There is a lot of misinformation about soy out there. Will guys develop man boobs if they consume soy? Do the plant estrogens in soybeans affect the hormones in women and men? Some people believe that men who eat soybeans become more feminine, and hence refer to them as “soy boys”. Where did these concepts come from? Is soy really unhealthy? Let’s focus on high-quality evidence and separate fact from misconception.
What are isoflavones?
Soy contains a type of phytoestrogen (the prefix phyto meaning plant) called isoflavones. Phytoestrogen has a structure similar to mammalian estrogen, but is indeed a different chemical and has somewhat different effects on the body. Cells have two types of estrogen receptors, α and β. Type α estrogen receptors influence cell division, while type β estrogen receptors affect cell apoptosis (the programmed self-destruction of cells). Isoflavones can potentially influence these receptors.1 So, phytoestrogens can potentially influence processes related to our estrogen levels. Some argue that because of this, isoflavones could lead to increased cell division, potentially causing issues like cancer and other hormonal problems. These claims are based on mechanisms. As discussed in this post, focusing too much on mechanisms without thoroughly examining the overall effects and consumed dosages of a food can be very misleading. Hence, in the next section, we will discuss the overall effects of soy consumption, based on higher quality evidence.
Where does the fear regarding soy stem from?
The fear associated with soy seems to come primarily from two case studies. One case study looked at a 60 year old man who drank 3 L of soy milk every day and developed gynecomastia (the technical term for man boobs).2 Another case study looked at a 19 year old man with type 1 diabetes. This man suffered from erectile dysfunction and low libido seemingly due to his excessive soy consumption.3 In both cases, their symptoms gradually disappeared after they consumed less soy. So based on this evidence, should we conclude that eating soy is bad for our health? That would be a big logical leap. As explained in this post, case studies are not representative of the general population because they are usually conducted on just one person. The studies did not control for other variables and cannot prove cause and effect. We also need to consider dosage. These people were consuming 12-14 servings of soy every day. Most people do not consume anywhere near that amount of soy. Like any food, eating too much of it can be unhealthy. Even though drinking too much water is dangerous, we do not go ahead and conclude that water is bad for our health in all cases. These case studies do not prove that eating 2-4 servings of soy daily is unhealthy. So in summary, we can only conclude from these studies that in the particular case of these men, eating such high doses of soy can lead to negative outcomes.
Similarly, the fear surrounding soy stems from animal studies where animals were forced to consume high doses.4 In order to obtain useful information, we need studies that draw conclusions by observing large populations of humans consuming more standard quantities of soy while controlling for confounding variables. Fortunately, we already have this higher quality evidence, which I will present in the next section. As explained in this post, focusing on high-quality meta-analyses, randomised controlled trials, and prospective cohort epidemiological studies provides us with the most useful information.
Soy and men’s health
A meta-analysis of clinical studies concluded that consuming soy, soy-derived proteins, or plant estrogen has no significant impact on hormones such as testosterone and estradiol in men.5 At typical dosages, the isoflavones from soy do not lead to feminising effects in men. Isoflavones also do not have a significant effect on estrogen or sperm. Even though Asian men consume much more soy compared to men living in the West, it is still considered safe to exceed the typical consumption levels of Asian men.4
A meta-analysis of epidemiological studies concluded that consuming soy reduces the risk of prostate cancer. In the case of Western men, the incidence of prostate cancer remained consistent, however, Asian men had a 48% lower risk of developing prostate cancer. As mentioned previously, Asian men generally consume more soy consistently throughout their lifetime and may therefore reap greater benefits.6 In particular, non-fermented soy foods like tofu, soy milk, and edamame reduce the risk of prostate cancer. However, fermented soy foods like miso and soy sauce do not seem to reduce the risk.7
Soy and women’s health
A meta-analysis of epidemiological studies concluded that consuming soy reduces the risk of breast cancer. In the case of Western women, the incidence of breast cancer remained consistent, but postmenopausal Asian women had a 54% lower risk of developing breast cancer. Pre-menopausal Asian women had a 37% lower risk of breast cancer.8 A prospective cohort study from China found that for every increase in daily intake of isoflavones by 10 mg, the risk of breast cancer decreased by 3%.9 10 mg of isoflavones can be found in about 3 grams of tofu or 86 mL of soy milk. Another meta-analysis of randomised controlled trials concluded that consuming soy may also increase the likelihood of survival for women diagnosed with breast cancer.10 In addition to breast cancer, soy consumption may be beneficial for reducing symptoms associated with menopause. A meta-analysis of randomised controlled trials concluded that consuming soy can reduce the frequency and intensity of hot flashes.11
Soy and chronic disease
A review of meta-analyses concluded that compared to animal-based foods, soy-based foods reduce the risk of heart disease, cancer, and type 2 diabetes.12
A meta-analysis of epidemiological studies concluded that consuming soy reduces the risk of heart disease and stroke. In Asian populations, soy had an even greater protective effect, likely due to the high average intake.13 A meta-analysis of randomised controlled trials showed that consuming soy lowered LDL cholesterol levels in adults by 3-4%. As will be explained in a future post, high LDL cholesterol is an independent risk factor for heart disease. Individual trials may not always capture the full effect of a single food, which is why many individual studies have concluded that soy has little effect. However, meta-analyses, which combine and analyse data from multiple studies, provide a more comprehensive view of the overall effects.14 Similarly, another meta-analysis of randomised controlled trials concluded that consuming soy can lower blood pressure.15
A meta-analysis of prospective cohort studies found that people who consumed more legumes had a 9% lower risk of developing colorectal cancer. In Asian populations, the decrease in the incidence of colorectal cancer was 17%. For those who consumed soy in particular, the decreased risk was 15%.16 However, it is important to note that not all forms of soy have the same cancer preventing effect. For example, consuming 1-5 servings of miso soup daily may increase the risk of stomach cancer due to its high salt content.17
Another meta-analysis of randomised controlled trials concluded that soy isoflavones can increase bone density and decrease bone resorption related to osteoporosis.18 Additionally, soy is high in protein, iron, calcium, zinc, and fiber, and is low in saturated fat. Overall, consuming 2-4 servings of soy daily can provide health benefits.19
Does soy affect thyroid hormone?
If a healthy individual consumes soy, there is no effect on the thyroid. However, soy-based foods can lower the absorption of thyroid hormone. Therefore, individuals with hypothyroidism may need to consume higher doses of thyroid hormone if they also consume soy. Additionally, it is important for these individuals to ensure sufficient iodine intake.20 Overall, most people do not need to worry about this effect, and only those with this condition should give more consideration to their soy, thyroid hormone, and iodine intakes.
What about genetically modified soybeans?
Most commercially grown soybeans for human consumption are not genetically modified. In fact, most genetically modified soybeans are used as feed for farm animals. We do not yet know if genetically modified soybeans are harmful to our health. If you would like, you can take a cautious approach until more data is published and check the type of soy in a product before purchasing.
Can children and babies safely consume soy ?
Soy-based infant formulas support the healthy growth of infants at a similar rate to dairy milk-based formulas. There is no strong evidence suggesting negative effects of phytoestrogens on babies, but it would be prudent to take a cautious approach until more research is published. Therefore, it is not recommended for infants under 6-24 months old to consume soy-based formulas. If possible, breastfeeding is the best option.21
What if I have a soy allergy?
If you have a soy allergy, it is important to avoid consuming soy. Instead, you can obtain protein and a variety of health benefits from other legumes such as lentils, chickpeas, mung beans, and kidney beans.
Conclusion
We need more research on the effects of consuming soy by pregnant women and infants. However, current evidence suggests that consuming an appropriate amount of soy does not show any significant negative effects. Concerns about soy have arisen from low-quality case studies, mechanistic evidence, and animal experiments. There is ample high-quality evidence opposing these fears which show that soy consumption at normal doses does not lead to issues like gynecomastia (man boobs), erectile dysfunction, low libido, or hormonal problems in men. In fact, consuming 2-4 servings of soy daily can provide health benefits. For instance, incorporating soy into our diet can reduce the risk of developing conditions such as heart disease, colorectal cancer, breast cancer, prostate cancer, type 2 diabetes, and osteoporosis. However, soy varieties that are high in salt, such as miso and soy sauce, are less healthy compared to soy products such as tofu, soy milk, tempeh, and edamame. If you are diagnosed with hypothyroidism, you can still consume soy, but you should also increase iodine intake and avoid taking thyroid hormone close to soy-based meals. Even though humans generally do not eat genetically modified soybeans, there is no strong scientific evidence to justify fearing the consumption of genetically modified soy.
References
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