Soy is a very interesting food to discuss. Very few foods fall into this category where one group of people claim it is one of the healthiest foods to eat, while others claim it is one of the worst foods to eat. What’s going on here? Well, to some degree they are both correct. Soy does contain phyto-estrogens (isoflavones) which have been shown to support cardiovascular and bone health, and to have anti-cancer properties. However, because this particular phytonutrient is a phyto-estrogen, it can affect our hormones.
Mixed Research on Soy's impact on Cancer and Hormones
There has been a great deal of research done on these phytoestrogens (isoflavones). Results from these studies are very mixed when it comes to cancer risk. A review article showed that some studies reported an increase in cancer risk while others show a decrease in cancer risk (1). Quite a bit of research shows that hormones may be negatively affected by soy consumption. Of particular interest is research on how soy negatively impacts the reproductive system/infertility (2, 3, 4, 5, 6, 7, 8, 9, 10) and libido (11, 12, 13, 14).
Still, research in this area is not conclusive since not all studies report a negative impact on reproductive function (15). On the other hand, research which focuses on the impact of phytoestrogens on cardiovascular disease and bone health are positive or neutral with no negative effects reported (1). Soy is often recommended to help improve menopausal symptoms, but research in this area is also mixed. Some studies show improvements in symptoms such as hot flashes, while others have shown no effects (1). It is interesting to note that the placebo effect is very strong in many of these studies that focus on the effects of phytoestrogens on menopausal symptoms (16, 17, 18, 19, 20). Just to connect the dots, when the placebo effect is strong, it is likely that the belief in the supplement (not the supplement itself) is what is causing a large degree of the benefits.
Genetics and Gut Health Impact
Only 30-50% of people can convert one of these phytoestrogens into its active form, equol (21-22), which may result in different health outcomes in equol producers versus non-equol producers (23). It is believed that the ability to convert to equol is based on genetics, diet, and gut physiology. This may be one reason why outcomes to studies on soy are very mixed.
Soy's Impact on the Thyroid
Soy also may cause problems with the thyroid and have a goitrogenic effect. Remember, goiter is the condition where one’s neck appears swollen due to the thyroid being enlarged. An enlarged thyroid is associated with thyroid dysfunction. Since the thyroid regulates metabolism, growth, development, body temperature, and how our body responds to other hormones, we want to make sure it is functioning optimally. In most cases, adequate iodine in the diet or in supplement form will offset this goitrogenic effect (24). In 1980 there was an Infant Formula Act that required infant formula to contain a certain amount of iodine. After manufacturers started adding iodine to baby formulas, a study on children with autoimmune thyroid disease found that a significantly higher percentage of these children were on soy formula as infants (when compared to children without thyroid disease) (25). Multiple studies have shown that those with congenital thyroid disease had to increase their thyroid medication when on soy formula (26, 27, 28). Results from these studies could imply that iodine supplementation was not sufficient, or that in the presence of iodine, the thyroid can still be negatively affected by soy.
A study on rats showed that the size of the thyroid was substantially larger in rats that were iodine deficient and consuming soy verses those who were iodine deficient and not consuming soy (29). Again, this may imply that soy can contribute to thyroid issues over and above iodine deficiency. With that being said, compared to human studies, animal studies have shown much more significant thyroid issues connected to soy consumption. This includes potential for thyroid cancer (30). However, based on differences in thyroid physiology of rats versus humans (31), it is postulated that soy does not impact the human thyroid to the same extent that it does an animal thyroid.
The most important study is the one that was done on healthy humans. Researchers gave healthy adults 30 grams of soybeans every day for 30 or 90 days (32). Results showed that all subjects had increases in TSH levels (in the high-normal range), and 50% of the subjects eating soybeans for 90 days developed goiter. Within one month of stopping daily soybean intake, goiter disappeared. It seems that having soy as a staple in the diet could negatively impact the thyroid, especially in the presence of iodine deficiency.
Phytates: Inhibit absorption of some minerals
The compound phytic acid is highly concentrated in soy and inhibits the absorption of various essential minerals such as calcium, zinc, magnesium and iron. These nutrients may need to be increased in the diet of those who consume soy regularly in order to offset any potential deficiencies. Many vegetables, seeds, beans and nuts also contain phytates. However, they seem to be highest in soy concentrate (33). This is also dependent on how the soy is processed, which explains why other reports of phytates in soy concentrate may differ greatly. For instance, fermenting soy has been shown to be the most effective way to decrease phytate content (34). Fermented soy also seems to be easier to digest, and better tolerated in general. For this reason, organic fermented soy such as miso, soy sauce/tamari, natto and tempeh can be a reasonable option. Other anti-nutrient components in soy can further impact vitamin, mineral, and protein digestibility (35).
The Immune System, Allergies, & Food Sensitivities
Some think that one of these anti-nutrients, lectin, could actually be responsible for an immune response that causes food allergies/sensitivities (36). Many people with dairy intolerance also have a soy intolerance due to cross-reactivity (37, 38, 39). This is when the body mistakenly recognizes two different foods as the same thing, thus having a similar immune reaction to both foods. This is why someone that needs to be dairy-free may benefit from being soy-free as well, and vice versa. It is suggested to avoid soy products due to the high amounts of processing, food additives, problems with digestibility, and higher quantities of questionable nutrients. Examples of processed soy include soymilk, soy yogurt, soy ice cream, textured soy protein, soy protein powder, most veggie burgers, many protein bars, most fake meats and many grain products that say “high protein” which may have soy flour added. Tofu may not be heavily processed, but it also isn’t fermented so it is suggested to limit tofu as well.
GMO's and Herbicides
Most of the soy in this country is genetically modified, and the health risks associated with GMO’s is not yet clear. For those who need the evidence, there are plenty of research studies showing negative potential health effects from GMO’s in general (40, 41, 42, 43, 44, 45, 46, 47). What about all the studies saying that GMO’s are safe? A critique of these studies reveals their shortcomings and leaves us to demand better testing to ensure safety (48). Soy specifically is genetically modified to withstand higher amounts of herbicides. For this reason, soy typically is laden with herbicide residue. Roundup is the herbicide of choice. The chemical found in Roundup called glyphosate has tons of research showing it's toxic impact (49, 50, 51, 52, 53, 54, 55). Here is another great resource that gives references to studies against Roundup and glyphosate. GreenMedInfo is also a great resource with research on glyphosate and Roundup.
The research on soy is very mixed when it comes to cancer and hormones.
The research on cardiovascular and bone health seems to be mostly positive.
The research on soy's impact on the thyroid is probably the most consistent and compelling for avoiding soy.
The research on GMO's and Roundup are also compelling enough to suggest only buying organic.
Research on lectins, phytates, and food sensitivities are other reasons to consider limiting soy in the diet.
There is enough research on the potential negative health impacts of soy to suggest avoiding frequent consumption of soy products.
Many processed soy products, such as veggie burgers, often contain MSG, yeast extract, and other junky ingredients as well.
In general, it is a good idea to try to avoid genetically modified soy. Look for organic soy products if you choose to eat these foods. Remember, if it is only non-GMO and not organic, it can still contain Roundup.
Organic fermented forms of soy such as miso, natto, soy sauce/tamari, and tempeh are likely fine in moderation as long as you do not have a sensitivity to soy.
Soy is in the top 8 for most common foods to have an allergy or sensitivity to. Food sensitivities are not always obvious so you may need to work with a knowledgable healthcare provider.
The content of this blog post came directly from the research section of my book, The Meat & Potatoes of a healthy meal plan...no bun intended
Reference where no link was provided:
31. Hashcheck WM, Rousseaux CG. Thyroid follicular cells. In Fundamentals of Toxicologic Pathology, 1998. Academic Press, New York, pp. 418-428.