Updated: Mar 1
After 15 years of medical practice and thousands of questions regarding nutrition, this is my favorite one: "is soy healthy?". Why did soy get such a bad reputation? Why do some actual real life medical doctors discourage their patients from consuming soy?
In this article, I'll summarize findings from a massive undertaking where international scientists and researchers came together and reviewed hundreds of trials and finally came to a scientific consensus about soy. I'll also answer all of your burning questions about soy, including:
is soy linked to cancer?
is soy adequate for children?
does soy consumption in men lead to "man-boobs"?
does soy affect thyroid function?
What is soy?
Soy foods, including soy milk and tofu, all originate from soy beans. Soy beans are a type of legume, like other beans, chickpeas or lentils and immature soy beans in the pod are called edamame. Soy beans are also used to make fermented soy foods, like miso, tempeh, natto and soy sauce. Although some processing goes into the making of soy derived foods, the processing is minimal. Food processing exists along a continuous spectrum. For example, tofu is made from dried soybeans that have been soaked, crushed and boiled in water. Although this is considered a processed food, this form of processing is minimal and doesn't change the chemistry of the food and so the nutrition contained inside has been preserved. For reasons that I’ll mention in a future post, I try to choose organic soy products whenever I can!
As a side note, you’ll notice that the previous illustration mentions that soy is a “complete“ protein. It also mentions that soy is “the” protein in the soy beans. Soy protein is a term given to soy isolate, where protein is obtained from soybean meal that has been dehulled and defatted. In terms of a protein being “complete”, this means that it contains all 9 essential amino acids. What we need to clarify is that all plant foods contain all of the 9 essential amino acids in different ratios and if you’re getting enough calories from a variety of plant foods, then you’re getting enough protein! The body can then recycle the different amino acids in order to synthesize any protein you need! So the terms “complete” or “incomplete” should be avoided since many seem to associate these terms with “good” or “bad”. This creates unjustified fears regarding getting adequate protein quantity and quality from plant foods. So let’s ditch the terms “complete” and “incomplete” and focus on diversity and whole plant sources of protein.
Why does soy get such a bad reputation?
Every year, thousands of scientific research papers are published on soy foods alone. Taking a look at the totality of data in humans is a daunting task. Soy unfortunately got its bad reputation from old research done on animals. That's correct, most of the research done on soy, in the early days, was done on rodents. It turns out that rodents don't digest soy and the isoflavones they contain the same way we do. Who knew!? Isoflavones, also called plant estrogens, are protective phytochemicals that are widely found in the plant kingdom, but are present in particularly high concentrations in soy. Isoflavones are interesting compounds and have been linked to protective properties in breast cancer, prostate cancer and help reduce bone density loss with aging. Sadly, when you study a rodent that metabolizes isoflavones differently than the subjects that you actually need the data for, it's no surprise that trying to interpret the results can create mass confusion. When researchers scrapped these flawed animal studies and focused solely on human research, the story was quite different and the public needs to be made aware of this. After the rodent studies came out and people found out that these compounds were "plant estrogens", the implications of the rodent studies were generalized to many hormone dependent medical conditions. This isn't necessarily a bad thing. Researchers simply made conclusions and recommendations based on the available science at the time, but science evolves and recommendations change. That's the beauty of science! But when old outdated recommendations linger, cue soy panic.
Is soy healthy?
Let's start by breaking down soy into its nutrients. It's a great source of plant-protein. Soy contains a higher quality protein than other plant sources and contain more protein than other legumes. They're also packed with phytochemicals, like isoflavones, and many vitamins and minerals. They contain both essential fatty acids and are a great source of healthy fats as well as a great source of fiber. Just looking at the individual nutrients it contains, soy seems to hit it out of the park. But as real scientists, only studies where soy foods are tested on actual people should be considered to emit reliable conclusions. Just so happens that there are tons of studies like that around the globe.
Soy can be studied by looking at populations that consume soy foods, or in a clinical setting, where soy isolates or isoflavone extracts can be tested against placebo. It would be hard to disguise tofu as a placebo and so population or cohort-based studies are easier with whole foods, while isolates or extracts are easier to study in a clinical setting. This way, the participants can be blinded to receiving either soy or placebo.
Soy and thyroid health
There are many speculations that soy consumption could negatively impact thyroid. The first paper published on thyroid function and soy consumption dates back almost 100 years. In clinical studies, it's crystal clear that soy foods do not impact the levels of T3 or T4, the two main thyroid hormones, even in patients with thyroid disorders.
Soy and estrogen
More than 10 years ago, a popular men's magazine published an article that stated that soy consumption could possibly lead to feminization in males. The magazine later retracted the article and corrected that statement, since it wasn't in line with the scientific literature. But soy's reputation was already damaged. That same magazine later published an article describing tofu as the new "king of protein". After looking at more than 400 human studies, studies looking specifically at soy consumption and testosterone levels in males found absolutely no association. There was no effect of soy on estrogen levels, fertility, menstrual cycles, ovulation, sperm counts or the growth of breast tissue. And no, men consuming soy will not have "man-boobs", or gynecomastia. Studies specifically in athletes found no difference in muscle gains and protein synthesis when comparing soy and whey protein isolates, but bro-science is pervasive in the gym, and the fear of soy remains. Over and over, soy has been found to be neutral or beneficial when looking at hormone dependent conditions and cancers, like breast cancer, endometriosis and menopause, including an almost 60-70% reduction in frequency and severity of menopausal hot flashes.
Soy and breast cancer
In the 1990's there was a study done on rodents that suggested that compounds in soy might stimulate the growth of existing breast cancer tumors. This damaged soy's reputation and many have never regained the confidence required to incorporate soy foods regularly in their diets. Since there were no well-designed human studies at that point, the scientific community adopted a reasonable, appropriate and cautious stance in regards to soy consumption. That was almost 30 years ago and scientific research has come a long way since then. Human studies have since refuted the previous animal study findings. Soy studies done in humans now clearly suggest that higher soy consumption is associated with a decrease in breast cancer incidence and recurrence, as well as the risk of dying from the disease in those with a breast cancer diagnosis. The official position of the American Cancer Society, the Canadian Cancer Society and the American Institute for Cancer Research is that soy is perfectly safe to consume in women that have received a breast cancer diagnosis. I would strongly recommend that everyone include soy foods in their daily diet.
Soy in children
Conducting clinical and placebo controlled trials in 6 year olds is complicated. That's why most studies on soy in children are more observational ones. Nevertheless, the scientific consensus is that soy is perfectly safe and healthy to consume in children, and suggests that consumption of as little as one serving a day in childhood is protective against getting cancer in adulthood.
My soy food prescription
There's a lot of misinformation out there about soy. Even doctors and health professionals seem confused about the message they should convey to their patients. I recently had a young patient and athlete that told me his personal trainer insisted that he should avoid soy to ensure he'd build adequate muscle. That's what's wrong with bro-science. It spreads like wildfire, yet no one is able to quote a single study backing these claims. Out in the real science world, there are researchers, toxicologists, scientists and physicians that have dedicated their careers and lives to studying soy. Let's just say that these people, the actual experts on all things soy, eat soy daily and feed it to their kids. That's where I choose to look for answers, not at my local gym.
My family eats soy foods daily and whether it's soy milk, tofu or edamame, we make sure to include 1-2 servings everyday. The evidence suggests that 1-2 servings per day are sufficient to reap the benefits, which is pretty realistic for most people. A serving is the equivalent of 1 cup of soy milk, 1/2 cup of tofu or an ounce of soy beans. Just on the basis of its nutrient content alone, soy should be regarded as a power house of nutrition. It contains high quality protein, healthy fats, fiber, vitamins, minerals and cancer fighting isoflavones.
Even in people already convinced in soy's benefits, the challenge is still finding delicious ways of preparing meals that contain it. That's why I've created a completely free recipe eBook containing over 20 of my family's favorite plant-based recipes. You can download it for free at plantbaseddrjules.com.
Thanks so much for reading! Spread the awesome news about the benefits of consuming soy!
Check out my website plantbaseddrjules.com and look for the “How To” section in the menu. There, you’ll find tips and tricks that helped me on my journey towards a plant-predominant diet. Everything there is completely free, no catches!
Look for me on the socials, @plantbased_dr_jules on Instagram and go like my Facebook Page, Plant-based Dr. Jules. If you’re looking for some fitness motivation and are curious to see what a plant-based athlete can accomplish, follow me, @maritimeninja, on my fitness account on Instagram or check out my fitness group on Facebook, called Maritime Ninja Warrior.
Thanks so much for reading!
Plant-Based Dr. Jules 💚🌱
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. doi: 10.1097/GME.0000000000001812. Epub ahead of print. PMID: 34260478.
Messina M. Soy foods, isoflavones, and the health of postmenopausal women. Am J Clin Nutr. 2014 Jul;100 Suppl 1:423S-30S. doi: 10.3945/ajcn.113.071464. Epub 2014 Jun 4. PMID: 24898224.
Martín Salinas C, López-Sobaler AM. Beneficios de la soja en la salud femenina [Benefits of soy in women's health]. Nutr Hosp. 2017 Oct 15;34(Suppl 4):36-40. Spanish. doi: 10.20960/nh.1569. PMID: 29156930.
Mourouti N, Kontogianni MD, Papavagelis C, Panagiotakos DB. Diet and breast cancer: a systematic review. Int J Food Sci Nutr. 2015 Feb;66(1):1-42. doi: 10.3109/09637486.2014.950207. Epub 2014 Sep 8. PMID: 25198160.
Kazemi A, Barati-Boldaji R, Soltani S, Mohammadipoor N, Esmaeilinezhad Z, Clark CCT, Babajafari S, Akbarzadeh M. Intake of Various Food Groups and Risk of Breast Cancer: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies. Adv Nutr. 2021 Jun 1;12(3):809-849. doi: 10.1093/advances/nmaa147. PMID: 33271590; PMCID: PMC8166564.
Fraser GE, Jaceldo-Siegl K, Orlich M, Mashchak A, Sirirat R, Knutsen S. Dairy, soy, and risk of breast cancer: those confounded milks. Int J Epidemiol. 2020 Oct 1;49(5):1526-1537. doi: 10.1093/ije/dyaa007. PMID: 32095830; PMCID: PMC8453418.
Messina M. Impact of Soy Foods on the Development of Breast Cancer and the Prognosis of Breast Cancer Patients. Forsch Komplementmed. 2016;23(2):75-80. doi: 10.1159/000444735. Epub 2016 Apr 12. PMID: 27161216.
Buja A, Pierbon M, Lago L, Grotto G, Baldo V. Breast Cancer Primary Prevention and Diet: An Umbrella Review. Int J Environ Res Public Health. 2020 Jul 1;17(13):4731. doi: 10.3390/ijerph17134731. PMID: 32630215; PMCID: PMC7369836.
Dandamudi A, Tommie J, Nommsen-Rivers L, Couch S. Dietary Patterns and Breast Cancer Risk: A Systematic Review. Anticancer Res. 2018 Jun;38(6):3209-3222. doi: 10.21873/anticanres.12586. PMID: 29848668.
Maumy L, Harrissart G, Dewaele P, Aljaber A, Bonneau C, Rouzier R, Eliès A. Impact des régimes alimentaires sur la mortalité et le risque de récidive de cancer du sein : revue de la littérature [Impact of nutrition on breast cancer mortality and risk of recurrence, a review of the evidence]. Bull Cancer. 2020 Jan;107(1):61-71. French. doi: 10.1016/j.bulcan.2019.08.009. Epub 2019 Sep 26. PMID: 31564476.
Xie M, Liu J, Tsao R, Wang Z, Sun B, Wang J. Whole Grain Consumption for the Prevention and Treatment of Breast Cancer. Nutrients. 2019 Aug 1;11(8):1769. doi: 10.3390/nu11081769. PMID: 31374888; PMCID: PMC6723813.
Mourouti N, Panagiotakos DB. Soy food consumption and breast cancer. Maturitas. 2013 Oct;76(2):118-22. doi: 10.1016/j.maturitas.2013.07.006. Epub 2013 Aug 2. PMID: 23916376.
Marcondes LH, Franco OH, Ruiter R, Ikram MA, Mulder M, Stricker BH, Kiefte-de Jong JC. Animal foods and postmenopausal breast cancer risk: a prospective cohort study. Br J Nutr. 2019 Sep 14;122(5):583-591. doi: 10.1017/S0007114519000072. Epub 2019 Mar 5. PMID: 30832747.
Zhang L, Huang S, Cao L, Ge M, Li Y, Shao J. Vegetable-Fruit-Soybean Dietary Pattern and Breast Cancer: A Meta-Analysis of Observational Studies. J Nutr Sci Vitaminol (Tokyo). 2019;65(5):375-382. doi: 10.3177/jnsv.65.375. PMID: 31666473.
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; PMCID: PMC5793271.
Messina M, Hamilton-Reeves J, Kurzer M, Phipps W. Effect of soy protein on testosterone levels. Cancer Epidemiol Biomarkers Prev. 2007 Dec;16(12):2795; author reply 2795-6. doi: 10.1158/1055-9965.EPI-07-2543. PMID: 18086792.
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.
Messina M, Lynch H, Dickinson JM, Reed KE. No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise. Int J Sport Nutr Exerc Metab. 2018 Nov 1;28(6):674-685. doi: 10.1123/ijsnem.2018-0071. Epub 2018 Oct 26. PMID: 29722584.
Messina M, Rogero MM, Fisberg M, Waitzberg D. Health impact of childhood and adolescent soy consumption. Nutr Rev. 2017 Jul 1;75(7):500-515. doi: 10.1093/nutrit/nux016. PMID: 28838083.
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.