Soy isoflavones are plant compounds found naturally in soybeans that have gained attention as a dietary supplement, particularly for women approaching or going through menopause. These compounds belong to a class called phytoestrogens because they can weakly mimic the effects of oestrogen in the body.

If you’ve been considering soy isoflavone supplements or simply want to understand what the research actually says, this article covers the evidence for various health claims, along with important safety information you should know before taking them.

What are soy isoflavones?

Soy isoflavones are a type of flavonoid naturally occurring in soybeans. The two main isoflavones are daidzein and genistein, with smaller amounts of glycitein also present.

What makes isoflavones interesting from a biological standpoint is their structural similarity to human oestrogen, specifically 17-β oestradiol. This resemblance allows them to bind to oestrogen receptors in the body, though with much weaker effects than actual oestrogen. Depending on your existing hormone levels and which tissues are involved, isoflavones can produce either weak oestrogenic effects or actually block oestrogen activity.

Beyond their hormone-related properties, isoflavones also function as antioxidants and have been studied for anti-inflammatory, antibacterial, and anti-cancer effects in laboratory settings.

One observation that sparked interest in soy isoflavones: Asian populations traditionally consume far more soy foods than Western populations (15-47 mg of isoflavones daily compared to just 0.15-1.7 mg in typical Western diets), and they also have notably lower rates of breast and prostate cancer [1]. Whether this correlation reflects a genuine protective effect of soy intake remains an area of ongoing research.

Food sources

Before reaching for supplements, it’s worth knowing that soy isoflavones occur naturally in many foods. Here’s the approximate content per 100 grams:

Soybeans (whole): 26-381 mg
Soy flour: 83-466 mg
Tofu: 8-67 mg
Miso: 25-89 mg
Tempeh: 86.5 mg

The wide ranges reflect natural variation between different soybean varieties and processing methods. Fermented soy products like miso and tempeh may have better bioavailability because fermentation converts isoflavone glycosides into their more easily absorbed aglycone forms.

10 researched benefits of soy isoflavones

1. May reduce arterial stiffness

As arteries become stiffer with age, they lose their ability to expand and contract smoothly with each heartbeat. This arterial stiffness has been linked to higher risks of cardiovascular problems including systolic hypertension, coronary artery disease, stroke, and heart failure.

A 2020 systematic review and meta-analysis examined 8 randomised controlled trials and found that soy isoflavone supplementation reduced various measures of arterial stiffness compared to placebo, including pulse wave velocity, systemic arterial compliance, and cardio-ankle vascular index [2].

The evidence is promising but limited. Most studies were relatively short with small participant numbers, so larger and longer trials would help confirm these cardiovascular benefits.

2. May help preserve memory

A 2019 meta-analysis pooled data from 16 randomised controlled trials involving 1,386 participants [3]. Participants taking soy isoflavones (doses ranging from 60 mg to 160 mg daily over a median of 17 weeks) showed improvements in cognitive function compared to those taking placebo, with memory being the area where improvements were most consistent.

Cognitive decline tends to begin subtly and progress gradually, so anything that might slow that process is worth investigating. The catch: some of the individual studies were quite small, so the findings need replication.

3. Reduces hot flashes in menopause

Hot flashes are probably the most common reason women turn to soy isoflavone supplements. These sudden episodes of intense heat, sweating, and flushing typically affect the head, neck, chest, and upper back, often accompanied by rapid heartbeat and anxiety.

A meta-analysis of 17 double-blind randomised controlled trials found that soy isoflavone supplements (median dose 54 mg, taken for 6 weeks to 12 months) reduced hot flash frequency by 20.6% and severity by 26.2% compared to placebo [4].

An interesting detail: supplements containing more than 18.8 mg of genistein specifically were about twice as effective as those with lower genistein levels. So not all soy isoflavone products are equally effective. The mechanism behind this benefit relates to isoflavones’ weak oestrogenic activity and their ability to modulate oestrogen receptors.

4. Cholesterol effects are limited

Many people hope soy isoflavones will improve their cholesterol levels, but the evidence here is disappointing.

A meta-analysis of 12 randomised controlled trials in menopausal women with normal cholesterol found that taking soy isoflavones (averaging 70 mg daily in aglycone form for 1-3 months) did not significantly improve total cholesterol or LDL cholesterol compared to placebo [5].

If you’re specifically looking to manage cholesterol, soy isoflavone supplements alone probably won’t help. Whole soy foods might have modest benefits, but that’s likely due to other components like soy protein and fibre rather than the isoflavones themselves.

5. May benefit bone health

Osteoporosis becomes increasingly common after menopause, with fracture rates rising from about 4% in women aged 50-59 to over 52% in women aged 80 and above. Could soy isoflavones help?

A systematic review and meta-analysis of 28 randomised controlled trials found that menopausal women taking soy isoflavones (56 mg aglycone equivalents daily for 10 weeks to 12 months) showed significant reductions in deoxypyridinoline (DPD), a marker of bone breakdown [6].

Another meta-analysis of 12 trials involving 1,240 menopausal women found that soy isoflavone intake (averaging 82 mg daily for 6-12 months) increased spinal bone mineral density by about 2.4% [7]. However, no significant improvements were seen in hip or femoral neck bone density.

The spine benefits but not the hip. Why? Nobody’s entirely sure yet. And whether a 2.4% increase in bone density actually translates to fewer fractures is still an open question.

6. May improve blood sugar control

Menopause often accelerates the development of glucose intolerance, partly because declining oestrogen affects how the body handles insulin and stores fat.

A 2016 meta-analysis examined 17 randomised controlled studies involving 1,529 menopausal women and found that soy isoflavone supplements improved fasting blood glucose, insulin resistance, and insulin sensitivity [8]. The effects were particularly notable with supplements containing genistein alone rather than mixed isoflavones.

Isoflavones may work through several mechanisms: inhibiting alpha-glucosidase (an enzyme that breaks down carbohydrates), inhibiting tyrosine kinase, and regulating pathways involved in glucose metabolism.

7. Associated with lower breast cancer risk in Asian populations

A meta-analysis of 35 epidemiological studies found an association between soy isoflavone intake and lower breast cancer risk in Asian women, both before and after menopause [9].

However, this association was not seen in Western populations. The difference might relate to lifelong soy consumption starting from childhood in Asian cultures versus occasional adult intake in Western countries, or there could be other dietary and lifestyle factors at play.

This is observational data showing correlation, not causation. Randomised controlled trials would be needed to prove that soy isoflavones actually prevent breast cancer, and those studies haven’t been done.

8. May help regulate blood pressure

A 2010 systematic review and meta-analysis of 14 randomised controlled trials involving 789 participants found that soy isoflavone supplementation reduced systolic blood pressure by about 1.9 mmHg in people with normal or slightly elevated blood pressure [10].

The blood pressure reduction was most pronounced in participants who took supplements for more than 3 months. The mechanism likely involves isoflavones increasing nitric oxide levels, which helps blood vessels relax and dilate.

While statistically significant, a reduction of under 2 mmHg is quite modest. Soy isoflavones probably shouldn’t be your primary strategy for managing blood pressure.

9. May reduce prostate cancer risk

Prostate cancer rates are about 10 times higher in North America than in Asian countries. While genetics and screening practices contribute to this difference, diet may also play a role.

A systematic review of 8 randomised controlled trials found that men at potential risk of prostate cancer who consumed soy products or soy isoflavone supplements had a reduced risk of developing the disease (by up to 51%) [11].

Interestingly, there were no significant changes in prostate-specific antigen (PSA) levels, testosterone, or other sex hormones, suggesting the protective effect works through other mechanisms, possibly relating to cell proliferation and apoptosis.

10. May help with weight in menopausal women

Hormonal changes during menopause often lead to increased fat accumulation, particularly around the abdomen. Studies have reported that abdominal fat can increase by 49% after menopause, with subcutaneous fat up 22% [12].

A meta-analysis of 30 studies involving approximately 2,800 non-Asian postmenopausal women found that soy isoflavone supplementation helped with weight loss and blood sugar control compared to placebo [13].

The weight loss effects were most significant in women with a BMI under 30, taking less than 100 mg daily, for no more than 6 months. After that, benefits seemed to taper off.

Side effects

Soy isoflavones have been part of human diets for centuries and generally have a good safety profile when consumed in amounts typical of Asian diets or in supplements at moderate doses.

Possible side effects include:

Gastrointestinal discomfort
Nausea
Bloating or gas

Allergic reactions to soy are possible, though relatively uncommon. If you experience skin rash, itching, swelling of the face, tongue or throat, severe dizziness, or difficulty breathing after consuming soy products, seek medical help immediately.

Safety precautions

Tolerability. A study found that 100 mg of soy isoflavones daily for 6 months was well tolerated in elderly participants [14]. That’s a reasonably high dose, so supplements at standard doses appear safe for most adults.

Thyroid function. Laboratory and animal studies suggest that soy intake might increase the risk of clinical hypothyroidism in people who already have underactive thyroid or insufficient iodine intake [15]. If you have thyroid problems, talk to your doctor before taking soy supplements.

Pregnancy and breastfeeding. Safety during pregnancy and breastfeeding hasn’t been established. It’s best to avoid concentrated supplements during these times, though moderate consumption of soy foods is generally considered acceptable.

Breast cancer history. This is where things get complicated. While some research suggests soy isoflavones might actually reduce breast cancer recurrence [16], women with oestrogen receptor-positive (ER+) breast cancer are often advised to limit phytoestrogen intake, including soy isoflavones. The concern is that even weak oestrogenic activity could potentially stimulate hormone-sensitive cancers. Discuss this with your oncologist.

Endometrial concerns. Some researchers have raised questions about whether long-term, high-dose soy isoflavone use might affect the uterine lining, though this remains unproven [17].

Soy allergies. If you’re allergic to soybeans or soy products, avoid isoflavone supplements as they may trigger allergic reactions.

Antibiotic interactions. Since isoflavone metabolism depends on gut bacteria, antibiotic treatment may reduce their bioavailability and effectiveness.

Drug interactions. Avoid using soy isoflavone supplements with anti-oestrogen medications like tamoxifen. Animal studies suggest that high doses of soy isoflavones might interfere with how these drugs work [18].

References

Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016;8(12):754.
Li N, Wu X, Zhuang W, et al. Soy and Isoflavone Consumption and Multiple Health Outcomes: Umbrella Review of Systematic Reviews and Meta-Analyses of Observational Studies and Randomized Trials in Humans. Mol Nutr Food Res. 2020;64(4):e1900751. PubMed
Cui C, Birru RL, Snitz BE, et al. Effects of soy isoflavones on cognitive function: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2020;78(2):134-144. PubMed
Taku K, Melby MK, Kronenberg F, Kurzer MS, Messina M. Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause. 2012;19(7):776-790. PubMed
Nikander E, Tiitinen A, Laitinen K, Tikkanen M, Ylikorkala O. Effects of isolated isoflavonoids on lipids, lipoproteins, insulin sensitivity, and ghrelin in postmenopausal women. J Clin Endocrinol Metab. 2004;89(7):3567-3572. PubMed
Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake inhibits bone resorption and stimulates bone formation in menopausal women: meta-analysis of randomized controlled trials. Eur J Clin Nutr. 2008;62(2):155-161. PubMed
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;27(1):57-64. PubMed
Fang K, Dong H, Wang D, Gong J, Huang W, Lu F. Soy isoflavones and glucose metabolism in menopausal women: A systematic review and meta-analysis of randomized controlled trials. Mol Nutr Food Res. 2016;60(7):1602-1614. PubMed
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Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018;10(1):40. PubMed
Toth MJ, Tchernof A, Sites CK, Poehlman ET. Effect of menopausal status on body composition and abdominal fat distribution. Int J Obes Relat Metab Disord. 2000;24(2):226-231. PubMed
Ricci E, Cipriani S, Chiaffarino F, Malvezzi M, Parazzini F. Soy isoflavones and bone mineral density in perimenopausal and postmenopausal Western women: a systematic review and meta-analysis of randomized controlled trials. J Womens Health (Larchmt). 2010;19(9):1609-1617. PubMed
Basaria S, Wisniewski A, Dupree K, et al. Effect of high-dose isoflavones on cognition, quality of life, androgens, and lipoprotein in post-menopausal women. J Endocrinol Invest. 2009;32(2):150-155. PubMed
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