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Soy Isoflavones

What are soy isoflavones?

Soy isoflavones, which include daidzin, genistin and glycitin, are flavonoids occurring in soybeans and soybean products [1]. Soy isoflavones are considered phytoestrogens–plant-derived compounds with a mild estrogenic activity [2]. In the human intestine, they are digested or fermented into daidzein, genistein and glycitein [2].

Soy Products High in Isoflavones

  • Soy protein concentrate
  • Boiled soybeans
  • Soy milk
  • Miso
  • Tempeh
  • Tofu
  • Reference: [2]

Soy Infant Formulas

Soy Isoflavones Supplements

Soy isoflavones supplements without prescription (over-the-counter) are available.

Soy Isoflavones Health Benefits

Soy isoflavones from foods or supplements are POSSIBLY INEFFECTIVE [5] in the prevention or treatment of cancer (colorectal) [5], fibromyalgia, male infertility [3] (high intake of soy may decrease sperm concentration) [4] and rheumatoid arthritis, or in reducing muscle soreness after exercise.

There is INSUFFICIENT EVIDENCE [2,5] about the effectiveness of  soy isoflavones supplements in the prevention or treatment of asthma, atherosclerosis (artery hardening), breast pain (mastalgia), cancer (breast, endometrial, lung, prostate, thyroid), Crohn’s disease, diabetes mellitus type 2 [9,10], viral diarrhea in infants [11], heart disease, high blood pressure, high cholesterol [8], infant colic, hot flashes in menopausal women [6,7], inflammation [10], kidney disease [12], metabolic syndrome, migraines, osteoarthritis [2], osteoporosis, premenstrual syndrome (PMS), sun-damaged skin, wrinkled skin or in improvement of cognitive function [13], memory or muscle strength or in promoting weight loss.

Soy Isoflavones Safety: Toxicity, Side Effects

Soy extract supplements are POSSIBLY SAFE when used up to 6 months [5]. The long-term safety of consumption of large amounts of soy products is not known [2].

Side effects may include nausea, constipation or bloating [5].

During Pregnancy

Soy isoflavones in the medicinal amounts are POSSIBLY UNSAFE during pregnancy [5]. Not enough is known about the safety of  soy isoflavones supplements during breastfeeding, so women in this period should avoid them [5].

Soy Infant Formulas

No overt harmful effects of soy-based infant formulas have been recognized, so far [14].

Who else may need to avoid soy isoflavones supplements?

Individuals with the following health conditions may need to avoid soy isoflavones supplements [5]:

  • Breast, bladder or endometrial cancer
  • Hypothyroidism
  • Asthma or hay fever
  • Kidney stones
  • Kidney failure
  • Diabetes treated with glucose-lowering drugs
  • Cystic fibrosis

Soy Isoflavones-Drug Interactions

MAO inhibitors (antidepressants). Soy products, which contain tyramine, may cause dangerous increase of blood pressure if taken along with MAO inhibitors [5].

Consuming large amounts of soy products may decease the effectiveness of estrogen pills, tamoxifen and warfarin [5].

  1. Flavonoids  Linus Pauling Institute
  2. Soy isoflavones  Linus Pauling Institute
  3. Hamillton Reeves JM et al, 2010, Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis  PubMed
  4. Chavarro J et al, 2008, Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic  PubMed
  5. Soy uses, side effects  WebMD
  6. Seidl MM et al, 1998, Alternative treatments for menopausal symptoms. Systematic review of scientific and lay literature  PubMed
  7. Lethaby A et al, 2013, Phytoestrogens for vasomotor menopausal symptoms  Cochrane
  8. Qin J et al, 2013, Isoflavones for hypercholesterolaemia  Cochrane
  9. Gilbert ER et al, 2013, Anti-diabetic functions of soy isoflavone genistein: mechanisms underlying effects on pancreatic β-cell function  PubMed Central
  10. Charles C et al, Effects of High-Dose Isoflavones on Metabolic and Inflammatory Markers in Healthy Postmenopausal Women  PubMed Central
  11. Donovan SM et al, 2009, Soy formula and isoflavones and the developing intestine  PubMed
  12. Jing Z et al, 2015, Effects of soy protein containing isoflavones in patients with chronic kidney disease: A systematic review and meta-analysis  PubMed
  13. Zhao l et al, 2007, WHI and WHIMS follow-up and human studies of soy isoflavones on cognition  PubMed
  14. Canadian Paediatric Society, 2009, Concerns for the use of soy-based formulas in infant nutrition  PubMed Central

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