart disease.
- To reduce high cholesterol
Clinical trials show that soy protein (not soy pills) reduces LDL (bad) cholesterol levels and may increase HDL (good) cholesterol levels.
- To treat menopause symptoms such as hot flashes, vaginal dryness, sleep disturbances, mood problems
Clinical studies have conflicting results.
- To prevent bone loss
Some clinical trials and animal studies support this use.
- You are hypersensitive to soy products.
- You are taking tamoxifen: Animal studies have shown that genistein, a soy isoflavone, may reduce the effects of tamoxifen on estrogen-dependent breast cancer.
- You are taking aromatase Inhibitors: Soy-based supplements may reduce the effects of aromatase inhibitors used in breast cancer treatment.
- You are taking cytochrome P450 substrate drugs: Soymilk and miso may make them less effective.
- You are taking P-Glycoprotein substrate drugs: Soymilk and miso may make them less effective.
- You are taking UGT (Uridine 5’-diphospho-glucuronosyltransferase) substrate drugs: Soy can increase the side effects of these drugs.
- Allergic reactions
- Breast enlargement: In a 60-year-old man following consumption of soy milk over a period of 6 months. Symptoms resolved after discontinuing soy use.
- Abnormal uterine bleeding: In 3 women after a high intake of soy products. Symptoms improved following withdrawal of soy use.
- Loss of sex drive and erectile dysfunction: In a 19-year-old diabetic man who was otherwise healthy, following intake of large amounts of soy-based products in a vegan-style diet. Symptoms improved 1 year after discontinuing the diet.
- Death related to salt poisoning: A 55-year-old woman died following ingestion of a large quantity of Japanese shoyu soy sauce.
- Seizure-like activity from elevated blood sodium levels: In a 19-year-old man 2 hours after ingesting a quart of soy sauce.
For Healthcare Professionals
Soy is derived from the seed of Glycine max. Soybeans are rich in protein and other essential nutrients and are widely consumed as food. Soy also contains significant amounts of isoflavones — genistein (4’,5,7-trihydroxyisoflavone), daidzein (4’,7-dihydroxyisoflavone), and glycitein (4’,7-dihydroxy-6-methoxyisoflavone) (1). They are marketed as dietary supplements to treat hot flashes, high cholesterol, and for cancer prevention.
Isoflavones in soy are thought to exhibit both selective estrogen receptor modulator activity and non-hormonal effects. Studies on the effect of soy for menopausal symptoms are inconclusive (2)(3)(4)(5)(6)(7)(8)(9). While some studies indicate that soy may slow bone density loss (10)(11), others report no effects on bone mineral density in postmenopausal women (12)(13). Soy was shown to reduce low-density-lipoprotein (LDL)(14) in postmenopausal women (15) and blood pressure in hypertensive postmenopausal women (16), suggesting a benefit for cardiovascular health (17). Genistein also reduced fasting blood glucose and insulin levels as well as improving insulin resistance (18). Soy intake also reduced proteinuria in type 2 diabetic patients with nephropathy (19). However, conflicting data suggest no such protective effects of soy consumption against diabetes (20).
Soy may reduce risk of prostate (21)(22)(23), lung (24)(80), and endometrial (25)(26) cancers, but can increase the risk of bladder cancer (27) and endometrial hyperplasia (28). It was also shown to prevent breast cancer (29)(30) but conflicting data suggest that soy supplementation may have adverse effects (31)(32). A study done in mice suggests that consumption of soy products may increase metastasis (33).
Genistein, the most estrogenic soy isoflavone (34), demonstrates antiproliferative effects in multiple cell lines, including breast cancer (ER-positive and negative) (35), prostate cancer (androgen-dependent and independent) (36), nasopharyngeal carcinoma (37), neuroblastoma, sarcoma, and retinoblastoma cells (36). Animal studies also show that genistein antagonizes the effects of tamoxifen (38)(39) and promotes tumor progression in advanced prostate cancer (40). Soy supplements may decrease serum PSA levels in prostate cancer patients before prostatectomy (41), but this effect is not observed in patients after prostatectomy (82) . Further research is needed to evaluate the role of genistein in cancer prevention.
Soy consumption reduced mortality and recurrence in breast cancer patients, regardless of tamoxifen use (42). However new data suggest that supplementation may stimulate overexpression of breast cancer genes in patients with invasive breast cancer (87).
In other studies, soy prolonged survival among women with lung cancer (81). Supplementation with soy isoflavones may reduce the adverse effects associated with chemotherapy (43) and radiotherapy (44), but when used together with vitamin E and selenium, did not prevent prostate cancer progression (45). It was also ineffective in reducing hot flashes in patients with prostate cancer (83).
Patients should consult their physicians about the use of soy supplements.
Animal studies suggest that genistein and daidzein have an ability to prevent or reduce bone loss in a manner similar to synthetic estrogen due to increased beta versus alpha estrogen receptor (ER) binding (10). Both isoflavones may modulate bone remodeling through ERs by regulating target gene expression (50). Soy may also contribute to maintaining bone density by causing less calcium to be excreted in the urine (35).
Proposed mechanisms of soy’s cholesterol-lowering effect include phytoestrogen-induced hyperthyroid state and increased excretion of bile acids, which may enhance removal of LDL. Isoflavones may inhibit oxidation of LDL and may alter hepatic metabolism with enhanced removal of LDL and VLDL by hepatocytes (17). Serum lipids may also be regulated through modified transcription factor and downstream gene expression and by promoting antioxidant enzyme activity (51).
The phytochemicals in soybeans also exhibit anticarcinogenic activity, for which many mechanisms have been proposed. Genistein affects microRNA expression—targeted translation inhibitors for multiple proteins implicated in the regulation of various pathobiological processes in cancer (52). In addition, genistein demonstrated an anti-minichromosome maintenance (MCM) effect, a gene family frequently upregulated in various cancers and considered a promising anticancer drug target (53).
In breast cancer, genistein acts as an agonist to estrogen receptor (ER)-alpha in ER-alpha-predominant breast cancer cells, but likely acts as an antiestrogen in cells with ER-beta alone, suggesting therapeutic potential for premenopausal women with ER-alpha-negative/ER-beta-positive tumors (54). However, in an in vitro breast model, genistein also induced estrogen-dependent MCF-7 tumor cell growth and increased breast cancer-associated aromatase expression and activity, suggesting that soy-based supplements may affect the efficacy of aromatase inhibitors used in breast cancer treatment (55). Genistein is also known to negate the inhibitory effect of tamoxifen on MCF-7 tumor growth and increase expression of estrogen-responsive genes (38). Alternatively, soy isoflavones may reduce breast cancer risk by decreasing endogenous ovarian steroid levels (56). Studies suggest that some benefits ascribed to dietary isoflavones may depend on early life exposure, thereby impacting gene expression at an epigenetic level. (47)(48)(49)
In prostate cancer, soy protein extracts appear to influence the progression of established tumors rather than inhibit etiologic factors. Furthermore, soy protein consumption reduces androgen receptor expression in prostate tumors (57). Other proposed prevention mechanisms include genistein-induced prostate cancer cell adhesion, direct growth inhibition, and induction of apoptosis (22). Growth inhibition appears to be independent of genistein’s estrogenic effects. In human prostate cancer cell lines, both genistein and daidzein affect microRNA regulation (58) and induce decreased methylation of gene promoters including BRCA1(59). In tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-resistant prostate cancer cells, soy isoflavones enhance TRAIL-mediated apoptosis by engaging apoptotic pathways and regulating NF-κB activity (60)(61). However, in a patient-derived prostate cancer xenograft model, increased proliferation and metastasis in genistein-treated groups were linked to enhanced activities of tyrosine kinases, the epidermal growth factor receptor, and its downstream Src (40). Both genistein and daidzein also act as radiosensitizers for prostate cancer in vitro and in vivo, but pure genistein increased lymph node metastasis, whereas the combination of genistein, daidzein, and glycitein did not. Daidzein may protect against genistein-induced metastasis, and its ability to inhibit cell growth and potentiate radiation appears to be androgen-receptor-independent (62). In addition, soy isoflavones radiosensitized human A549 NSCLC cells, and decreased hemorrhages, inflammation, and fibrosis caused by radiation suggesting protection of normal lung tissue (63). The combination of genistein, daidzein, and glycitein also mediated growth-suppressive effects via ER-beta in DLD-1 human colon adenocarcinoma cells (64). 7,3’,4’-trihydroxyisoflavone (THIF), a daidzein metabolite, targets Cot and MKK4 to inhibit UVB-induced skin cancer (65) and cyclin-dependent kinases and phosphatidylinositol 3-kinase to inhibit EGF-induced proliferation and transformation in JB6 P+ mouse epidermal cells (66).
The intestinally derived isoflavone metabolite R-equol, but not S-equol, was also found to be potently chemopreventive (67). However, neonatal and prepubertal exposure to equol showed no long-term chemoprevention against DMBA-induced mammary tumors even though an equol-exposure ’imprinting’ effect resulted in a decrease in immature terminal end structures and an increase in mature lobules (68).
- Flatulence, allergic reactions
- Gynecomastia: In a 60-year-old man following consumption of soy milk over a period of 6 months. Symptoms resolved after discontinuing soy use (76).
- Abnormal uterine bleeding: With endometrial pathology in 3 women after a high intake of soy products. Symptoms improved following withdrawal of soy use (77).
- Loss of libido and erectile dysfunction: In a 19-year-old diabetic man who was otherwise healthy, following intake of large amounts of soy-based products in a vegan-style diet. Symptoms improved 1 year after discontinuing the diet (78).
- Death from massive pulmonary edema: A 55-year-old woman died following ingestion of a large quantity of Japanese shoyu soy sauce (79).
- Seizure-like activity due to acute hypernatremia: In a 19-year-old man 2 hours after ingesting a quart of soy sauce. Symptoms improved following treatment with large quantities of water (85).
- Tamoxifen: Animal studies suggest that genistein, a soy isoflavone, may antagonize the effects of tamoxifen on estrogen-dependent breast cancer (MCF-7) (38)(39).
- Aromatase inhibitors: Genistein also induced MCF-7 tumor cell growth and increased breast cancer-associated aromatase expression and activity in an in vitro breast model, suggesting that soy-based supplements may affect the efficacy of aromatase inhibitors used in breast cancer treatment (55).
- Cytochrome P450 substrates: Soymilk and miso were shown to induce CYP3A4 in vivo, and can affect the intracellular concentration of drugs metabolized by this enzyme (86).
- P-Glycoprotein: Soymilk and miso were shown to induce P-Gp in vivo, and can affect the intracellular concentration of certain drugs (86).
- UGT (Uridine 5’-diphospho-glucuronosyltransferase) substrates: Soy modulates UGT enzymes in vitro and can increase the side effects of drugs metabolized by them (75).