What Are Isoflavones?

Plant compounds with estrogen-like and antioxidant effects

Isoflavone powder, edamame, tofu, capsules, tempeh, soymilk

Verywell / Anastasia Tretiak 

Isoflavones are plant-based compounds found almost exclusively in beans, like soybeans, that mimic the action of the hormone estrogen. They may be useful in alleviating menopause symptoms or preventing osteoporosis in postmenopausal women.

Foods rich in isoflavones may offer other health benefits in that they are both anti-inflammatory and antioxidant (meaning they prevent damage to a cell's DNA). Some studies have even suggested that isoflavones may help prevent heart disease or have cancer-fighting properties.

Even so, the evidence supporting these health claims is generally weak.

This article describes the potential benefits of isoflavones, including what they do and how they work. It also lists some of the foods high in isoflavones along with the possible risks, side effects, and interactions.

Hormone Effects of Isoflavones

Isoflavones are also known as phytoestrogens. That's because they are plant-based (phyto means "from plants") and they have a similar structure to estrogen. This allows isoflavones to bind with estrogen receptors.

Depending on the hormone status of a person, isoflavones can affect a person in the same way that estrogen does by producing either estrogenic or antiestrogenic effects.

In studies involving isoflavone supplements for menopausal symptoms, some benefits have been shown, such as: 

  • Improving fatigue
  • Improving irritability
  • Decreasing hot flashes

But, according to The Pharmaceutical Journal, although isoflavones are being marketed as an effective product for natural hormone replacement therapy (HRT), further research is needed. Consumers should not use isoflavones for long-term HRT until more research is able to prove the safety and efficacy of the products.  

Some case reports indicate that isoflavones in red clover help reduce hot flashes and anxiety during menopause. Although the herb is marketed as a dietary supplement called Promensil, the National Women’s Health Network reports that there is a lack of sufficient clinical research data to back up these claims.

Medical Uses for Isoflavones

There are many common medical uses for isoflavones. Conditions that may improve with the use of isoflavones vary.

Breast cancer: The research is mixed. Those who eat a high soy diet during adolescence may have a lower risk of breast cancer later in life, but some studies show that isoflavones from soy can increase the risk of cancer.  

Type 2 diabetes: The research says that eating soy protein or fermented soy may reduce blood sugar in those with diabetes.

Diarrhea in infants: Soy formula supplementation may shorten the duration of diarrhea (compared to drinking formula from cow’s milk). It’s important to note that in adults, soy fiber was NOT found to improve diarrhea.

High cholesterol: Research strongly suggests that isoflavones from soy reduce bad cholesterol called LDL. Only protein-based food sources of isoflavones such as tofu or tempeh were found to lower cholesterol; isoflavone supplements were not effective. Red clover has also been found to have a cardiovascular benefit, raising HDL, or good cholesterol.

High blood pressure: Eating soy may lower the blood pressure slightly and is suggested for those with slight increases in blood pressure, but NOT in people with severely elevated blood pressure.

Irritable bowel syndrome (IBS): Some research found that soy isoflavones may improve symptoms of IBS, such as abdominal pain.

Menopausal symptoms: Symptoms such as irritability, depression, and hot flashes may be lessened with the use of isoflavones. However, isoflavones were NOT found to be beneficial for night sweats.

Osteoporosis: In studies, soy protein from food sources and isoflavones in supplement form were both found to increase bone density.

There is NOT enough evidence to back the claims of using isoflavones to treat many medical conditions, including:

What Research Says

A 2016 review published in the British Journal of Pharmacology, reports that it has not been well-established whether plant-derived compounds, like isoflavones, provide potential benefits that outweigh the risk factors. 

Various studies have been done on isoflavones:

  • A study published in 2016 in the Journal of the American Medical Association examined 60 studies and found that some plant-based therapies—such as isoflavones—worked to provide a modest reduction in hot flashes and vaginal dryness, but weren't effective for reducing night sweats.
  • Several studies, of both animals and humans, have shown that soy protein supplementation containing isoflavones may reduce both the total and LDL cholesterol.
  • A study published in the journal Nutrients reports that, "...isoflavones exhibit impressive anti-inflammatory properties in various animal models, and even in humans, through increased antioxidative activities." The study authors also explain that safety hasn't been established and there may be harmful side effects of isoflavones, like potentially lowering the immune response and causing cancer.

Possible Side Effects of Isoflavones

Most of the side effects of isoflavones are associated with long-term use of supplements and not from dietary sources such as soy products.

  • Some data shows that there is a link between long term soy consumption and Kawasaki disease (KD). Soy isoflavones play a role in this.
  • Another study discovered that exposure to soy-based infant formula resulted in negative effects on the long-term development of infants.
  • Breast cancer cell growth has been linked with long-term exposure to genistein, resulting in what is called “soy protein isolate-induced tumors and advanced growth phenotypes."
  • Animal studies have shown that isoflavone genistein may have adverse effects on the developing reproductive system of assigned females.
  • Long-term use of soy extract supplements may result in abnormal tissue growth in the uterus.

Short-Term Use

When ingested on a short-term basis (up to six months in duration) soy is considered possibly safe. Common side effects may include:

  • GI upset
  • Constipation, bloating, and nausea
  • Allergic reactions (involving rash, itching, and in severe instances, anaphylaxis)
  • Loss of appetite

Swelling of the ankles and abdominal tenderness have been noted in high doses of isoflavones of four to eight milligrams per kilograms (mg/kg).

Do Isoflavones Cause Weight Gain?

Research suggests that, in general, isoflavones from soy do not cause weight gain. However, it may cause slight weight gain in young individuals, as well as individuals who are obese.

Precautions and Contraindications

There are precautions associated with soy products and isoflavones.

  • There is not enough clinical research data to support the safe use of soy products when pregnant or breastfeeding, particularly at higher doses.
  • There have been some findings that link infant soy formula with delayed development in infants. However, the National Toxicology Program concluded that there is little concern for developmental effects in infants given soy formula. Long-term use of soy formula should always be discussed with a healthcare provider.
  • Children should not eat soy in large amounts. It’s uncertain whether soy is safe for kids at high doses.
  • Those with asthma or hay fever may have a higher risk of an allergic reaction to soy.  
  • Those with breast cancer should discuss the use of isoflavone supplements with their oncologist or healthcare provider. Research findings are mixed, and it’s possible that soy may act like estrogen and promote cancer cell growth in certain types of breast cancer.
  • Children with cystic fibrosis should avoid taking isoflavones because they may interfere with the way protein is processed.
  • The use of isoflavones in people with diabetes should be discussed with a healthcare provider prior to use because soy products may lower blood sugar, interfering with diabetes medication.
  • Hypothyroidism may be worsened when using soy isoflavones.
  • Those with kidney stones should avoid taking soy isoflavones as they may increase a chemical, called oxalates, that contributes to kidney stones.
  • Those with kidney conditions should avoid the use of phytoestrogens, such as soy isoflavones, which could be toxic at high doses for those with kidney failure

Drug Interactions

Isoflavones may adversely interact with some medications:

  • Monoamine oxidase inhibitors (MAOIs) are a type of antidepressant which interacts adversely with fermented soy products.
  • Antibiotics may reduce the effectiveness of soy products by negatively impacting the natural gut flora needed to effectively process isoflavones.
  • Estrogens such as Premarin, estradiol, and other HRT for menopause should not be taken with isoflavones because the isoflavones may decrease the effect of estrogen.
  • Soltamox (tamoxifen) is a drug used in the treatment of the type of cancer influenced by estrogen and should not be taken with isoflavones.
  • Coumadin (warfarin) should not be taken with soy products as it may lower the effectiveness of this drug. Red clover may have blood-thinning properties and should not be taken with Coumadin.

Anyone taking prescription or over the counter medications should consult with their healthcare provider before taking isoflavones or any other herbal or nutritional supplement.

Isoflavones may adversely impact the speed in which the liver processes certain drugs:

  • Glipizide (hypoglycemic agent)
  • Phenytoin (anticonvulsant)
  • Flurbiprofen (anti-inflammatory agent)
  • Warfarin (anticoagulant)
Isoflavones capsules

Verywell / Anastasia Tretiak 

Sources and Composition

While soybeans have the highest level of isoflavones, herbal sources like red clover (Trifolium pratense) and alfalfa (Medicago sativa) are also rich in isoflavones. Like soy, red clover is considered a legume that contains phytoestrogens.

In their most common form, soy isoflavones include:

  • Genistein
  • Daidzein
  • Glycetein

Isoflavones found in red clover include:

  • Formononetin
  • Biochanin A
  • Daidzein
  • Genistein

You may get these be consuming these foods or by using supplements made from them.

In Asia, where soy is a staple in the diet, the rate of heart disease, breast cancer, and prostate cancer is lower than in the U.S. Many health experts feel that eating fermented soy in moderation may increase bone density, lower bad cholesterol, improve mental functioning and more.

It is thought that fermentation helps with the digestion of soy and may even promote the body’s ability to absorb isoflavones.

It’s important to note that taking a supplement source of isoflavones and eating a food/protein source of isoflavones—like tofu, tempeh, soy milk, miso, or other soybean products—produces different results in terms of benefits and side effects.

Supplement Dosages

The following doses of isoflavones are backed by clinical research studies:

  • For postmenopausal individuals: A supplement with least 54 mg of genistein (a soy isoflavone) per day is suggested for hot flashes.
  • For IBS: Take a supplement of 40 mg of isoflavones per day for six weeks.
  • For protection against osteoporosis: A supplement of 80 mg per day of soy isoflavones was associated with a dosage that reduced bone loss in postmenopausal individuals (protecting against osteoporosis).  
  • For general supplementation: Drugs.com suggests using a daily dose of 40 to 120 mg of isoflavones per day (from soy) or 40 to 80 mg per day of isoflavones (from red clover) for various conditions.

The safety of the use of isoflavones supplements cannot be guaranteed when taken for a duration of longer than six months.

What to Look For

Since supplements are not regulated by the FDA, there are several things to look for to ensure quality and effectiveness in isoflavones and other natural supplements.

  • Look for a product that is made into an extract and avoid powder supplements which may be much weaker in strength (but cheaper to make). 
  • Ensure that the strength and dosage of the isoflavone supplement align with recommendations from clinical research data. Those who are unsure should consult with their healthcare professional or pharmacist.
  • Check the label to make sure that the product has active ingredients, such as natural phytoestrogens contained in isoflavones (in the extract form).
  • Check to ensure that the product is quality-tested for safety and strength by a third party and made in the USA.
  • Choose non-genetically modified isoflavones products.
  • Choose a company that backs its products with a 60-day guarantee to allow enough time to evaluate the effectiveness of the product.

A Word From Verywell

Although much of the clinical research data on isoflavones support its health benefits, such as promoting cardiac health, reducing menopausal symptoms and more, there is quite a bit of contradictory information. 

Because of the number of mixed study data reports on isoflavones, it's strongly suggested you consult with your healthcare provider before using this nutritional supplement in any form, including eating large amounts of soy products, ingesting herbal sources of isoflavones from red clover, or taking any supplements with any other form of isoflavones. 

Frequently Asked Questions

  • What foods are high in isoflavones?

    Other than unprocessed sources of soy, isoflavones are found in:

    • Fruits like currants and raisins
    • Chickpeas
    • Fava beans
    • Nuts like pistachios and peanuts
  • Can soy isoflavones affect fertility?

    It's possible, but the evidence is still unclear. Some studies have found that soy can help boost fertility, while others show that large amounts may have a negative effect.

  • Does red clover work better than soy for menopausal symptoms?

    The isoflavones from red clover have phytoestrogens, which are known to help balance estrogen levels, but the research is mixed on how well red clover helps reduce symptoms of menopause. There have been more studies on soy isoflavones that support their use for menopausal symptoms.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Rietjens IMCM, Louisse J, Beekmann K. The potential health effects of dietary phytoestrogens. Br J Pharmacol. 2017 Jun;174(11):1263-80. doi:10.1111/bph.13622

  2. Ahsan M, Khurram Mallick AK. The effect of soy isoflavones on the menopause rating scale scoring in perimenopausal and postmenopausal women: a pilot study. J Clin Diagn Res. 2017 Sep;11(9):FC13–16. doi:10.7860/JCDR/2017/26034.10654

  3. Mason, P. Isoflavones. The Pharmaceutical Journal. 2001;266(7129):16-19.

  4. Franco OH, Chowdhury R, Troup J, et al. Use of Plant-Based Therapies and Menopausal Symptoms: A Systematic Review and Meta-analysis. JAMA. 2016;315(23):2554-2563. doi:10.1001/jama.2016.8012

  5. Yu J, Bi X, Yu B, Chen D. Isoflavones: anti-inflammatory benefit and possible caveats. Nutrients. 2016;8(6):361. doi:10.3390/nu8060361

  6. Akhlaghi M, Zare M, Nouripour F. Effect of soy and soy isoflavones on obesity-related anthropometric measures: a systematic review and meta-analysis of randomized controlled clinical trialsAdv Nutr. 2017;8(5):705-717. doi:10.3945/an.117.015370

  7. National Toxicology Program. NTP Brief On Soy Infant Formula: Peer-Review Comments and NTP Response September 2010.

  8. American Heart Association. Isofavones, in tofu and plant proteins, associated with lower heart disease risk.

Additional Reading

By Sherry Christiansen
Sherry Christiansen is a medical writer with a healthcare background. She has worked in the hospital setting and collaborated on Alzheimer's research.