Soy Protein and Hormonal Health

Doctor’s Advice
Ask the Expert…
Bob Blair, Ph.D. & Aaron Tabor, M.D
Soy has attracted a great deal of attention for its use in alleviating menopausal symptoms because of its use in other cultures. For example, the Japanese have no word for “hot flash”; the same is true for other far eastern cultures. The women in these cultures have a very low incidence of hot flashes and other menopausal symptoms. Researchers have studied the reason for the low incidence of menopausal symptoms in these cultures and determined that dietary differences appear to be a major factor. Eastern women consume significantly more vegetables, grains, and beans and far less animal protein. A major component in their diet is soy. Soy contains isoflavones, which aid in reducing menopausal symptoms, specifically hot flashes.

Isoflavones are not estrogens as commonly defined by medical professionals; however, they do have estrogen-like effects. Isoflavones bind to estrogen receptors and through an adaptogenic process work to restore homoeostasis of estrogen levels. Isoflavones have a very low level of estrogen-like activity. It is not possible to arrive at a single estimate of potency, as potency varies by tissue. However, potency has been estimated at approximately 2% or less of endogenous estrogen. What is well established is that serum isoflavone levels in response to even modest soy food consumption can reach the low micromolar range, about 100 to 1,000 times that of estrogen. This suggests, even assuming very weak potency, that isoflavones have the potential to exert biological effects. That they do so has been reported in several trials, which are discussed later in this section.

Over 25 clinical studies have been conducted on the efficacy of soy isoflavones in alleviating the discomfort of hot flashes. Two major analytical reviews were recently published. A review of the data by Krebs et al. (2004) concluded that soy isoflavones were not effective in menopause relief in comparison to a placebo treatment. A regression analysis conducted by Messina and Hughes (2003) looked at 19 studies (13 using a parallel design) involving more than 17,000 women. The trials exposed women to intakes of 34 to 100 mg isoflavones per day (in most cases equal to or greater than 70 mg/day). Of the 13 studies used in the final analysis only 4 showed negative results while 9 reported positive results. The final results revealed a statistically significant relationship (P = 0.01) between initial hot flash frequency and treatment efficacy. The results of this regression analysis suggest that patients with frequent hot flashes might want to consider soy foods or soy supplements for the alleviation of the menopausal symptoms.

In addition to the published review and regression-analysis several other studies have recently been published that suggest soy isoflavones, while not a replacement for HRT, are a viable option for women who do not wish to take hormone therapy.

Han et al. (2002) conducted a double blind, placebo-controlled study in which 80 women were randomly assigned to isoflavone (100 mg aglycone) and placebo treatments (n=40 per group) for 4 months of treatment. The data demonstrated that isoflavone treatments resulted in a significant decrease in menopausal symptoms compared to baseline (P < 0.01) and compared to the placebo treatment (P < 0.01). In this study, isoflavone treatment also significantly reduced total cholesterol and LDL cholesterol. This study suggests that a 100-mg isoflavone aglycone treatment may be a safe and effective alternative therapy for menopausal symptoms, while offering cardiovascular benefits. This is an important finding since heart disease is the number one killer of women.

Nahas et al. (2004) conducted a prospective, double blind, placebo-controlled, randomized study of 50 postmenopausal women. In this study (60 mg per day, capsules) the isoflavone treatment was significantly more effective than placebo (P < 0.05) in regards to the percentage of women who became asymptomatic for hot flashes (44% versus 10% for the isoflavone and placebo treatments, respectively) and in regards to percent change in hot flashes from baseline (-57% vs. -19% for the isoflavone and placebo treatments, respectively). Isoflavone treatment also resulted in a reduction of LDL by 11.8% and an increase in HDL by 27.3% (P < 0.05) suggesting a beneficial effect on the cardiovascular system.

A study by Crisafulli et al. (2004) measured the effects of hot flashes in 90 early postmenopausal women in a randomized, double blind, placebo-controlled, positively controlled trial in which the women were treated for one year with 54 mg genistein per day. Compared to the placebo treatment, the percent change in daily hot flashes was reduced significantly (P < 0.01) by a mean of 22% after 3 months, 29% after 6 months, and by a mean of 24% after 12 months of genistein treatment. Additionally, genistein administration did not induce any changes in endometrial thickness. It was concluded that genistein administration might have beneficial effects by reducing hot flashes without causing adverse effects on the endometrium. The overall results suggest that genistein may be a viable natural option in the management of postmenopausal symptoms.

Most of the attention to soy isoflavones is directed towards menopause and soy’s ability to reduce the discomfort of menopause. However, soy isoflavones may also have a positive role in reducing PMS. Soy has been shown to lower premenopausal estrogen levels (Lu et al., 2000; Wu et al., 2000) and increase sex hormone binding globulin (Wu et al., 2000), which is important because high estrogen levels cause more severe PMS symptoms and soy can decrease luteal phase length (high estrogen levels during luteal phase produces severe PMS symptoms). Preliminary evidence from two ongoing clinical trials suggest that soy isoflavones significantly improve PMS symptoms (Bryant et al., 2003; Ishiwata et al., 2003)

In summary, numerous studies have shown that soy isoflavones can have a positive effect (reduced number and intensity) on menopausal hot flashes and that these beneficial effects may be related to initial hot flash frequency and intensity. Also, soy isoflavones have the potential to help premenopausal women with PMS. Therefore, soy can serve a positive role in the overall health of women.


  1. Krebs EE, Ensrud,KE, MacDonald R, Wilt TJ. Phytoestrogens for treatment of menopausal symptoms: a systematic review. Obstet Gynecol 2004; 104:824-836
  2. Messina M, Hughes C. Efficacy of soy foods and soybean isoflavone supplements for alleviating menopausal symptoms is positively related to initial hot flash frequency. J Med Food 2003; 6:1-11
  3. Han KK, Soares Jr JM, Haidar MA, de Lima GR, Baracat EC. Benefits of soy isoflavone therapeutic regimen on menopausal symptoms. Obstet Gynecol 2002; 99:389-394
  4. Nahas EP, Neto JN, De Luca L, Traiman P, Pontes A, Dalben I. Benefits of soy germ isoflavones in postmenopausal women with contraindication for conventional hormone replacement therapy. Maturitas 2004; 48:372-380.
  5. Crisafulli A, Marini H, Bitto A, Altavilla D, Squadrito G, Romeo A, Adamo EB, Marini R, D’Anna R, Corrado F, Batolone S, Frisina N, Squadrito F. Effects of genistein on hot flushes in early postmenopausal women: a randomized, double-blind, EPT- and placebo-controlled study. Menopause 2004; 11:400-404.
  6. Lu LJ, Anderson KE, Grady JJ, Kolhen F, Nagamani M. Decreased ovarian hormones during a soya diet: implications for breast cancer prevention. Cancer Res 2000; 60:4112-4121
  7. Wu AH, Stanczyk FZ, Hendrich S, Murphy PA, Zhang C, Wan P, Pike MC. Effects of soy foods on ovarian function in premenopausal women. Br J Cancer 2000; 82:1879-1886
  8. Bryant M, Dye L, Hill C, Powell J, Talbot D, Cassidy A. Role of Phytoestrogens on Menstrual Cycle Symptoms. UK 5th International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Sept. 21-24th, 2003 Orlando, FL.
  9. Ishiwata N, Uesugi S, Uehara M, Watanabe S. Effects of Soy Isoflavones on Premenstrual Syndrome, 5th International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Sept 21-24th, 2003 Orlando, FL.

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