Key Topics Covered
Common foods that contain soy:
- 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
- Feminizing effects in men (increase estrogen, decrease testosterone)
- Infertility in women
- Effects on thyroid function
- 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?
- 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
- Protein rich food, beneficial for vegans and vegetareans
- Taste + also adding another option in general
- Complete amino acid profile
- Great leucine content for plant based option
- Breast cancer? Prostate cancer?
- 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
- Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature
- Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis
- Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study
- Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function
- Vertebral fracture risk reduction with risedronate in post-menopausal women with osteoporosis: a meta-analysis of individual patient data
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