soy isoflavones

What Is It?
Health Benefits
Dosage Information
Guidelines for Use
General Interaction
Possible Side Effects
Evidence Based Rating Scale

What Is It?

Westerners have been slow to put tempeh, miso, tofu, soy milk, and other soybean products on the table. But as evidence mounts that compounds in soy--isoflavones--have impressive health benefits, so too does consumption of this Asian dietary staple. Edamame, those popular pop-out-of-the-shell green bean snacks showing up on appetizer menus, are actually soy beans.

Soy isoflavones are powerful plant substances chemically similar to the female Hormone estrogen; in fact, they are also known as plant estrogens, or phytoestrogens. (1) Their presence in soy foods may help to explain why people in countries where soy is a big part of the diet suffer from relatively few menopausal symptoms such as hot flashes. It may also explain why rates of certain hormone-related cancers are much lower in Asia than they are in many parts of the West, including the United States.

While soy isoflavones are now available in supplement form, supplements have not provided the same health benefits as soy isoflavones consumed as part of the diet. (2) In fact, most research has been done on people who eat soy products rather than take soy supplements. It's possible that isoflavones are just one of several therapeutic compounds in soy. (3) Still, while it's ideal to get soy isoflavones from one or two daily servings of soy foods, supplements are now an option for soy-leery eaters.

Health Benefits

Two particularly important isoflavones in soy--genistein and daidzein--appear to protect against hormone-related disorders such as breast cancer and endometriosis. They do this by competing for the same place on cells (receptor sites) that the body's own estrogen does. (1,4) Some of the risks of excess estrogen, including breast and uterine cancer, can apparently be lowered in this way.

Similarly, when the body's natural levels of estrogen drop, as they do with menopause, soy isoflavones can compensate by binding to some of the cell receptor sites that estrogen once did. Menopausal symptoms may improve as a result.(5)

Research findings suggest that soy isoflavones may also inhibit cancer-causing enzymes, provide Antioxidant protection, and enhance the immune system. (6,7)

Specifically, soy isoflavones may help to:

  • Control symptoms of menopause and perimenopause. When regularly ingested, soy may reduce the frequency and severity of hot flashes and other menopausal difficulties. In one study, women who consumed 30 mg of soy isoflavones or 15 mg of genistein   daily reduced their hot flashes by up to 50%.(5)

  • Guard against osteoporosis. While a number of studies indicate soy isoflavones may aid women in maintaining bone Mineral density, there are also studies where soy isoflavones have no effect on bone mineral density. The inconsistency in data may be related to study design, estrogen levels, and dietary, exercise or other factors. A synthetic isoflavone known as ipriflavone has shown promise in the prevention and treatment of osteoporosis.(8-14)

  • Reduce the risk of endometriosis. In 1999, a study was done on 138 Japanese women to determine the effects of soy isoflavones on endometriosis. The women were in various stages of endometriosis ranging from stage I (no endometriosis) to stage III-IV (advanced endometriosis). In this sample of Japanese women, it was found that including more soy in the diet may reduce the risk of endometriosis. (4)

  • Protect against prostate problems. Eating soy products may protect against enlargement of the male prostate gland. (15) The size of the prostate gland tends to increase with age, causing various types of urinary difficulties, including frequent nighttime awakenings.

  • Prevent various cancers. Preliminary studies show that regular consumption of soy foods or supplements may protect against hormone-related cancers of the breast, prostate, and endometrium (uterine lining). According to one study, women who ate the most soy products and other foods rich in phytoestrogens reduced their risk of endometrial cancer by 54%. (16-21)

    Integrating soy products may be especially important for women who have never been pregnant. One analysis found that women in this category who consumed little, if any, soy--less than a quarter ounce in a given day, on average--were at four times the risk for developing endometrial cancer. (16) In animal studies, adding soy protein to the diet significantly reduced tumor formation and the likelihood that cancer, once developed, would spread.(19,22) So even though more research is needed on soy's cancer-fighting properties, researchers are hopeful. They speculate that the isoflavone genistein may block a protein called tyrosine kinase, which promotes the growth and proliferation of tumor cells.(23)

  • Reduce heart disease risk. Heart-healthy actions have been attributed to isoflavone-rich soy. In 1999 the Food and Drug Administration declared that soy foods can be billed as products that reduce the risk of heart disease by lowering harmful cholesterol. Specifically, soy products have been shown to lower LDL ("bad") cholesterol and significantly increase HDL ("good") cholesterol.(2,6)

    In one study, people who consumed 25 g of soy protein for nine weeks experienced, on average, a 4% reduction in LDL cholesterol. And people with the highest LDL levels experienced n 9% drop. (24) (For each 10% to 15% drop in the LDL level, the risk of a heart attack decreases 20% to 25%.) Because soy's effects are less marked in people with near-normal cholesterol levels, individuals in this category will need to take larger amounts of soy to produce the same reductions. In addition, soy products appear to inhibit the oxidation of LDL cholesterol that can eventually clog arteries.

  • Reduce alcohol consumption. The isoflavone daidzein, an extract of the kudzu plant, has been shown to reduce the amount of alcohol consumed. It has been used by the Chinese for centuries as a natural treatment for alcoholism. (25,26)

  • Help control the effects of type II diabetes. A study was done to compare the use of estrogen with soy isoflavones on blood glucose levels and insulin levels in thirty post-menopausal Taiwanese women with type II diabetes. The study concluded that soy isoflavones and estrogen equally lowered fasting blood glucose levels and insulin levels in post-menopausal women. (31) In addition, other studies indicate soy isoflavones reduce lipid levels in post-menopausal women with type II diabetes. (32)

  • Provide benefits in kidney disease. Studies show that soy protein may reduce proteinuria (high levels of protein in the urine) in patients with kidney disease. More studies are needed to determine the mechanism of action. (33-34)

Note: Soy isoflavones have also been found to be useful for a number of other disorders. For information on these additional ailments, see our Dosage Recommendations Chart for Soy Isoflavones.


  • tablet
  • powder
  • liquid
  • capsule

Dosage Information

Special tips:

  • The best way to get soy isoflavones is through soy foods. Such foods provide abundant protein without the saturated fat found in many other protein-rich sources, such as red meat.

  • Experts have yet to pinpoint the amount of soy isoflavones needed to produce health benefits. In Asian countries, where the prevalence of disorders such as breast cancer and invasive prostate cancer is relatively low, people consume soy foods in amounts that provide an estimated 20 to 200 mg of soy isoflavones a day.

  • If you decide to take supplements, choose products that supply a mixture of isoflavones including genistein and daidzein.

  • High-quality soy-based protein powders are widely available and easy to use; simply add the recommended number of scoops to a cold or hot drink or a food such as oatmeal or soup.

    For the majority of conditions mentioned: Try to ingest a total of 100 mg twice a day from supplements, foods, or a combination of the two.

    Note: Soy isoflavones have also been found to be useful for a number of other disorders. For information on these additional ailments, see our Dosage Recommendations Chart for Soy Isoflavones.

  • Guidelines for Use

  • Take a pill containing soy isoflavones with a large glass of warm water before eating breakfast and dinner.

  • A high-fiber diet may interfere with the absorption of soy isoflavones. Therefore, if your diet is high in fiber, boost your soy consumption.

  • General Interaction

    • Do not consume soy products when taking monoamine oxidase inhibitors (MAOI) such as Nardil or Parnate. Soy can increase hypertension in people taking MAOI's. (30)

    • A recent study indicates genistein may reduce the tumor-inhibiting effects of aromatase inhibitors such as letrozole in breast cancer patients. Soy isoflavones should be avoided by breast cancer patients taking aromatase inhibitors. (35)

    Possible Side Effects

  • Even large amounts of soy isoflavones obtained through foods pose no apparent health risks. However, much less is known about the potential side effects of taking soy isoflavones in concentrated capsule form, and research into the increased risk for breast cancer and other reactions is ongoing.

  • A woman with a strong family history of breast cancer in particular should be aware that in laboratory studies, soy stimulates the growth of breast cells. Whether this directly translates into an increased risk for breast cancer in humans is poorly understood, however. Consult your doctor for guidance.

  • Cautions

  • A woman's decision to take soy isoflavone supplements for any condition must be an individualized one made in consultation with a doctor familiar with her medical background. This is certainly the wisest approach until research can reveal more details about how soy isoflavones affect the body--in both positive and negative ways.

  • Because of their estrogenlike effects, pregnant or nursing women should not take soy isoflavone supplements. (Soy-rich foods pose no such problems, however.)

  • Avoid all soy supplements and soy-based foods if you have an allergy to soybeans.
  • References

    1. Messina MJ, Wood CE. Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and commentary. Nutr J. 2008 Jun 3;7:17.
    2. Michelfelder AJ. Soy: a complete source of protein. Am Fam Physician. 2009 Jan 1;79(1):43-7.
    3. Cassidy A, Hooper L. Phytoestrogens and cardiovascular disease. J Br Menopause Soc. 2006 Jun;12(2):49-56.
    4. Tsuchiya M, Miura T, Hanaoka T, Iwasaki M, Sasaki H, Tanaka T, Nakao H, Katoh T, Ikenoue T, Kabuto M, Tsugane S. Effect of soy isoflavones on endometriosis: interaction with estrogen receptor 2 gene polymorphism. Epidemiology. 2007 May;18(3):402-8.
    5. Kurzer MS. Soy consumption for reduction of menopausal symptoms. Inflammopharmacology. 2008 Oct;16(5):227-9.
    6. Moţa M, Gârgavu S, Popa S, Schiopu S, Panduru NM, Moţa E. Soya--the medicine food product. Rom J Intern Med. 2007;45(1):113-21.
    7. Sakai T, Kogiso M. Soy isoflavones and immunity. J Med Invest. 2008 Aug;55(3-4):167-73.
    8. Liu J, Ho SC, Su YX, Chen WQ, Zhang CX, Chen YM. Effect of long-term intervention of soy isoflavones on bone mineral density in women: A meta-analysis of randomized controlled trials. Bone. 2009 Jan 3. [Epub ahead of print]
    9. Zhang Y, Chen WF, Lai WP, Wong MS. Soy isoflavones and their bone protective effects. Inflammopharmacology. 2008 Oct;16(5):213-5.
    10. Atmaca A, Kleerekoper M, Bayraktar M, Kucuk O. Soy isoflavones in the management of postmenopausal osteoporosis. Menopause. 2008 Jul-Aug;15(4 Pt 1):748-57.
    11. Marini H, Minutoli L, Polito F, Bitto A, Altavilla D, Atteritano M, Gaudio A, Mazzaferro S, Frisina A, Frisina N, Lubrano C, Bonaiuto M, D'Anna R, Cannata ML, Corrado F, Adamo EB, Wilson S, Squadrito F. Effects of the phytoestrogen genistein on bone metabolism in osteopenic postmenopausal women: a randomized trial. Ann Intern Med. 2007 Jun 19;146(12):839-47.
    12. Kreijkamp-Kaspers S, Kok L, Grobbee DE, de Haan EH, Aleman A, Lampe JW, van der Schouw YT. Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial. JAMA. 2004 Jul 7;292(1):65-74.
    13. Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomized controlled trials. Clin Nutr. 2008 Feb;27(1):57-64.
    14. Head KA. Ipriflavone: an important bone-building isoflavone. Altern Med Rev. 1999 Feb;4(1):10-22.
    15. Geller J, Sionit L, Partido C, Li L, Tan X, Youngkin T, Nachtsheim D, Hoffman RM. Genistein inhibits the growth of human-patient BPH and prostate cancer in histoculture. Prostate. 1998 Feb 1;34(2):75-9.
    16. Goodman MT, Wilkens LR, Hankin JH, Lyu LC, Wu AH, Kolonel LN. Association of soy and fiber consumption with the risk of endometrial cancer. Am J Epidemiol. 1997 Aug 15;146(4):294-306.
    17. Michelfelder AJ. Soy: a complete source of protein. Am Fam Physician. 2009 Jan 1;79(1):43-7.
    18. Gardner CD, Oelrich B, Liu JP, Feldman D, Franke AA, Brooks JD. Prostatic soy isoflavone concentrations exceed serum levels after dietary supplementation. Prostate. 2009 Jan 29. [Epub ahead of print]
    19. Perabo FG, Von Löw EC, Ellinger J, von Rücker A, Müller SC, Bastian PJ. Soy isoflavone genistein in prevention and treatment of prostate cancer. Prostate Cancer Prostatic Dis. 2008;11(1):6-12.
    20. Messina M, Kucuk O, Lampe JW. An overview of the health effects of isoflavones with an emphasis on prostate cancer risk and prostate-specific antigen levels. J AOAC Int. 2006 Jul-Aug;89(4):1121-34.
    21. Steiner C, Arnould S, Scalbert A, Manach C. Isoflavones and the prevention of breast and prostate cancer: new perspectives opened by nutrigenomics. Br J Nutr. 2008 May;99 E Suppl 1:ES78-108.
    22. Helferich WG, Andrade JE, Hoagland MS. Phytoestrogens and breast cancer: a complex story. Inflammopharmacology. 2008 Oct;16(5):219-26.
    23. Ravindranath MH, Muthugounder S, Presser N, Viswanathan S. Anticancer therapeutic potential of soy isoflavone, genistein. Adv Exp Med Biol. 2004;546:121-65.
    24. Crouse JR 3rd, Morgan T, Terry JG, Ellis J, Vitolins M, Burke GL. A randomized trial comparing the effect of casein with that of soy protein containing varying amounts of isoflavones on plasma concentrations of lipids and lipoproteins. Arch Intern Med. 1999 Sep 27;159(17):2070-6.
    25. Lowe ED, Gao GY, Johnson LN, Keung WM. Structure of daidzin, a naturally occurring anti-alcohol-addiction agent, in complex with human mitochondrial aldehyde dehydrogenase. J Med Chem. 2008 Aug 14;51(15):4482-7.
    26. Lukas SE, Penetar D, Berko J, Vicens L, Palmer C, Mallya G, Macklin EA, Lee DY. An extract of the Chinese herbal root kudzu reduces alcohol drinking by heavy drinkers in a naturalistic setting. Alcohol Clin Exp Res. 2005 May;29(5):756-62.
    27. Williamson-Hughes PS, Flickinger BD, Messina MJ, Empie MW. Isoflavone supplements containing predominantly genistein reduce hot flash symptoms: a critical review of published studies. Menopause. 2006 Sep-Oct;13(5):831-9.
    28. Kurzer MS. Hormonal effects of soy in premenopausal women and men. J Nutr. 2002 Mar;132(3):570S-573S.
    29. Kim HW, Khil JM. [A study on isoflavones intake from soy foods and perimenstrual symptoms] Taehan Kanho Hakhoe Chi. 2007 Apr; 37 (3):276-85.
    30. Shulman KI, Walker SE. Refining the MAOI diet: tyramine content of pizzas and soy products. J Clin Psychiatry. 1999 Mar;60(3):191-3.
    31. Cheng SY, Shaw NS, Tsai KS, Chen CY. The hypoglycemic effects of soy isoflavones on postmenopausal women. J Womens Health (Larchmt). 2004 Dec;13(10):1080-6.
    32. Jayagopal V, Albertazzi P, Kilpatrick ES, Howarth EM, Jennings PE, Hepburn DA, Atkin SL: Beneficial effects of soy phytoestrogen intake in postmenopausal women with type 2 diabetes. Diabetes Care 25:1709–1714, 2002.
    33. Velasquez MT, Bhathena SJ. Dietary phytoestrogens: a possible role in renal disease protection. Am J Kidney Dis. 2001 May; 37 (5): 1056-68.
    34. Ranich T, Bhathena SJ, Velasquez MT. Protective effects of dietary phytoestrogens in chronic renal disease. J Ren Nutr. 2001 Oct;11(4):183-93.
    35. Ju YH, Doerge DR, Woodling KA, Hartman JA, Kwak J, Helferich WG. Dietary genistein negates the inhibitory effect of letrozole on the growth of aromatase-expressing estrogen-dependent human breast cancer cells (MCF-7Ca) in vivo. Carcinogenesis. 2008 Nov;2(11):2162-8.

    Evidence Based Rating Scale  

    The Evidence Based Rating Scale is a tool that helps consumers translate the findings of medical research studies with what our clinical advisors have found to be efficacious in their personal practice. This tool is meant to simplify which supplements and therapies demonstrate promise in the treatment of certain conditions. This scale does not take into account any possible interactions with any medication/ condition/ or therapy which you may be currently undertaking. It is therefore advisable to ask your doctor before starting any new treatment regimen.














    Date Published: 04/19/2005
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