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Soy isoflavones

Supplement phytoestrogen women's healthbone & joint
Aka: genistein, daidzein, isoflavones
Brands seen in Singapore: NOW Foods, Nutricost, Swanson, Life Extension, 21st Century
Not medical advice. Check with your doctor or pharmacist before you start, stop or change the timing of anything you take.
How to use
What it’s for
Menopausal hot flushes and postmenopausal bone density
When to take
Daily
Studied for
Pooled trials had a median duration of 24 weeks
Pros & cons
Pros
  • NCCIH: probably helps postmenopausal bone density
  • 2024 meta-analysis of 63 RCTS: bmd up at 3 sites
  • 18 RCTS, 40–200 mg/day: no effect on thyroid hormones
Cons
  • NCCIH: supplement form not shown to cut cancer risk
  • Hot-flash efficacy disputed; nams does not endorse it
  • Only 1 of 4 RCTS lasting ≥2 years saw bmd improve
Sources
  1. NCCIH — Soy (what soy supplements contain) · read 2026-08-27
    what it says
    “Soy supplements may contain soy protein, isoflavones (compounds similar in structure to the hormone estrogen), or other components.”
  2. NCCIH — Soy (menopausal hot flashes) · read 2026-08-27
    what it says
    “Soy isoflavone supplements or soy protein may help to reduce the frequency and severity of menopausal hot flashes, but the effect may be small.”
  3. NCCIH — Soy (bone density) · read 2026-08-27
    what it says
    “Soy isoflavones probably have a beneficial effect on bone density in postmenopausal women.”
  4. NCCIH — Soy (breast cancer) · read 2026-08-27
    what it says
    “soy products in supplement form have not been shown to reduce breast cancer risk.”
  5. NCCIH — Soy (side effects and allergy) · read 2026-08-27
    what it says
    “Side effects of soy may include bloating, constipation, diarrhea, and nausea. Some people are allergic to soy.”
  6. NCCIH — Soy (pregnancy and breastfeeding) · read 2026-08-27
    what it says
    “The use of soy during pregnancy may be unsafe in amounts greater than those commonly found in foods.”
  7. NCCIH — Soy (interactions with medications) · read 2026-08-27
    what it says
    “Interactions between certain soy products containing high amounts of tyramine (such as tofu and soy sauce), and monoamine oxidase inhibitors … have been reported.”
  8. Perspective: Isoflavones — Intriguing Molecules but Much Remains to Be Learned about These Soybean Constituents (Adv Nutr 2025) — bone mineral density · read 2026-08-27
    what it says
    “a meta-analysis of 63 RCTs … showed that in postmenopausal women, isoflavones significantly improved BMD at the lumbar spine, femoral neck, and distal radius … of the 4 longer-term (≥2 y) RCTs … only 1 reported BMD improved.”
  9. Adv Nutr 2025 isoflavones perspective — hot-flash dose threshold (Taku 2012 meta-analysis of 13 RCTs) · read 2026-08-27
    what it says
    “These authors concluded that taking 50 mg/d total isoflavones is efficacious if at least ∼20 mg that amount is genistein.”
  10. Adv Nutr 2025 isoflavones perspective — professional society positions · read 2026-08-27
    what it says
    “With respect to hot flash alleviation, there continues to be disagreement about the efficacy of isoflavones … because of the variable findings, the North American Menopause Society does not endorse isoflavones.”
  11. Adv Nutr 2025 isoflavones perspective — Viscardi et al. systematic review of 40 RCTs on estrogenicity · read 2026-08-27
    what it says
    “in postmenopausal women, isoflavones (median trial duration, 24 wk; median isoflavone dose, 75 mg/d) were without significant effect on endometrial thickness, vaginal maturation index, and circulating levels of follicle-stimulating hormone and estradiol”
  12. Adv Nutr 2025 isoflavones perspective — thyroid function · read 2026-08-27
    what it says
    “a meta-analysis of 18 RCTs published in 2019 showed that neither soy nor isoflavone intake (doses ranging from 40 to 200 mg/d) affects circulating levels of free thyroxine or free triiodothyronine”
  13. HSA — Guidelines on Prohibited and Restricted Ingredients in Health Supplements and Traditional Medicines, GL-CHPB-1-012 Rev. No. 001 (Oct 2025) · read 2026-08-27
    what it says
    “The objective of this guideline is to highlight the substances of concern when used in HS and TM and is not meant to be an exhaustive list”
  14. iHerb Singapore — Soy Isoflavones category (default order, 1 - 14 of 14 results); first 5 distinct brands in page order · read 2026-08-27
    what it says
    “NOW Foods, Soy Isoflavones, 120 Veg Capsules … Nutricost, Soy Isoflavones, 150 mg … Swanson Vitamins, Soy Isoflavones, 750 mg … Life Extension, Soy Isoflavones … 21st Century, Soy Isoflavones Extract, Standardized”
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Updated 2026-08-29. Every line above traces to a numbered source. Kozen-X is not a medical device and does not give medical advice. It lists what published sources have written down — nothing more. A pair not listed is unrecorded, not cleared. Before you start, stop or re-time anything, talk to your doctor or pharmacist.