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Soy – Healthy or not?

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Soy, healthy or not?

A question I am often asked is whether soy foods are healthy or not, many people avoiding soy due to fears of breast cancer promotion, hormone disruption, thyroid issues or development of “man boobs”. But what does the science say?

Soy too much

What are soy foods?

Soy foods are derived from the soybean, a type of legume native to Asia, and include immature soybeans (edamame), mature soybeans, and products made from them e.g., tofu, tempeh, miso, natto, soy sauce, and soy milk. Soy can also be found in more processed protein isolate forms in supplements, some soy milks and tofu and vegan food products including cheeses, burgers, sausages and other meat substitutes.

Soybeans are one of 300 odd plant foods which contain phytoestrogens, naturally occurring plant compounds which when consumed, have a mildly estrogenic effect i.e., they have a similar chemical structure and function in a similar way to our own reproductive hormone estrogen. Estrogen is produced by women and to a lesser extent by men.

There are four main groups of phytoestrogens:

  1. Isoflavones – found in soybeans
  2. Lignans – found in nuts, seeds, legumes, cereals, fruits and vegetables
  3. Coumestans – found in sprouted beans, alfalfa, spinach
  4. Stilbenes – found in red wine, peanuts

Soybeans contain the main isoflavone components genistein, daidzein and glycitein.

How do phytoestrogens act in the body?

The concern around soy causing cancer relates to its mildly estrogenic properties with some forms of breast cancer originating from estrogen receptors found in breast tissue. However, we now understand that phytoestrogens act in a different way within the body compared to estrogen, having both estrogenic and anti-estrogenic capabilities. Soy can be consumed in moderate amounts without fear as part of a healthy plant-predominant dietary pattern.

Estrogen undertakes its role within the body by binding to proteins called estrogen receptors (ER’s) located in the cytoplasm of cells within our reproductive tissue. Upon activation by estrogen, ER’s are able to regulate the activity of various genes and tell the cells what to do and when.

Reproductive tissue cells contain two types of ER’s, each with different biological functions: ER-alpha (ER-A) and ER-beta (ER-B). When stimulated by estrogen, ER-A has the capabilities to tell cancer cells to keep growing. When stimulated, ER-B has an anti-estrogenic effect. Estrogen preferentially binds to ER-A whereas the isoflavones in soy tend to bind to ER-B. When isoflavones do happen to bind to ER-A, their signalling capacity is much lower than estrogen.

Soy & Breast Cancer Risk

Long-term clinical studies on women who consume high levels of soy (Asian populations) confirm that soy consumption is not linked to an increased risk of cancer, in fact it may have a protective effect. Women consuming the most soy are less likely to get breast cancer. This effect is thought to be partly due to the antiestrogenic, anticarcinogenic and anti-angiogenic properties of the isoflavones in soy.

One study found that women consuming 1 cup of soy milk or half a cup of tofu daily have a 30% lower risk of developing breast cancer compared with women who eat little or no soy.1

The Shanghai Women’s Health Study looking at the diets of more than 73,000 Chinese women shows strong evidence for a protective effect of soy foods against pre-menopausal breast cancer.2

Soy can also be protective of post-menopausal cancer with a 2014 meta-analysis finding that soy isoflavone intake could lower the risk of breast cancer for both pre- and post-menopausal women in Asian countries.3

Another study looking at the diets of Asian women in the US showed soy consumption at as little as one portion per day, beginning in childhood, resulted in the strongest reduction in breast cancer risk.4

Note: Women in western countries don’t typically eat much soy as Asian women so it’s difficult to compare between high and low levels of intake. Hence an association between intake of soy isoflavone and breast cancer in western women has not been found.

Soy & Breast Cancer Risk & Recurrence

What about women with a genetic predisposition or already diagnosed with breast cancer? Research suggests women who consume soy are less likely to have their cancer return vs those that avoid it with soy isoflavones potentially helping to keep estrogen signalling at healthy levels.

Soy is also considered safe for women with the BRCA positive gene and for women who have been diagnosed with breast cancer and are taking tamoxifen.5,6

The Women’s Healthy Eating and Living study found that breast cancer survivors who ate the most soy reduced their risk of cancer recurrence and death from breast cancer by 54%.7 Another study found that cancer survivors consuming the most soy milk and tofu had a 60% reduction in breast cancer recurrence and were 29% less likely to die from cancer vs women who consumed less.8

The American Institute for Cancer Research (IARC) states that it is safe to consume soy foods as part of a health dietary pattern, even after breast cancer diagnosis.

Protective Effects for Other Cancers

What about other cancers? Results from a 2022 meta-analysis of 81 prospective cohort studies identified an inverse relationship between higher intake of soy and soy isoflavones and cancer risk in a dose responsive manner i.e., the more soy, the less cancer.9

Another large analysis of all the human data on soy consumption reported a significant reduction in risk of multiple cancers including ovarian (48% reduction), gastric (37%), prostate (29%), breast (13%), colorectal (24%), endometrial (19%) and lung (17%).10 These observations may be explained by the fact that people consuming more soy foods have decreased circulating estrogen, hence reduced cancer risk.11

Thyroid Health

In healthy people, clinical trials show that soy products do not affect thyroid health. 12,13 There is a possibility that soy isoflavones may bind to iodine, which the body uses to make thyroid hormones although this is based on in vitro and animal studies, and older reports of goiter (enlarged thyroid) in babies fed soy formula not fortified with iodine.14 To take the cautious approach, people who consume soy may need slightly more iodine in their diets.

Iodine is found in seafood, cow’s milk (due to processing), packaged bread, sea vegetables (like nori and dulce), and iodized salt. If you eat a plant exclusive diet you may wish to have your iodine levels tested (a simple urine test) and supplement if required.

Male Hormones

The myth about growing “man boobs” from consuming soy originates from a clinical case of a 60-year-old man who presented to an endocrinology clinic with gynecomastia (breast enlargement). He also reported erectile dysfunction and decreased libido. It was later found that he was consuming nearly 3L of soy milk every day! After ceasing the soy milk, the gynecomastia resolved.15

According to clinical studies, soy products do not have adverse effects on men and may help prevent cancer. A 2010 meta-analysis found that neither soy products nor soy isoflavone supplements affect testosterone levels in men.16

Another 2009 meta-analysis showed that consuming more soy resulted in a 26% lower risk of prostate cancer (hormone-related cancer).17 A 2018 systematic review and meta-analysis showed the same results for prostate cancer: a statistically significant association between soy consumption and decreased prostate cancer risk.18

Reduced Menopausal Hot Flashes

In the 2021 WAVS (Women’s study for the Alleviation of Vasomotor Symptoms) study, post-menopausal women reporting two or more hot flashes per day were randomly assigned to a low-fat vegan diet including half cup cooked mature soybeans per day or to no diet changes for 12 weeks.

The combination of the low-fat vegan diet and the soybeans was associated with a 79% reduced frequency in total hot flashes with 59% of women becoming free of mod-severe hot flashes within 12 weeks.19 This effect is proposed to be due to the phytoestrogens in soy improving estrogen balance.

Other Benefits

Other potential health benefit from consuming soy may include:

  • Improved bone strength20
  • Reduced hip fracture risk from osteoporosis21,22
  • Reduced risk of fibroids23
  • Anti-inflammatory effects24
  • Reduced LDL cholesterol levels25,26

How much soy should I consume?

Breast cancer surgeon and author of Breasts: The Owner’s Manual, Dr Kristi Funk, encourages women to consume 1 cup soy milk (made from whole soybeans) or half cup of tofu per day starting in childhood or prior to the preteen and teenage years when breast tissue is developing.

For reduced menopausal hot flashes, Dr Nitu Bejekal, author of Finding Me in Menopause recommends consuming approximately 2-4 portions soy foods per day e.g.,1 cup soy milk, 80g tofu or tempeh, half cup mature soybeans, in conjunction with a low-fat plant-predominant diet for best results.

Two-4 portions per day of minimally processed soy foods would also be suitable for boys and men.

Focus on the least processed forms of soy for maximum health benefits e.g., edamame, tofu, tempeh and soy milk. Due to potentially adverse effects of high doses of phytoestrogens on cancer risk, Cancer Council Australia cautions against supplementary sources of soy and phytoestrogens, including isoflavone supplements and soy protein isolate, particularly for women with a history of breast cancer.

Additionally, salt-preserved fermented soy foods, such as miso, natto, and soy sauce, should be consumed only in small amounts, as there is some evidence linking these foods to an increased risk of stomach cancer.

References

  1. Wu AH, Yu MC, Tseng CC, Pike MC. Epidemiology of soy exposures and breast cancer risk. Br J Cancer. 2008 Jan 15;98(1):9-14. doi: 10.1038/sj.bjc.6604145. Epub 2008 Jan 8. PMID: 18182974
  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 Jun;89(6):1920-6. doi: 10.3945/ajcn.2008.27361. Epub 2009 Apr 29. PMID: 19403632
  3. Chen M, Rao Y, Zheng Y, Wei S, Li Y, Guo T, Yin P. Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies. PLoS One. 2014 Feb 20;9(2):e89288. doi: 10.1371/journal.pone.0089288. PMID: 24586662
  4. Korde LA, Wu AH, Fears T, Nomura AM, West DW, Kolonel LN, Pike MC, Hoover RN, Ziegler RG. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev. 2009 Apr;18(4):1050-9. doi: 10.1158/1055-9965.EPI-08-0405. Epub 2009 Mar 24. PMID: 19318430.
  5. Ko KP, Kim SW, Ma SH, Park B, Ahn Y, Lee JW, Lee MH, Kang E, Kim LS, Jung Y, Cho YU, Lee B, Lin JH, Park SK. Dietary intake and breast cancer among carriers and noncarriers of BRCA mutations in the Korean Hereditary Breast Cancer Study. Am J Clin Nutr. 2013 Dec;98(6):1493-501. doi: 10.3945/ajcn.112.057760. Epub 2013 Oct 23. PMID: 24153343.
  6. Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009 Nov;118(2):395-405. doi: 10.1007/s10549-009-0321-5. Epub 2009 Feb 17. PMID: 19221874
  7. Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011 May;20(5):854-8. doi: 10.1158/1055-9965.EPI-10-1041. Epub 2011 Feb 25. PMID: 21357380
  8. Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43. doi: 10.1001/jama.2009.1783. PMID: 19996398
  9. Fan Y, Wang M, Li Z, Jiang H, Shi J, Shi X, Liu S, Zhao J, Kong L, Zhang W, Ma L. Intake of Soy, Soy Isoflavones and Soy Protein and Risk of Cancer Incidence and Mortality. Front Nutr. 2022 Mar 4;9:847421. doi: 10.3389/fnut.2022.847421. PMID: 35308286; PMCID: PMC8931954.
  10. Li N, Wu X, Zhuang W, Xia L, Chen Y, Zhao R, Yi M, Wan Q, Du L, Zhou Y. Soy and Isoflavone Consumption and Multiple Health Outcomes: Umbrella Review of Systematic Reviews and Meta-Analyses of Observational Studies and Randomized Trials in Humans. Mol Nutr Food Res. 2020 Feb;64(4):e1900751. doi: 10.1002/mnfr.201900751. Epub 2019 Oct 14. PMID: 31584249.
  11. T. Kellis Jr. and L. E. Vickery, “Inhibition of Human Estrogen Synthetase (Aromatase) by Flavones,” Science 225 (1984): 1032–35.
  12. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar;16(3):249-58. doi: 10.1089/thy.2006.16.249. PMID: 16571087.
  13. Rizzo G, Baroni L. Soy, Soy Foods and Their Role in Vegetarian Diets. Nutrients. 2018 Jan 5;10(1):43. doi: 10.3390/nu10010043. PMID: 29304010
  14. Bruce B, Messina M, Spiller GA. Isoflavone supplements do not affect thyroid function in iodine-replete postmenopausal women. J Med Food. 2003 Winter;6(4):309-16. doi: 10.1089/109662003772519859. PMID: 14977438.
  15. Martinez J, Lewi JE. An unusual case of gynecomastia associated with soy product consumption. Endocr Pract. 2008 May-Jun;14(4):415-8. doi: 10.4158/EP.14.4.415. PMID: 18558591.
  16. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007. doi: 10.1016/j.fertnstert.2009.04.038. Epub 2009 Jun 12. PMID: 19524224.
  17. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr. 2009 Apr;89(4):1155-63. doi: 10.3945/ajcn.2008.27029. Epub 2009 Feb 11. PMID: 19211820.
  18. Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018 Jan 4;10(1):40. doi: 10.3390/nu10010040. PMID: 29300347
  19. Barnard ND, Kahleova H, Holtz DN, Del Aguila F, Neola M, Crosby LM, Holubkov R. The Women\’s Study for the Alleviation of Vasomotor Symptoms (WAVS): a randomized, controlled trial of a plant-based diet and whole soybeans for postmenopausal women. Menopause. 2021 Jul 12;28(10):1150-1156. doi: 10.1097/GME.0000000000001812. PMID: 34260478
  20. Lanou AJ. Soy foods: are they useful for optimal bone health? Ther Adv Musculoskelet Dis. 2011 Dec;3(6):293-300. doi: 10.1177/1759720X11417749. PMID: 22870487
  21. Koh WP, Wu AH, Wang R, Ang LW, Heng D, Yuan JM, Yu MC. Gender-specific associations between soy and risk of hip fracture in the Singapore Chinese Health Study. Am J Epidemiol. 2009 Oct 1;170(7):901-9. doi: 10.1093/aje/kwp220. Epub 2009 Aug 31. PMID: 19720865
  22. Zhang X, Shu XO, Li H, Yang G, Li Q, Gao YT, Zheng W. Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women. Arch Intern Med. 2005 Sep 12;165(16):1890-5. doi: 10.1001/archinte.165.16.1890. PMID: 16157834.
  23. Nagata C, Takatsuka N, Kawakami N, Shimizu H. Soy product intake and premenopausal hysterectomy in a follow-up study of Japanese women. Eur J Clin Nutr. 2001 Sep;55(9):773-7. doi: 10.1038/sj.ejcn.1601223. PMID: 11528492.
  24. Wu SH, Shu XO, Chow WH, Xiang YB, Zhang X, Li HL, Cai Q, Ji BT, Cai H, Rothman N, Gao YT, Zheng W, Yang G. Soy food intake and circulating levels of inflammatory markers in Chinese women. J Acad Nutr Diet. 2012 Jul;112(7):996-1004, 1004.e1-4. doi: 10.1016/j.jand.2012.04.001. PMID: 22889631
  25. Pipe EA, Gobert CP, Capes SE, Darlington GA, Lampe JW, Duncan AM. Soy protein reduces serum LDL cholesterol and the LDL cholesterol:HDL cholesterol and apolipoprotein B:apolipoprotein A-I ratios in adults with type 2 diabetes. J Nutr. 2009 Sep;139(9):1700-6. doi: 10.3945/jn.109.109595. Epub 2009 Jul 15. PMID: 19605528.
  26. Li SS, Blanco Mejia S, Lytvyn L, Stewart SE, Viguiliouk E, Ha V, de Souza RJ, Leiter LA, Kendall CWC, Jenkins DJA, Sievenpiper JL. Effect of Plant Protein on Blood Lipids: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. J Am Heart Assoc. 2017 Dec 20;6(12):e006659. doi: 10.1161/JAHA.117.006659. PMID: 29263032

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