Episode 43 – Is Soy Good or Bad for Your Health?

Key Topics Covered


Common foods that contain soy:

  • Edamame
  • Tofu
  • Tempeh
  • Textured Vegetable Protein
  • Soy milk
  • Foods fortified with soy protein  

Australians consume on average 2 serves of soy or soy-containing foods per week which were equal to 1-2mg of phytoestrogens per day.

**The terms phytoestrogens and isoflavones are interchangeable.

Proposed negative impacts

  1. Feminizing effects in men (increase estrogen, decrease testosterone) 
  2. Infertility in women  
  3. Effects on thyroid function  
  4. Cancer risk  

Soy and Men’s Health 

  • A case study several years ago spiked an interest, suggesting that soy has feminising effects of males.
  • 60 year old man drank 3L of soy milk per day (equivalent ~360mg isoflavone) 
  • Developed gynecomastia and his circulating estrogen levels had increased – erectile dysfunction and decrease libido 
  • Once he stopped drinking soy milk, his breast tenderness resolved and his estradiol concentration slowly returned to normal. 
  • This one case study is the only evidence we have that soy intake at these levels may effect men’s health negatively 
  • Oestrogen levels: A 2010 review found that the data from nine different clinical studies showed that isoflavone exposure from eating soy did not affect circulating estrogen levels in men.  
  • Prostate cancer: Isoflavones have also been associated with lower rates of prostate cancer. This was found in a meta-analysis of over 30 case-control and cohort studies from the US, Japan, Europe, and China.  
  • Fertility: Research into soy and male fertility has also not shown no significant correlation or impact 
  • Take-away for men: Consuming soy in moderate amounts is safe and may be beneficial to heath  

Soy and Women’s Health 


  • All reproductive tissues in the body can theoretically be acted upon by phytoestrogens.  
  • Increased estrogen signaling in these tissues may disrupt the usual hormonal cascade and the menstrual cycle.  
  • Generally research in this space shows a positive impact on fertility and pregnancy outcomes  
    • BUT, 3 separate case studies of women consuming an estimated amount of 100g of isoflavones/day had abnormal uterine bleeding, loss of period and infertility.
    • In all 3 cases, symptoms improved after soy was taken out of their diet.
  • Maybe some people are more sensative than others?

Breast Cancer

  • The largest and most detailed study to investigate soy intake and the risk of breast cancer is The Shanghai Women’s Health Study.  
  • This study followed over 70,000 Chinese women for over seven years. 
  • Women who ate the most soy had a 59% lower risk of  breast cancer compared to those who ate the lowest amounts of soy. 

Soy and Thyroid Function  

  • A systematic review from 2019 showed that there was a significant change in TSH as a result of soy supplementation although it was not clinically significant. 
  • In one other study with patients who had subclinical hypothyroidism compared the effect of low dose phytoestrogen (2mg comparable to Western diet) with higher dose phytoestrogen (comparable to a vegetarian diet), there were no signifcant changes in thyroid function test as a group.  
    • However, there was a 3-fold increase in progression from subclinical to overt hypothyroidism in the vegetarian group (2 VS 6) 
    • This suggests that soy could have more effect on people with compromised thyroid function 
  • Key take away – Those with low thyroid function may benefit from limiting soy intake, although if it is managed with medication, it is likely okay.
  • NOTE soy can interfere with thyroid medication and should not be taken within a 3-4 hour window when medication is taken 

Proposed Benefits of Soy 

  1. Protein rich food, beneficial for vegans and vegetareans
  2. Taste + also adding another option in general 
  3. Complete amino acid profile  
  4. Great leucine content for plant based option 
  5. Breast cancer? Prostate cancer? 

Menopausal Symptoms

  • Soy isoflavone may reduce episode of hot flushes 
  • Four trials found that 30 mg or greater of isoflavones in supplement form consistently reduced the frequency of hot flashes.  
  • But evidence is not conclusive  

Post-Menopausal Bone Loss

  • The mild estrogen effect of soy isoflavone may be a positive thing for postmenopausal women who see a decrease in estrogen levels  
  • Link between estrogen levels and bone loss 
  • In one cross-over study, over a 50-day period, isoflavones (105 mg/day) increased bone calcium content by 7.6%, approximately half the increase noted in response to risedronate, a treatment commonly used in cases of osteoporosis. 


  • >100mg of isoflavones from soy may interfere with female fertility  
  • Feminizing effects in men have not been found in the research 
  • Phytoestrogens in soy may be protective against breast and prostate cancer  
  • In post-menopausal women soy intake may reduce hot flashes and assist with maintaining bone health  
  • For those with hypothyroidism and those taking thyroid medication and that wish to consume soy products should consult with their doctor first for individualized advice  

Useful Links/ Resources

Studies Mentioned:

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