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Is soy bad for you?

Soy often gets a bad rap as people are worried about the effect it has on oestrogen levels.

In fact, the evidence actually shows that eating soy is beneficial for health. Soy has been shown to have protective effects against some cancers, heart disease, stroke and has positive effects on bone health.

Soy is an excellent source of plant protein, as it contains all 9 essential amino acids. It is rich in fibre, vitamins and minerals. Soy may also help to lower LDL cholesterol (bad cholesterol) and has shown positive effects on bone health and menopausal symptoms.

Let's address the myths and the current evidence:

MYTH: Soy causes "man boobs" because soy has estrogen.

Truth: Soy contains a compound called phytoestrogens. Phytoestrogens are isoflavones, which is the plant form of estrogen. This is different from estrogen that is found in our body and does not cause feminising effects in men.

Evidence: A 2010 review of 9 studies found that soy did not affect estrogen levels in men.

"There is essentially no evidence from the nine identified clinical studies that isoflavone exposure affects circulating estrogen levels in men." (1)

MYTH: Soy Causes breast cancer

Truth: Phytoestrogens found in soy may reduce the risk of the development of breast cancer.

Evidence: A study conducted in 2009 with over 73,000 participants showed that those who consumed soy had a 59% lower risk of breast cancer compared to those who consumed lower amounts of soy foods. No significant association with soy food consumption was found for postmenopausal breast cancer. (2)

Findings also reported that women who consumed a high amount of soy foods consistently during adolescence and adulthood had a substantially reduced risk of breast cancer.

A 2013 review highlights that studies that highlight consuming soy decreases the risk of developing breast cancer are done in Asian rather than Western cultures. (3)

This is likely attributed to the greater amount of soy consumed in Asian diets compared to Western diets. In a typical Asian diet, the average daily intake of isoflavones is 25-50mg, whereas in Western diets the amount of soy isoflavones is less than 1mg per day. (3)

How much soy contains 50mg of isoflavones?

50mg of soy isoflavones:

  • About 200g of Tofu

  • About 100-150g of Tempeh

  • About three cups of whole bean soy milk

  • About 100g of cooked/canned soybeans

What about thyroid health?

Those who take thyroid medication do need to be mindful of the timing of their soy consumption. This is because soy can interact with thyroid medication.

It is recommended to avoid soy consumption within 3-4 hours of taking thyroid medication. Consult your health care professional regarding individualised advice.

There is little evidence to suggest that people who have hypothyroidism should avoid soy completely.

Hot Flushes:

There is also a small amount of evidence that shows soy foods may reduce the severity and frequency of hot flushes in post-menopausal women.

A review of 43 randomized controlled trials looked at the effects of soy phytoestrogens on hot flushes and night sweats in perimenopausal and postmenopausal women. (4)

Four of these trials found that an isoflavone supplement of 30mg reduced the frequency of hot flashes.

Summary & Recommendations:

  • The consensus is that consuming moderate amounts of soy foods (1-2 servings of soy a day) as part of a healthy and balanced diet does not pose any harmful effects and can in fact be beneficial for health.

  • Soy based foods are nutritious, as they are rich in protein, fibre, calcium and low in saturated (unhealthy) fats.

  • Include 1-2 servings of minimally processed soy-based foods into your diet daily, including: > Tofu > Tempeh > Edamame > Soy milk > Miso > TVP 1 serving is equivalent to approximately 150g of tofu/tempeh, 1 cup of soy milk and 1 cup of edamame.

  • Approach soy supplements with caution, as they contain concentrated sources of Isoflavones. Consult a healthcare professional prior to beginning any supplement regimen.

  • It is recommended to avoid soy consumption within 3-4 hours of taking thyroid medication. Consult your health care professional regarding individualised advice.

References: 1. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010 2. 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. Am J Clin Nutr. 2009 3. Mourouti N, Panagiotakos DB. 2013) 'Soy food consumption and breast cancer.' Maturitas (,

4. Br J Clin Pharmacol. Quantitative efficacy of soy isoflavones on menopausal hot flashes; 2015 Apr;79(4):593-604.

Author: This blog post was written by Melissa D'Elia (APD)

Book your free discovery call now to find out how Melissa can assist you.

Disclaimer: The content of this blog is not intended to be a substitute for professional medical advice, diagnosis or treatment. Please consult with your Dietitian or GP for individualised advice.

Copyright: 2022 Sprout Nutrition & Dietetics.


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