Ahh yes, soy: the legume that has been eaten by human beings for nearly 5000 years, and has somehow had quite the divisive reputation in recent years. Some claim soy is good for you, while others claim soy is bad for you, stating it has harmful to the thyroid, disruptive to hormones, and can turn men into women.
But what does the science say about soy? Let’s take a look at the data.
Disclaimer: As always, this is general information intended for healthy adults. Your needs may vary based on medical status, lifestyle, or life-stage. Please never replace generalized health information you’ve read online with individualized clinical care.
So, first of all, why has there been conflicting information about soy all over the media in recent years?
This is a great question. There are several reasons why the soy-health narrative seems inconsistent.
First, many of the studies that have either found positive or negative health benefits have been different in design.
Some have been epidemiological in nature, meaning scientists collect information about dietary intake and health outcomes and look for patterns, and others have been randomized control studies (the gold standard, where one ‘experimental’ group gets a soy product, and the other ‘control’ group gets no soy product), and others have been animal studies, which do not always translate to humans.
Additionally, some of these studies have been well-designed…others, not so much. Several studies that have made major headlines about soy have been relatively poor in design, or have had very small sample sizes or short durations, and therefore should be interpreted with caution.
Furthermore, different studies have looked at different forms of soy consumption. There is a difference between consuming whole soy foods (like tofu, tempeh, and edamame), which not only contain soy compounds but also fiber, protein, and micronutrients, compared to consuming ultra-processed soy protein isolates, which are often components of ultra-processed foods with other ingredients.
And some studies that make headlines have actually tested soy isoflavone supplements, not soy foods. Soy foods and soy supplements are different in terms of their isoflavone content, as well as their fiber, protein, and micronutrient content. Keep this in mind as we discuss!
What even is soy? What foods contain soy?
Soy is a legume that’s been eaten for a thousands of years. It’s found in its most whole food form as soybeans, or edamame.
Soy is also found in soymilk, tofu, and tempeh (a fermented soy food).
You can also find soy in oodles of ultra-processed foods. Soy protein isolate is added to many packaged foods as a source of protein, and derivatives of soy, such as soy lecithin, are used to alter the texture or shelf-life of other products.
Does soy really mimic human hormones? Or does soy disrupt hormones?
You may have heard that soybeans contain estrogen-like compounds called isoflavones. Isoflavones are plant estrogens that have a similar function to estrogens in humans, but with a substantially weaker effect.
In the human body, soy isoflavones can bind to estrogen receptors and cause either weak estrogenic or anti-estrogenic activities.
This has caused many people to be fearful of soy, as some believe it may interfere with female hormones.
Studies examining soy and female hormones have been a bit all over the place, with some examining soy isoflavone supplements, and others using soy protein products, and some examining whole soy food. Regardless, most of the existing studies on the topic suggest that soy consumption does not have a significant impact on TSH and LSH (female hormone) levels.
In fact, environmental exposure to BPA (found in many plastics) is a true endocrine disruptor, and studies have found soy consumption may actually be helpful to mitigate the effects of BPA’s endocrine-disruption in women undergoing infertility treatments.
Does soy impact thyroid function?
For most people, no. Scientific reviews have found that soy has no ill effects on thyroid function in healthy adults who were not iodine-deficient (iodine deficiency is rare in the developed world).
That said, soy supplements might interfere with thyroid hormone medication used to treat hypothyroidism, which is a common condition in which a person’s thyroid is under-active.
A 2002 study found that giving soy supplements to animals and cells may cause an interaction with thyroid medications when the two were taken too close together.
There is still a lot to be learned about this topic, but if you do take thyroid medication, talk to your doctor about whether or not spacing your thyroid medication and your consumption of soy supplements or foods may be something to consider.…