Even after having a number of health benefits of soybeans, some people have concerns about the consumption of soy products. These concerns are common, few of them are supported by sound science. While those observed as having negative effects, often followed by very large amounts of soy consumption.

Thyroid function

There is some concern that soy intake may interfere with thyroid function, possibly inducing hypothyroidism (low thyroid function).(5)

The thyroid is a large gland that regulates growth and controls the rate at which your body expends energy.(3)

A comprehensive review in 2006, of 14 studies in humans does not report any significant adverse effects of soy foods and Isoflavones consumption on thyroid activity in healthy adults (with normal thyroid function and sufficient iodine intake), whereas people with compromised thyroid function or insufficient iodine intake, also infants born with thyroid hormone deficiency may be considered at greater risk of developing hypothyroidism if they consume soy foods.(6, 1)

Since the publication of this review, two more long term randomized controlled trials (RCT’s) have been published. It also reported that daily supplementation with 54 mg of genistein had no significant effect on thyroid-antibody concentrations, thyroid-hormone concentrations, or thyroid-receptor activity.(6)

Soy is a goitrogenic food that prevents proper thyroid hormone production and causes the thyroid gland to enlarge. It may also impact the thyroid by other mechanisms such as, preventing the action of thyroid hormones throughout the body and reducing absorption of thyroid medications from the intestines.(4)

The human evidence suggests that soy foods do not have any significant impact on thyroid health. Neither the review nor the two more recent RCTs had thyroid activity as their primary outcome.(6)

The authors of the 2016 study indicate that soy is likely to be safe. But consuming excessive amounts may create health risks in some people, such as those with undiagnosed hypothyroidism. However, there’s no evidence that people who have hypothyroidism should avoid soy completely.

Summary 

There is no effect of Soy supplementation on the thyroid hormones and very modestly raises TSH levels, the clinical significance, if any. The rise in TSH level is unclear.(7) The evidence regarding the impact of soy on thyroid health is limited, also studies do not show any link between soy and TSH or thyroid function.

Cancer risk

In modern medicine, cancer diseases are the biggest challenge. Breast cancer is one of the largest problems in women.

Therefore, in recent times, phytoestrogens due to its broad spectrum of biological activity, is taken into consideration in the context of chemoprevention.(8)

Phytoestrogens are estrogen receptor agonists which can attach themselves to estrogen receptors and activate them, which is similar in function to human estrogen but with much weaker effects.(6)

As per a meta-analysis of epidemiological studies a negative effect was found between the soy isoflavone consumption and the risk of breast cancer to the pre and postmenopausal Asian women. This relationship was not observed in western women due to high intake of isoflavone by Asians not only in adulthood, but also from childhood. In addition, studies have reported that consuming soy and isoflavone from childhood may reduce the risk of breast cancer in later life.(8)

A meta analysis of randomized, placebo-controlled clinical trials determined the effect of soy isoflavone on breast density and breast parenchyma in post menopausal women. After 10 months it was found that daily 100 mg intake of isoflavone supplementation did not affect the breast density and breast tissues.

In post menopausal women who have already been diagnosed with breast cancer, the effects of soy depends on the type of breast cancer, the amount of isoflavones and its source, also the age at which consuming soy has been started.(6)

Summary

Some studies in the 2019 review, found that asian women had a reduced risk of cancer both before and after menopause who consumed soy isoflavones. Soy isoflavones may help reduce the growth and spread of hormone associated cancers.

Genetically modified Organisms

According to the World Health OrganizationTrusted Source (WHO), genetically modified organisms (GMOs) are “organisms (i.e. plants, animals or microorganisms) in which the genetic material (DNA) has been altered in a way that does not occur naturally by mating and/or natural recombination.”

People are concerned about potential side effects and long-term health consequences of eating GMO soy foods.

As per FDA, any genetically modified food has to take the same safety standards like traditionally grown plants. The nutritional content of genetically modified food is also taken into account. These nutritional content includes the macronutrient profile of protein, carbohydrates and fats, also the micronutrients such as vitamins, and minerals which are compared with traditionally grown food.

In the November 2018 study issue of the journal Food and chemical Toxicology found that GMO soybeans are as safe and nutritious as traditionally grown varieties.(9) No adverse effects were observed over the 90 days of study on rats who fed GMO soybean.

In 2014, according to the USDA Economic Research Service farmers used 94% of soybean farmland in the U.S. to grow genetically engineered soybeans. (10)

In one review, from the National Institutes of Health, researchers suggest that Genetically modified foods could potentially lead to illnesses that are resistant to antibiotics.(11) The amount of isoflavones may be lower in genetically modified soybeans. In addition, risks of GMOs are the transfer of allergens and the formation of new allergens.

Though there are many concerns about the health impacts and environmental safety of genetically modified crops. The WHO also state that “GM foods currently available on the international market have passed safety assessments and are not likely to present risks for human health.” (5)

Summary

More research is needed on the real health impact of genetically modified crops.If you are still concerned about GMO soybeans, choose 100% organic soy products or those labeled as non-GMO. The food which is certified as USDA organic cannot be made with genetically modified foods.

Feminizing effects in Men

The number of reports demonstrating adverse effects of soy isoflavones due to their estrogen-like properties, has increased.

There are some concerns that soy isoflavones may reduce the production of the male hormone testosterone. As per the theory, in the journal of acupuncture and meridian studies, it was observed that isoflavones can inhibit the activity of enzymes involved in testosterone production, thus lowering testosterone levels and increasing estrogen levels.(12)

Some research shows that regularly eating soy products like edamame, tofu, soy milk, and miso may cause a drop in testosterone levels. For example, one study in The Journal of nutrition, found that 35 men drinking soy protein isolate for 54 days resulted in decreased testosterone levels. (13) 

Two case reports describing feminizing effects occurred as a result of soy food consumption, have been published. The reports have documented adverse effects with ≈360 mg of soy isoflavones per day for 6–12 months. (14)

A 60-year-old man went to the endocrinology clinic for evaluation of bilateral gynecomastia of 6 months’ duration and he described a daily intake of 3 quarts of soy milk. He reported erectile dysfunction and decreased libido. This is a very unusual case of gynecomastia related to ingestion of soy products. Health care providers should review patients’ dietary habits to declare the analysis of medical conditions.(15)

Another case of a 19-year-old (type 1 diabetic) but otherwise healthy man vegan eating a soy-rich diet, indicates that soy product consumption is related to hypogonadism and erectile dysfunction.(16)

The two above mentioned case reports simply illustrate that consuming excessive amounts of essentially any food can potentially lead to abnormalities.(14)

A meta analysis of 15 placebo-controlled treatment groups with baseline and ending measures and an additional 32 reports involving 36 treatment groups reported no effects of soy protein or isoflavone intake on testosterone, sex hormone binding globulin, free testosterone or the free androgen index.(17) These studies used a variety of soy foods and protein powders that provided 10–70 g of soy protein and 60–240 mg of isoflavones per day.(6)

The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men.

A study in the Journal of the International Society of Sports Nutrition reported similar findings in young athletes. Evidence in the study suggests an inverse relationship between soy protein intake and serum concentrations of male sex hormones. 

Four groups of 20 young men, were supplemented with 50 g of protein from whey protein concentrate (no isoflavones), soy protein concentrate (138 mg of isoflavones), soy protein isolate (49 mg of isoflavones), or a whey-soy protein blend (24 mg of isoflavones) in combination with 12 week resistance-training program. At the end of week 12 Body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) were measured at baseline.(18)

There were no significant differences found in total or free testosterone, SHBG, estradiol, or estradiol-to-testosterone ratio between four groups of 20 young men after 12 weeks.(6)

Summary

Despite the fact that case reports have documented adverse effects from incredibly high daily intakes of soy for 6–12 months, appropriate intakes of soy foods and soy isoflavones do not affect men’s testosterone levels, estrogen levels, or fertility. Men who are at risk of developing prostate cancer might reduce their risk by eating soy foods, but soy foods do not appear to benefit men who already have prostate cancer.(6)

Soy infant formulas

Soy infant formula contains soy protein isolates and is fed to infants as a supplement to or replacement for human milk or cow milk. 

According to FDA, all baby formulas are required to fulfill certain criteria regarding their composition, purity, and nutrient content.(19)

Soy infant formulas include soy protein isolate with other nutrients. Also they contain the same amount of calories and important nutrients as other types of baby formulas. And hence, they have the same ability to meet a baby’s growth and developmental needs. Infants who suffer from dairy allergy or lactose intolerance may opt for soy infant formulas.(6, 20)

Due to the high amount of isoflavones present in soy based infant formulas (SBIF) the concern regarding its adverse effects are increasing. Babies fed soy formula (SF) receive more soy isoflavones than babies who are breastfed or fed with cow’s milk formula.(20)

Phytoestrogens are found in many food products in addition to soy infant formula, especially soy-based foods such as tofu, soy milk, and in some dietary supplements. 

In the relevant animal studies, various abnormalities in animals exposed to soy isoflavones were reported. This study examined the effects of isolated compounds, mainly genistein, whereas SF used for infant nutrition contains several other components, including different aglycones. Still  it’s important to note that estrogen is much more potent than soy isoflavones, and that animals metabolize soy isoflavones differently than humans.(21)

One study, of the journal of clinical endocrinology and metabolism (22) followed 410 infants fed only breast milk, cow-milk formula, or soy formula from birth to nine months. It was observed that serum estrogen levels were similar between groups, yet the uterine volume of the soy-fed infant girls was significantly altered in a way characteristic of high estrogen exposure.

As the implications of those findings are still unknown, the observed differences are not strong enough. This study serves as proof-of-principle for developmental effects of soy formulas. Studies are needed to further evaluate the effects of soy on child development.(6) 

As per the experimental studies, it results that SF-fed children, particularly the youngest, can have significant clinical problems. In most of the cases, poor or no negative effect has been observed. Soy infant formula was selected for National Toxicology Program (NTP) Evaluation of Risks to Human Reproduction (NTP CERHR) concludes that, there is “minimal concern for adverse effects on development in infants who consume soy infant formula” (21)

Finally, there is no conclusive evidence that soy formula (SF) can significantly affect human development, reproduction and endocrine function. Only hypothyroid children can have significant problems.(21)

Summary

Animal studies suggest that phytoestrogens of infants fed soy formulas are significantly higher than those of adults and may affect sexual development. Human studies being scarce, several researchers have warned that the use of soy formulas should be avoided or at least minimized. Soy formula is not an appropriate choice for babies born prematurely, nor those with low birth weights, poor renal function, or a cow’s milk allergy.(6)

Digestive issues

Despite the fact that soy is packed with lean protein (low in saturated fat), it also contains trypsin and protease inhibitors. These are the enzymes that make the digestion of protein incredibly difficult, which causes some gastric distress along with a deficiency in amino acid uptake if soy is eaten in excess. The only way to destroy these anti-nutrients is by soaking and cooking the beans.(23)

Soybean agglutinin (SBA) is a non-fiber carbohydrate-related protein and the main anti-nutritional factor that exists in soybean or soybean products. In one animal study of the International Journal of Molecular Sciences, researchers suggest that  the antinutrients in soy may reduce the gut’s barrier function, possibly resulting in inflammation and digestive issues. 

SBA declares negative effects on animal intestinal health by influencing the intestinal structure, barrier function, mucosal immune system, and the balance of the intestinal flora.But, still more human studies are needed to confirm this.(24, 2)

Summary

SBA negatively affects intestinal structure, intestinal permeability, mucosal immune system, and intestinal flora. With appropriate supplementation of soybean diets may improve the negative effects of SBA on the intestinal tract. Besides this, some oligosaccharide combinations can be selected to alleviate the intestinal structural and functional damage caused by SBA. (24)