Soy.
A simple word can evoke extremely different reactions from different people. Your vegan friends might get excited and start sharing soy-based recipes with you. Your weight-lifting friends might react as if you were trying to poison them. Your Aunt might tell you that she heard a doctor talk about the miracles of tofu on daytime TV before, but she’s still not sure what tofu is in the first place. So what is the real truth about soy?
Unfortunately, the truth behind most things is highly nuanced and isn’t exactly “sexy.” That’s where I come in: I’ve done the dirty work and dug through the research to paint a clear picture of what soy can (and can’t) do to your health. Let’s get into the facts, fiction and origins of some of the most popular myths surrounding soy.
NUTRIENT CONTENT OF SOY
When it comes to legumes, soy is the unicorn of the bunch. Soy tends to be higher in protein and significantly lower in carbs than its plant-based brethren, which sounds like a calorie-counter’s best friend. Protein for gains and satiety; low-carb for fat loss. From this standpoint, it’s a win-win. A frequently debated topic about soy involves its protein content.
AMINO ACID PROFILE OF SOY PROTEIN
All protein, including human muscle, is composed of 20 different individual Amino Acids (AA). Luckily, the body can synthesize some of these unique building blocks on its own. In fact, 11 different AA can be synthesized within your own body, which is why they’re considered to be “non-essential.” The remaining nine AA cannot be produced by the body, making them “essential” amino acids (EAA). They must be consumed through the diet in order to aid in building new muscle tissue. For this reason, good sources of protein should contain as much EAA as possible in order to supply the building blocks that your body can’t produce. Contrary to popular belief, the EAA profile of soy compares pretty favorably to that of human muscle tissue.
For more information on protein and AA requirements, check out my article “Vegan Protein For Maximum Muscle Gains – Complete Scientific Guide”
SOY AND HORMONES
If soy’s health impacts weren’t controversial enough, its impact on hormones (i.e. testosterone and estrogen) is on a whole different level. If conflict is your thing, just Google “soy” and “hormones” and sit back as the arguments and pseudo-science flood your screen. The culprits of this controversy are isoflavones (phytoestrogens), small molecules found in soy that have a similar molecular structure to the hormone estrogen. Because of these similarities, isoflavones can bind to estrogen receptors and mimic the function of estrogen.
Do men really grow breast when they eat soy? - what science says on the danger of soy estrogen.
Unfortunately much of the confusion behind the estrogenic effects of soy come from research done in vitro as opposed to in vivo. Understanding the difference between these two terms is paramount to explaining this controversy.
In vitro is research done outside of a living organism in a controlled environment such as a petri dish, therefore it does not show what will happen in the human body.
In vivo research refers to studies carried out within a living organism such as a human or a mouse. This is one reason why it is important to know how to choose which studies to base your conclusions on.
In Vitro and Isolated Cell studies are often behind the reports that depict negative effects of soy. The effects are often just theoretical and based on data showing what molecules bound to each other in a test tube. Some Isolated Cell studies even suggest soy can promote breast cancer. But in both scenarios, these findings have never been replicated nor confirmed in humans. Why do some studies use such high isoflavone doses that are impossible to consume on a daily basis? Isoflavones are often used for medical purposes, such as in drugs that regulate prostate cancer or post- menopausal symptoms. These astronomical doses only make sense in a medical context.
Now to get into the research and what we can extrapolate based on well-designed studies, reviews and meta analysis (these articles take research from multiple primary sources and put them into one article summarizing the findings).
One of the major concerns when it comes to soy and hormones: Will men really grow man boobs from consuming soy?
Some animal studies actually support this claim (4). One study reported that feeding male baby monkeys with soy formula decreased their testosterone (poor little guys...). But even though we share more than 95% of our DNA with our furry friends, monkeys have the unique ability to convert isoflavones into a molecule that has a higher estrogenic activity. Humans can’t produce this molecule, which is why this finding can’t be replicated in human studies with reasonable intakes of soy. So as you can see, extrapolating claims from soy research in animals is very misleading because it does not always apply to humans.
Based on the results of a meta-analysis analyzing 32 individual studies on the effects of soy protein/isoflavone consumption on male sex hormones, consuming up to 70 grams of soy protein per day doesn’t appear to have any negative effect (7). This means you can feasibly eat up to 350 grams (0.8 lbs) of tempeh, 700 grams (1.6 lbs) of tofu or 100 grams (0.23 lbs) of soy protein powder or drink up to 10 cups of soy milk each day and be just fine.
Now for women. Many of us have heard the claim that soy consumption can potentially increase breast cancer risk. Is this myth or fact?
The interesting (and confusing) thing about soy is that specific phytoestrogens can both mimic the function of estrogen and block it. This is particularly important in relation to breast cancer, since estrogen makes breast cancer tissue grow. High estrogen would be bad in this scenario, but would the phytoestrogens from soy exacerbate that problem or help cancel it out?
In the animal studies, cancer cells were implanted into the animals and a phytoestrogen-heavy diet made these cancer cells grow. This finding provides evidence that phytoestrogens can act as promoters for animals that already had cancer cells. It does not show that phytoestrogens cause cancer, as this experiment skips the first step of the cancer development (initiation) entirely. Phytoestrogens made the already present cancer cells grow but did not cause cancer.
Luckily, human studies show exactly the opposite. Soy food consumption seems to have either positive or neutral effect on women with breast cancer:
• Consuming soy products early in life can be protective against breast cancer (22, 23).
• A study examining 9,514 American and Chinese women who survived breast cancer found that consuming more than 10 mg isoflavones a day (>30 grams/1 oz tofu) can reduce the risk of cancer recurrence (24).
• A meta-analysis analyzing studies on 11,206 women who survived breast cancer concluded that soy consumption reduces mortality and cancer recurrence. It was not possible to say how much soy has to be consumed to have this beneficial effect (25). In sum, soy consumption seems to have either a neutral or positive effect when it comes to breast cancer development or reoccurrence in women.
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References for this article
4. Messina, M. Soybean isoflavone exposure does not have feminizing effects on men: A critical examination of the clinical evidence. Fertil. Steril. 93, 2095–2104 (2010).
7. Hamilton-Reeves, J. M. et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: Results of a meta-analysis. Fertil. Steril. 94, 997–1007 (2010).
22. Chen, M. et al. Association between Soy Isoflavone Intake and Breast Cancer Risk for Pre- and Post- Menopausal Women : A Meta-Analysis of Epidemiological Studies. 9, (2014).
23. Ziaei, S. & Halaby, R. Dietary Isoflavones and Breast Cancer Risk. 1–11 (2017). doi:10.3390/medicines4020018
24. Nechuta, S. J. et al. Soy food intake after diagnosis of breast cancer and survival : an in-depth analysis of combined evidence from cohort studies of US and Chinese women 1 – 3. Am J Clin Nutr 96, 123–32 (2012).
25. Chi, F. et al. Post-diagnosis Soy Food Intake and Breast Cancer Survival : A Meta-analysis of Cohort Studies. 14, 2407–2412 (2013).