Few foods have been subjected to as much nutritional fear-mongering as soy.
One minute we are told tofu is a healthy source of plant protein; the next, social media is warning us that soy will “mess up our hormones,” increase estrogen, cause breast cancer, damage the thyroid or feminise men.
But, when we move away from headlines and look at human clinical research, a very different picture emerges.
Soy is not only a nutrient-dense food; it is one of the most extensively researched plant foods in the world. Foods such as tofu, tempeh, edamame and soy milk provide high-quality protein alongside minerals and other beneficial plant compounds. And one group of compounds in particular, isoflavones, is responsible for both much of soy’s controversy and some of its most interesting potential health benefits.
First, let’s clear up the biggest misconception: soy is not estrogen
Soy contains isoflavones, including genistein and daidzein. These are classified as phytoestrogens.And this is where much of the confusion begins:
The word “estrogen” in phytoestrogen has led people ( including doctors) to assume that eating soy is equivalent to adding human estrogen to the body and it truly isn’t.
Isoflavones have a molecular structure that allows them to interact with estrogen receptors, but their biological activity is much weaker and more selective than our own estradiol. Their actions can also differ according to the tissue involved.
Researchers increasingly describe soy isoflavones as behaving more like selective estrogen receptor modulators than simply as “plant estrogen.”
A 2024 systematic review and meta-analysis of 40 randomized trials involving 3,285 postmenopausal women found that soy isoflavones did not significantly alter estradiol, FSH, endometrial thickness or vaginal maturation index compared with controls. The researchers concluded that soy isoflavones appear to act differently from endogenous or exogenous (pharmaceuticals) estrogen.
So the statement that “soy raises estrogen” is an enormous oversimplified inaccuracy, and in clinical trials, it simply isn’t what we see.
Myth: Soy causes breast cancer
This is probably the most damaging soy myth of all.
The concern originated from laboratory animal research in which high concentrations of isolated isoflavones sometimes stimulated estrogen, sensitive pathways.
A mouse is not a woman, and the metabolism of isoflavones differs between species. This is why ethical vegans like me, who love research, believe that we should test on consenting humans only. Eventually we have to anyway, but by that time, the wrong information is shared with the public and the medical field, which has most doctors already not educated in nutrition.
When we examine human evidence, the picture changes dramatically.
The American Cancer Society states that “soy foods do not increase breast cancer risk, including among women at higher risk” . It notes that: “ human research has found soy consumption either associated with a lower risk of certain cancers or with no effect on cancer risk” .
What about women who have already had breast cancer?
The evidence does not support avoiding tofu, tempeh or other traditional soy foods either. The American Cancer Society states that: “eating soy foods does not appear to increase the risk of breast cancer recurrence, while some research suggests it may actually be associated with a lower risk of recurrence”.
Important note: There is an important distinction here between whole soy foods and concentrated isoflavone supplements. The safety evidence for foods is considerably stronger, while high-dose supplements should not automatically be treated as equivalent to eating tofu or edamame. This thinking is also for other micronutrients, that in isolated form may be worse for our wellbeing.
Soy may be particularly interesting for women after menopause and PCRM, led by Dr Neil Barnard has found incredible benefits.
For women approaching and moving through menopause, the discussion around soy becomes even more relevant with ovarian estrogen production declining, changes in cardiovascular risk, bone turnover and body composition becoming increasingly important.
Soy offers an unusual combination: high-quality plant protein plus naturally occurring isoflavones... and a “nutrient” we so often forget: Fiber. We know the microbiome itself, the gut bacteria we are learning so much about, are responsible for a healthy immune system and response.
We often forget this: we ALL have cancer cells in our body at any time. Our immune system is responsible to seeing them as rogue and stopping them. A healthy gut microbiome contributes to their health.
Bone health is another area researchers have investigated extensively.
A large 2024 systematic review and meta-analysis examined 63 randomized controlled trials of isoflavone interventions in postmenopausal women. It reported significant improvements in bone mineral density in the lumbar spine, femoral neck and distal radius. Benefits were particularly apparent in interventions lasting at least 12 months and containing sufficient genistein.
A separate umbrella review of meta-analyses also found improvements in bone mineral density in the lumbar spine, femoral neck and total hip among peri- and postmenopausal women receiving isoflavone interventions.
That doesn’t mean tofu is a treatment for osteoporosis. Bone health depends on many factors and resistance and impact exercise, adequate protein, calcium, vitamin D and other nutrients among them. And protein is such a wonderful source of protein too.
Telling menopausal women to avoid soy because of its “estrogen” is not supported by the evidence and it shocks me when MDs say this to their patients. Especially in oncology. I truly believe that particularly in oncology, education with NEW and sound human trials is paramount.
Soy also stands apart nutritionally from many other legumes as it contains all nine essential amino acids and can make a substantial contribution to daily protein intake. I eat 400 to 500g of tofu and with all my vegetables i meet my protein needs fully. This makes tofu, tempeh, edamame and soy milk particularly useful for people eating predominantly or entirely plant-based diets
Unlike many animal protein sources, whole and minimally processed soy foods provide protein without dietary cholesterol and are low in saturated fat.
The American Cancer Society describes soy foods as sources not only of protein but also fibre, healthy fats and nutrients including potassium and magnesium, and if tofu is calcium set, it will also offer that benefit.
For women in midlife, maintaining adequate protein becomes particularly important for preserving muscles and bones.
Your heart may benefit too
Soy has one of the better researched relationships between an individual food protein and cholesterol.
A major meta-analysis examined 46 controlled trials picked by the US FDA, in its evaluation of soy protein.
At a median intake of approximately 25 g of soy protein per day, soy reduced LDL cholesterol by approximately 3–4% compared with non-soy protein controls.
That may sound modest, but nutrition does not work through one magical food.There is also a second benefit: what soy replaces.
Replacing foods high in saturated fat with tofu, tempeh, edamame or soy milk can improve the overall composition of the diet beyond any intrinsic cholesterol-lowering effect of soy protein itself.
But doesn’t soy feminise men?
This unfounded claim refuses to disappear. Fortunately, it is also remarkably easy to debunk.
If normal soy consumption meaningfully feminised men, controlled studies should reveal changes in testosterone or circulating estrogen and they don’t.
An updated meta-analysis evaluated 41 clinical studies examining soy protein and/or isoflavone exposure in men. Researchers assessed total testosterone, free testosterone, estradiol, estrone and sex hormone-binding globulin.
They found no significant effect on any of them. Importantly, the result remained unchanged when researchers examined different isoflavone doses and study durations. So no: eating tofu does not turn men into women....
I cannot believe I have to actually write this out....
What about the thyroid?
This myth requires a little more nuance.
A systematic review and meta-analysis of 18 randomized studies found that soy did not significantly alter the thyroid hormones free T3 or free T4. There was a very small increase in TSH, and the researchers concluded that the clinical significance of this change, if any, was unclear.
There is, however, an important practical consideration for people taking levothyroxine: foods, including soy, can interfere with absorption of the medication if consumed too close to the dose. That is an issue of medication absorption, not evidence that tofu is destroying the thyroid. Also, the solution is to wait before consuming soy after levothyroxine, and this can be discussed with your medical provider.
What about menopause symptoms?
Here we need to resist exaggerating the evidence in the opposite direction.
Soy is sometimes marketed almost as a natural replacement for menopausal hormone therapy. The research does not justify making that claim.
Clinical trials examining soy and isoflavones for hot flushes and other vasomotor symptoms have produced inconsistent results. The Menopause Society’s evidence review notes substantial variation between studies in the products, doses and formulations used, along with methodological limitations.
Some women may find soy helpful, but it shouldn’t be sold as a guaranteed treatment for hot flushes.
And that distinction matters. Defending soy from misinformation doesn’t require turning it into a miracle food. It simply requires representing the evidence accurately. Also, I want to repeat this: there is no one single silver bullet.
Whole soy foods versus isolated supplements
This distinction gets lost repeatedly online so let’s clarify:
tofu
tempeh
edamame
unsweetened soy milk
soy yoghurt
miso
Are not metabolically identical to swallowing a concentrated high-dose isoflavone extract.
Most people do not need to chase enormous doses of isolated phytoestrogens to obtain the benefits associated with soy-containing dietary patterns.
I favour whole or minimally processed soy foods as part of a varied plant-rich diet, as per list above and when I choose to consume a protein powder, I will choose hemp or pea, instead of soy.
Another increasingly popular argument is that soy should be avoided because some soy products are “processed.”
But processing is not a meaningful measure of a food’s health effects on its own. Also because there is a difference between processing and ultra processed foods.
Turning soybeans into tofu involves soaking, grinding, heating, coagulating and pressing them. Fermenting soybeans gives us foods such as tempeh and miso.
Humans have processed food for thousands of years.There is an enormous nutritional difference between minimally processed tofu and an ultra-processed product containing refined starches, sugars, oils and additives as a mix. Calling them both “processed” tells us remarkably little about their physiological effects.
The bottom line
Nutrition science rarely supports the extremes that perform best on social media.
Soy doesn’t need to be worshipped, and it certainly doesn’t need to be feared, but accumulated human evidence tells us that soy foods:
1. provides high-quality plant protein
2. can help lower LDL cholesterol
3. does not appear to increase breast cancer risk;
4. appears safe for breast cancer survivors when consumed as foods;
5. does not feminise men or lower testosterone;
6. do not behave in the body like simply consuming human estrogen;
7. do not appear to meaningfully disrupt thyroid hormones in people with normal thyroid function
8. There is a workaround for people on thyroid medication
9.May offer additional advantages for bone health during and after the menopausal transition.
10. It offers fiber as well as complete protein.
There are exceptions. Someone with a soy allergy obviously needs to avoid it.
But for most people, there is little scientific justification for fearing a block of tofu.
Please choose organic and non GMO for the planet and sustainability
Perhaps the more useful question isn’t: “What will soy do to my hormones?” but rather: “What happens when I replace some of the foods in my diet with beans and tofu? “
I will continue on another article for you! :)
Interested in how to actually cook with soy, build it into your daily meals, and understand exactly how much your body needs at this stage of life? That’s exactly what we go deep on inside the paid membership — along with monthly live sessions where food, nutrition, and lifestyle are based on fun and delicious science. Join us for €9/month.