Soy in the Spotlight: Sorting Facts from Fear

What foods contain soy?

Soy comes from the soybean, a food traditionally eaten in Asia for thousands of years—possibly dating back 9,000 years. It has long been prepared into foods like tofu, miso, soy sauce, tempeh, and edamame.

When discussing soy, it’s important to differentiate between whole food sources (edamame, soy nuts, tempeh, miso, tofu) and processed soy products (soy hot dogs, soybean oil, hydrogenated soy oils). Soy was not produced commercially for oil until the early 1900s.

Many traditional soy foods are fermented, which improves digestibility and supports the gut microbiome.

What makes soy unique?

Soy stands out because it has the highest isoflavone content of any plant. Isoflavones are a type of phytoestrogen.

Nutritionally, soy is well balanced:

• ~35% carbohydrates

• ~35% protein

• ~20% healthy fats

It is also one of the few plant foods that contains all nine essential amino acids.

Soy provides B vitamins, vitamin E, and minerals such as potassium, magnesium, calcium, phosphorus, and iron. Beyond isoflavones, soy contains beneficial phytochemicals like phytosterols, saponins, lecithin, lectins, α-Linolenic acid, and linoleic acid.

Isoflavones

Isoflavones are phytoestrogens with a chemical structure similar to estrogen, allowing them to bind (weakly) to estrogen receptors. Other phytoestrogens include lignans (flax, sesame) and coumestans/prenylflavonoids (alfalfa, hops).

The most studied soy isoflavones are genistein and daidzein:

• Genistein is metabolized in the gut into dihydrogenistein and 6′-OH-ODMA, which have gentle hormone-like effects.

• Daidzein is metabolized into equol and O-DMA. Equol can activate estrogen receptors, but only ~33% of people produce it naturally. O-DMA appears weaker but may support gut and metabolic health.

The gut microbiome plays a key role here—30–50% of people can produce equol via microbial metabolism.

Other plants rich in genistein and daidzein include:

• Kudzu root (Pueraria lobata / P. mirifica)

• Red clover (Trifolium pratense)

• Alfalfa (Medicago sativa)

• Chickpeas, lentils, peas, fava beans, mung beans, lupins, peanuts

Soy and Menopause

Because of their estrogen-like activity, soy products are often studied for menopause symptoms. But what does the evidence show?

Soy and Hot Flashes 

Some studies suggest 50–120 mg/day of soy isoflavones may reduce hot flashes and improve sleep, fatigue, and mood (Perna et al., 2016).

• A 2015 meta-analysis (Chen et al.) found isoflavones reduced hot flashes but not other symptoms.

• A 2024 systematic review (Gençtürk et al.) reported no benefit for hot flashes, but some improvement in depression.

• The North American Menopause Society (2023) does not recommend soy isoflavones as a reliable therapy for hot flashes.

Soy and Cognitive Function

Evidence shows little effect on memory or focus, except possible improvement in visual recall (Giampieri et al., 2022).

Soy and Bone Health 

Bone loss accelerates after menopause. Two meta-analyses (Akhlaghi et al., 2020; Inpan et al., 2025) found soy isoflavones may help preserve bone strength when taken for over a year.

Soy and Cardiovascular Health

A review of 56 RCTs (Wolters et al., 2020) suggests soy isoflavones may reduce LDL cholesterol and triglycerides while improving HDL. Effects on inflammation remain unclear.

Soy and Vaginal Health

Findings are inconsistent. Some studies show benefits (≥90 mg/day, especially with soy protein), while others do not. A review of 17 trials (Ghazanfarpour et al., 2016) concluded results depend on dose, duration, and product type.

Soy and Skin Health

Two RCTs (Tumsutti et al., 2022; Rizzo et al., 2023) found isoflavones may support skin health and reduce wrinkles.

Why is soy controversial?

Soy is a “tricky” food in the U.S. for several reasons:

• Soy production has increased >1,000% since 1960.

• Most U.S. soy is highly processed into oils and additives.

• Much of it is genetically modified and heavily treated with pesticides.

• Up to 60% of processed foods contain soy ingredients.

Soy and Breast Cancer

Many oncologists caution patients about soy, but what does research show?

• In animal and lab studies, isoflavones can stimulate breast cancer growth.

• However, population studies suggest the opposite—long-term soy intake appears protective.

For example:

• Women in Asia consume 40–50 mg/day of isoflavones, compared to 5–10 mg/day in the U.S.

• A meta-analysis found a 12% reduction in breast cancer deaths for every 5 g/day increase in soy protein.

• The Shanghai Women’s Health Study (13 years follow-up) found:

  • 22% lower breast cancer risk in women with the highest vs. lowest soy intake during adulthood.

  • Up to 47% lower risk for premenopausal breast cancer when soy was consumed during both adolescence and adulthood.

So far, randomized controlled trials have not confirmed a protective effect against breast cancer risk.

References

Akhlaghi, M., Ghasemi Nasab, M., Riasatian, M., & Sadeghi, F. (2020). Soy isoflavones prevent bone resorption and loss, a systematic review and meta-analysis of randomized controlled trials. Critical reviews in food science and nutrition, 60(14), 2327-2341.

Chen, M. N., Lin, C. C., & Liu, C. F. (2015). Efficacy of phytoestrogens for menopausal symptoms: a meta-analysis and systematic review. Climacteric, 18(2), 260-269.

Gençtürk, N., Bilgiç, F. Ş., & Kaban, H. U. (2024). The effect of soy isoflavones given to women in the climacteric period on menopausal symptoms and quality of life: Systematic review and meta-analysis of randomized controlled trials. Explore, 20(6), 103012.

Ghazanfarpour, M., Sadeghi, R., & Roudsari, R. L. (2016). The application of soy isoflavones for subjective symptoms and objective signs of vaginal atrophy in menopause: A systematic review of randomised controlled trials. Journal of Obstetrics and Gynaecology, 36(2), 160-171.

Giampieri, F., Godos, J., Caruso, G., Owczarek, M., Jurek, J., Castellano, S., ... & Grosso, G. (2022). Dietary phytoestrogen intake and cognitive status in southern Italian older adults. Biomolecules, 12(6), 760.

Inpan, R., Dukaew, N., Na Takuathung, M., Teekachunhatean, S., & Koonrungsesomboon, N. (2025). Effects of isoflavone interventions on bone turnover markers and factors regulating bone metabolism in postmenopausal women: A systematic review and meta-analysis of randomized controlled trials. Archives of Osteoporosis, 20(1), 1-20.

Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, Ji BT, Gao J, Gao YT, Zheng W. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study–. The American journal of clinical nutrition. 2009 Apr 29;89(6):1920-6.

Nachvak SM, Moradi S, Anjom-Shoae J, Rahmani J, Nasiri M, Maleki V, Sadeghi O. Soy, soy isoflavones, and protein intake in relation to mortality from all causes, cancers, and cardiovascular diseases: a systematic review and dose–response meta-analysis of prospective cohort studies. Journal of the Academy of Nutrition and Dietetics. 2019 Sep 1;119(9):1483-500.

Perna, S., Peroni, G., Miccono, A., Riva, A., Morazzoni, P., Allegrini, P., ... & Rondanelli, M. (2016). Multidimensional effects of soy isoflavone by food or supplements in menopause women: a systematic review and bibliometric analysis. Natural product communications, 11(11), 1934578X1601101127.

Rizzo, J., Min, M., Adnan, S., Afzal, N., Maloh, J., Chambers, C. J., ... & Sivamani, R. K. (2023). Soy protein containing isoflavones improves facial signs of photoaging and skin hydration in postmenopausal women: results of a prospective randomized double-blind controlled trial. Nutrients, 15(19), 4113.

Tumsutti, P., Maiprasert, M., Sugkraroek, P., Wanitphakdeedecha, R., & Bumrungpert, A. (2022). Effects of a combination of botanical actives on skin health and antioxidant status in post‐menopausal women: a randomized, double‐blind, placebo‐controlled clinical trial. Journal of cosmetic dermatology, 21(5), 2064-2072.

Wolters, M., Dejanovic, G. M., Asllanaj, E., Günther, K., Pohlabeln, H., Bramer, W. M., ... & Glisic, M. (2020). Effects of phytoestrogen supplementation on intermediate cardiovascular disease risk factors among postmenopausal women: A meta-analysis of randomized controlled trials. Menopause, 27(9), 1081-1092.