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.
Does soy cause ‘man-boobs’?
No. Science has shown that soy does not impact male testosterone levels, and it does not cause men to suffer hormonal issues nor does its consumption exert ‘feminizing effects,’ whatever that means.
What role does soy play in menopause?
There was once a myth that soy prevented menopause from happening, and therefore, could not be good for the human body. However, that’s untrue.
There is some evidence to support that soy supplements may alleviate certain menopause symptoms, but the sciences is pretty inconclusive.. Some reviews show phytoestrogens from soy have been found to reduce frequency of hot-flashes in menopausal women.
However, several other reviews have found inconsistent results, and for now, the use of soy supplements for menopause relief seems to be unclear, and generally isn’t recommended as there isn’t quite enough strong evidence to support their use.
What does the science say about soy and breast cancer?
Because soy may be estrogen-blocking in some, soy consumption could theoretically reduce the risk of breast cancer development.
Human epidemiological studies suggest that soy consumption either has a neutral or protective effect against developing breast cancer, and a 2019 meta-analysis found that soy consumption reduced risk of death from breast cancer.
So when it comes to human consumption of soy, soy from foods (not supplements) seems to at worst have a neutral effect on breast cancer risk, and at best have a significant protective effect on breast cancer risk.
Some highly-cited animal studies have shown high doses of isoflavones or isolated soy protein may stimulate breast cancer growth in those with existing cancer. However, these studies look at components of soy in isolation, rather than soy as it wound present in whole foods.
That said, the effect of large doses of soy protein isolate on existing breast cancer might be worthy of further investigation, but for now, the vast majority of evidence does seem to suggest soy consumption is protective from the development of breast cancer.
What about other cancers?
Although there was some buzz that soy causes cancer, this has not been proven to be true. In fact, many of the claims made about soy causing cancer are based on ultra-processed, multi-component food products that contain small amounts of soy, among many other ingredients.
Moderate soy consumption has actually been associated with lower risks of death from gastric (stomach), colorectal (colon) and lung cancer.
It has also been associated with a reduced risk of prostate cancer.
So, overall, it seems like soy consumption is protective against development of, and death from some cancers.
What about soy and heart disease?
The science suggests soy consumption from foods may be protective against the development of heart disease, and those who eat more soy have been found to have lower rates of death from cardiovascular diseases.
How? Soy is rich in polyunsaturated fatty acids, which have been shown to have favorable impacts on blood lipid profiles when used in place of saturated fats, and/or carbohydrates.
Additionally, meta-analysis data suggests that soy may reduce serum LDL (the ‘bad’) cholesterol, which are known to increase the risk of developing certain heart problems.
That said, it seems soy does not impact HDL levels, which are also important for cardiovascular health (HDL is ‘good’ cholesterol, and ideally, you want to elevate it while reducing LDL).
And the reductions in LDL observed in some studies have been small, so this probably isn’t soy’s best selling point. Still, overall, it seems soy may have a slightly favorable effect on blood cholesterol, and at the very worst, do no harm.
But soy can still benefit the heart in other ways, in addition to those mentioned above. Whole soy foods (like edamame and tempeh) are rich in fiber, which has been found to be protective against development of cardiovascular disease risk factors.
Soy is also rich in micronutrients, including iron, calcium, and magnesium, and when eaten in place of protein sources that may be rich in saturated fats and detrimental to heart health (like ultra-processed red meats), could have a substantial positive impact on cardiovascular disease-risk.
What about soy and diabetes?
There isn’t too much research on soy and diabetes quite yet.
The research that does exist seems to suggest that consumption of tofu, soy protein, and soy isoflavones is associated with reduced risks of type 2 diabetes, but some of the studies included in the linked review aren’t of the best quality, so more research is needed.
Despite the bad reputation it sometimes gets in the media, soy is fiber and micronutrient-rich source of plant-based protein.
Consuming soy foods does not seem to have a negative impact on male hormones, nor does it seem to increase the risk of breast cancer in females.
For most people, consuming soy has positive, or at worst, neutral impact on health (although based on existing evidence, it may interact with certain thyroid medications, so if you take these, speak with your doctor about timing of your soy consumption).
A lot of the research on soy that has suggested soy is bad for health has been low-quality in design or statistical analysis. There is certainly a need for more research in certain areas of the soy world, and I am excited to see where the research goes.
Lastly, soy seems to have the best impacts on human health when consumed in whole or fermented forms, meaning try to stick to things like edamame, tofu, and tempeh, overly heavily processed soy protein-isolate (and in general, lesser processed foods are better for health than ultra-processed foods).
But overall, soy is beneficial, or at worst, neutral, for most of us.
Have a great one! Millie loves you.
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