The Great Soy Formula Experiment

Soy milk and soy formula contain potent human hormone disruptors. We don’t know what this means for child development.

I’m hesitating over this one question I want to ask the scientist on the phone, a federal researcher studying the health effects of soy formula on infants. I worry that it’s going to sound slightly Dr. Frankenstein-esque. Finally, I spill it out anyway: “Are we talking about a kind of accidental experiment in altering child development?”

With our fondness for all things soy, have we created a kind of inadvertent national study?

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The line goes briefly silent. “I’m a little worried about the word ‘experiment,’” replies Jack Taylor, a senior investigator at the National Institute of Environmental Health Sciences, a division of the National Institutes of Health. Taylor and his colleagues in North Carolina have been comparing developmental changes in babies fed soy formula, cow-milk formula, and breastmilk. His group’s most recent paper, “Soy Formula and Epigenetic Modifications,” reported that soy-fed infant girls show some distinct genetic changes in vaginal cells, possibly “associated with decreased expression of an estrogen-responsive gene.”

But his first reaction is that my phrasing would, incorrectly, “make it sound like we were giving children a bad drug on purpose.” The research group, he emphasizes, is merely comparing the health of infants after their parents independently choose a preferred feeding method. No one is forcing soy formula on innocent infants.

“No, no, that’s not what I meant,” I explain with some hurry. “I wasn’t suggesting that you were experimenting on children.”

Rather, I was wondering whether we as a culture, with our fondness for all things soy, have created a kind of inadvertent national study. Soy accounts for about 12 percent of the U.S. formula market and I’ve become increasingly curious about what this means. Because the science does seem to suggest that we are rather casually testing the effect of plant hormones on human development, most effectively by feeding infants a constant diet of a food rich in such compounds.

Research shows that soy milk and soy formula contain up to 4,500 times the level of phytoestrogens found in cow’s milk or breastmilk. That’s a notable number. And it’s been associated with remarkably high levels of these compounds circulating in the bloodstreams of soy-fed infants. All of this matters when you consider that phytoestrogens are potent human endocrine disruptors, binding efficiently to the estrogen receptors found in both females and males. And consider further, that a baby on a soy formula diet is being repeatedly dosed every day.

It’s no wonder then that studies far beyond Taylor’s have found indicators of off-kilter developmental changes, ranging from unusually early menstruation to mammary gland effects.

In light of all this, Taylor reconsiders my point. “Well, you are absolutely correct that these babies are getting a lot higher dose of a known estrogenic compound than they’ll ever get from BPA or an endocrine disruptor like that.” And he considers a little more. “In that sense, it could be considered a kind of experiment.”

Let’s drop back for a minute. The idea that plant hormones — such as genistein, the primary phytoestrogen in soy — can interfere with mammalian development is not new. Biologists have been trying to sort out such effects for more than half a century; one of the first such studies followed the rather startling discovery that sheep grazing on fields dense with a hormone-rich clover could become temporarily sterile as a result of their diet.

Researchers now wonder whether a phytoestrogen-rich diet is entirely benign for children.

Heather Patisaul, a biology professor at North Carolina State University who specializes in the study of endocrine disruptors, notes that similar effects can be seen in humans: Young women who consume a diet exceptionally high in soy also occasionally “shut off their menstrual cycles” and become temporarily infertile. “When we think about endocrine disruptors, we have to remember that they aren’t all synthetic compounds,” she emphasizes. “Soy is both a natural food and a hormonally active one.”

Cultures with a long-time reliance on soy protein apparently realized this early, Patisaul adds. For instance, the compounds in soy are known to interfere with the body’s uptake of iodine, an element necessary for healthy functioning of the hormones produced by the thyroid gland. Chinese farmers first cultivated soybeans about 1100 B.C., so it’s not surprising, she notes, that many Asian diets gradually evolved to contain foods that contain extra high levels of iodine, such as seaweeds.

Evidence of soybean cultivation in North America dates back to colonial days, but the crop was largely considered one for animal feed. It wasn’t until the late 19th century that the first recipe for dining on soy “peas” (cooked with bacon, salt, and butter) was published by an agricultural research station. And it wasn’t until the 21st century that soy foods took off as a diet staple.

The Soyfoods Association of North America estimates that product sales rose from $1 billion a year in 1996 to $4.5 billion in 2013. “More than 75 percent of consumers perceive soy products as healthy,” according to one industry survey.

The embrace of soy appears driven both by an increased shift to vegetarianism and by high-profile research showing that a diet high in soy can have a positive effect on heart disease. The main unanswered question there, Patisaul says — and one that’s been difficult to tease out — is whether reductions in heart disease are due to some aspect of soy chemistry, or due to a reduction in eating meat.

The reasons infants are fed soy formula, though, are different. Doctors may recommend it as an alternative to breastmilk or cow-milk formulas if a baby appears lactose intolerant or has some other digestive upset related to feeding. Some parents choose it because they believe it is healthier and others because they themselves have rejected an animal protein diet for ethical reasons and want to raise their children in that model. It’s such decisions — and the resulting rise in numbers of soy-fed infants — that led researchers themselves to wonder whether a steady diet of a phytoestrogen-rich food would be entirely benign for children in the early stages of development.

A 2003 paper did conclude that the primary estrogenic hormone in soy was not as endocrine-disrupting as DES (diethylstilbestrol), a synthetic estrogen once used to prevent miscarriages and early pregnancy that was later found to put both mothers and children at risk of developing reproductive system cancers. But scientists still worried that they didn’t actually know how plant hormones might affect a developing human system. A review published that same year, comparing breastmilk and formula fed infants, warned of possible adverse effects but concluded that “the science is insufficiently developed at this time to allow a credible assessment of health risks to infants.”

Another more comprehensive review published the following year simply concluded that more research was needed.

Eventually, the National Toxicology Program — a federal project to assess potentially toxic chemical compounds — took on a comprehensive assessment and in 2010 sought to reassure scientists and parents alike. The NTP concluded that soy formula should be considered of “minimal concern” in terms of developmental toxicity, and while Andrew Rooney, an NIEHS researcher who worked on the evaluation, concedes that scientists of a decade ago lacked full ability to see the kind of minute genetic shifts that are detailed in the work of Jack Taylor and other researchers, he still sees soy as a minor health concern.

At the same time, Rooney acknowledges that as the science becomes more advanced, new questions about soy — and new research into its impacts on human development — are continually arising. In 2014, researchers from the NIEHS reported that six-month-old girls raised on soy formula showed clear signs of estrogen-driven changes in reproductive system cells. The study noted: “These vaginal cell changes suggest that an exclusive soy diet is associated with a response in young girls that is consistent with physiologically active estrogen exposure.”

Two studies published in the following years found a clear association between early soy exposure, the growth of large uterine fibroids later in life, and unusually heavy menstrual bleeding. The latter report made a point of emphasizing the vulnerable timing. “Our results support the idea that infancy is a susceptible developmental window for female reproductive function.”

Which brings me back to my original question.

I spent some time reading through a raft of these papers, including Taylor’s elegant discussion of a chemical mechanism by which phytoestrogens might tinker with human gene expression. And at some point, as one paper led to the next, their range and cumulative weight started to have the feel of a large-scale, if wholly inadvertent experiment in child development. It’s worth noting along these lines that while most studies suggest a more direct effect on girls — who, not surprisingly, possess more estrogen receptors — there are also some hints of subtle effects on boys, ones that the NIEHS scientists hope to study further.

Most people, Patisaul says, just aren’t aware that soy is such a hormonally active food.

While Taylor does not dismiss the idea that parents and their children may be participating in something of an ongoing, unsanctioned experiment on the impacts of soy, he quickly expresses caution about how strong the experimental results are at this point. “We have these hints in humans that this early exposure to estrogens may have long-term consequences,” he tells me, adding that it would be a mistake to scare parents into making some other choice, because the findings on soy are still so subtle, and so new. “We were very careful not to go to ‘don’t use’ in our paper,” he says. It’s too early to do that.

But there are indications that parents are becoming more wary. While soy accounts for 12 percent of the market now, in 1998 the American Academy of Pediatrics set that number at 25 percent. And some researchers — including Patisaul — have decided the enough is known to begin sounding additional warning notes. Most people, she says, just aren’t aware that soy is such a hormonally active food, and she’d like to raise a little awareness on that front.

“When I talk to parent groups, I know that a lot of people are choosing soy formula for vegetarian or ethical reasons,” she says. “I try to advise against that, to say that it shouldn’t be used unless it’s a medically necessary choice.”

Deborah Blum is a Pulitzer Prize-winning science writer, the director of the Knight Science Journalism Fellowship Program at MIT, and the publisher of Undark.

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30 comments / Join the Discussion

    My daughter now 21 had develop ovarian cysts, the doctors try to say that is genetic but I don’t believe it. Her problems started with her first menstruations.


    Thanks for the interesting article. Soy is big here in northern California, but fortunately, so is breastfeeding. People who feed their infants formula do so for various reasons, not all health-related. I think it’s super important for them to understand the trade-offs before making the decision. I’m waiting for the studies on boys. I’m cynical enough to believe we won’t see much progress until it is shown that boys suffer adverse effects.


    Isn’t all Soy grown with Monsanto’s seeds now? Monsanto holds the patents now on soy and corn, among others. What about Polysorbate 80 (from their herbicide Round up) being present in Soy products, including Soy formula?


    My 42 yr. old daughter was on soy formula and soy milk until about 5, as she was allergic to milk. just wondering if the have study adults that were raised on soy formula?


    Good question. Some of the studies (looking at fibroids, for instance) assess young women. Others, looking at menstruation, are more in the teenage years. I haven’t seen any that go into later adulthood but it would definitely be interesting and if I see any I’ll flag it. Having said that, if your daughter is basically healthy I won’t worry about this too much.


    Im 65 yrs old…Drs thot I was having a reaction to mommy milk and cow milk so they put me on soy from birth…when i was 5 yrs old I started developing soft tissue calcium deposits on my legs…i was experimented on for several yrs, but found NO reason for it…when I was 9 yrs old I had my 1st period and by 11 yrs old was regular as clock work…I developed very young, hair and I got older my cycle was 23days from start to finish…I had 2 sons and both were early..3 wks and 4wks, but were fine…when I was 38 I had already gone thru menopause, and found I had a thyroid condition, hyperthyroidism, my thyroid has nodules in both lobes…i didn’t treat, but they also said possibly calcium deposits… when I researched all this 20 plus yrs ago, said soy causes soft tissue calcium deposits, a thyroid condition can cause menopause like symptoms …i have NEVER treated for my thyroid…I go the more natural route in my health…on NO medications at all, Chiropractic and all natural supplements and NON GMO and organic eating….


    Is the diet of the children considered? After infancy I mean. Were the children in the study all eating meat-based diets or plant-based diets?


    I’d also like to hear more about this… And how a non-vegetarian diet with less emphasis on soy after, say, year 2 impacts child development. In other words, if the shifts they’re seeing early on can be reversed. Deborah, do you know of any longitudinal studies happening? Thanks for this really informative (and well-written!) piece!


    Thanks much, Sarah. Regarding soy, my impression is that the focus of concern is in the first year or so of child development but especially during the period where soy is the sole food, as with a diet of soy formula. After that, of course, it’s only part of the diet. Following the golden rule of a healthy diet which is mix it up so as to reduce chronic exposures. My understanding from conversations with scientists at NIEHS and others is that these recent results with soy formula have driven a new kind of explosion of interest and that there are a number of more comprehensive studies being planned, including one that’s more male-female inclusive, and others that are longer term. There’s a couple of researchers, like Kristen Upson, who are doing really interesting work in this regard. I think it makes a lot of sense to follow this work further and see what really holds up. Hope that makes sense.


    We adopted a baby in 1994 and felt that soy formula was the best choice. He drank the formula for almost 2 years, and we finally discontinued when our chiropractor suggested we stop the formula because he was beginning to vomit it up almost every time he drank it..It was several years later that we began to read about the hormonal effects of soy, and wondered if it could have any connection to the hyperactivity and other developmental issues we were seeing in our son…I would be most interested to know if there are any suggestions as to how to reverse any negative hormonal conditions that may have resulted from the soy formula.


    If it’s helpful, everything I’ve seen related to soy and child development is focused on reproductive system issues, mostly because of the possible effects of the phyto (plant) estrogens it contains. If the Taylor study I cited is right (and it should, of course, be confirmed independently) then there may be some chemical interaction with genes. If so, the easiest thing is to just take soy out of the diet because this would be an exposure-based temporary effect. So your chiropractor gave you excellent advice.


    Hello Ms. Blum,
    Thank you very much for the information, very interesting. I am an environmental engineer and I am finishing a masters degree in drinking water and emerging contaminants (Pharmaceuticals, pesticides and cyanotoxins MC-LR). One of the professors who advises me specializes in environmental toxicology mentioned that another problem little addressed in terms of soy milk in the first 5 years of life is the manganese contents. This can interfere with normal neural development in children. I share two scientific publications dealing with this topic:
    Manganese content of soy or rice beverages is high in comparison to infant formulas.

    Manganese, Monoamine Metabolite Levels at Birth, and Child Psychomotor Development


    Thanks so much for sharing that information. I knew that rice could contain high levels of inorganic arsenic and cadmium but I did not know about manganese. I really would like to come back to look at some of these naturally occurring elements in the diet so you’ve raised an excellent point here.


    Thanks for this piece–it’s super important to raise awareness of this issue. Nice job here.


    It is extremely rare that a baby would have a lactose intolerance since breastmilk is full lactose, what you mean is a cows milk protein allergy or intolerance about 1% of breastfeed babies have it and about 3-7 % of formula feed babies. Two different things. If you made this simple mistake I’m wondering what else you didn’t get right in your article. That being said thanks for writing it and I will be sharing it.


    Yes, I emphasized lactose because it is a sugar found in all mammalian milk. Ran that fact by a scientist who agreed that it’s a broad spectrum issue. But you are absolutely correct that the issue with milk-intolerance is often related to protein and may be even bigger. Sorry if it sounded confusing and thanks for the additional clarification.


    Soy products have been used in Dairy cattle milk replacements.
    Could a transcriptome model be created in calves/piglets be used as a model for humans?
    Transcriptome analysis reveals persistent effects of neonatal diet on the ileum gene expression in porcine neonates

    MinION is a cheap DNA analysis tool. Can it be used to model transcriptomes.

    MinION is used for agriculture and blood diagnostics.
    Could the aging of tissue show differences?​




    I don’t know that there is a great vegan alternative regarding formulas. And maybe we’re at a point that we should encourage formula makers to look at other protein-rich beans as a source. I’m not dismissing soy’s importance in the diet as we get older, only making the case that it may pose problems during the critical developmental period, from zero to six months at a minimum. If it wasn’t too much of an ethical conundrum, I’d lean toward a breast/cow milk formula diet until the point when more solid foods are possible. But I do get that’s a personal decision.




    I don’t think by pointing out that soy is a dodgy choice that it’s now the authors responsibility to provide a vegan friendly alternative.


    Breastmilk is a vegan alternative better support would prevent the need for such formulas on such a wide scale also Donor milk is available in the UK and US and would be much better for baby than any formula


    Babies are not designed to be vegetarian or vegan. They are designed to drink mammal milk. So any plant alternative is going to have issues.


    They are “designed” to drink HUMAN milk… not cow’s milk or any other mammalian milk other than their own species.


    Protein in the diet is highly overrated, and especially in that of an infant. It’s fat that the growing infant needs. If you look at the composition of breast milk it is contains over 55% fat and only about 6% protein with the rest being carbs. You can easily make your own infant formula’s using coconut milk and some high quality fats, just search around for recipes. This idea that we need to be feeding infants high amounts of protein is just simply incorrect. When in doubt, look to nature!


    I have been eating soy products for more than 20 years, and write about science, so I naturally wonder about the phytoestrogen connection. I try to vary my diet so that I’m not mainlining tofu on a daily basis, for example. Whatever its effects were during my fertile years, which ended 8 years ago, seem only to have been positive (e.g., an easy menopause). But that’s just me, and I was never tested, ergo, inconclusive, of course.
    This is an important article, especially for parents of young children of either sex. People of all dietary persuasions can get carried away with the magic bullet effect of certain foods. We cannot forget we are omnivores, primed to seek variety for some very, very good reasons, and can get into trouble if we act like koalas and try to specialize.
    One thing missing from this discussion (I hope only due to lack of space): the hormone load of feedlot (versus free-range, grass-fed) cows. Were the studies years ago of extremely early onset menarche in Costa Rican (?) girls just anecdotal and not worth pursuing? Surely having hormones (along with so much else) pass from meat to meat-eater will effect the eater’s biochemistry.


    yes , there is research – Maureen Minchen’s book Milk Matters – looks at the dairy industry and faormula – If cow’s are being fed hormones to produces more milk etc then cow’s milk formula must also contain hormones – babies are fed large quantities of this from day one , at least 6 times a day , –
    So surely one of the long term term effects could be hormonal disorders ?
    By not drinking species specific milk from birth the body has to adapt to another species milk – at what cost to the body ?


    Hi Louise – Thanks so much for your very thoughtful response. I did focus pretty specifically on the phytoestrogen factor with soy formula, partly for reasons of length and partly because it’s not an issue that gets enough attention (I think) so I wanted to explore it in some depth. It is true that different diets can affect the hormone load both in cow and breast milk so that’s a good point. But the studies I looked at noted that even in those cases, the estrogen loading in soy remains at a level some 2,000 times that of other forms of milk (compared to more than 4,000 in the “cleaner” comparison.) The real issue to me is trying to make parents aware that there is this potential to alter the normal development of the reproductive system so that they can factor that into their decisions. Thanks so much for writing.


    Thanks, Deborah. That makes a lot of sense.
    I am quite shocked to read it’s 2000 times!
    I agree that parents should be aware of the risks.
    Not a parent myself, but have to empathize with new parents, in particular, trying to navigate all the advice their families give them as well as whatever science appears in popular press. As you know, what filters down to the newspapers doesn’t always provide solid guidance, and can lead to info overload and a kind of numbness.
    As for ingesting hormone disruptors: I shudder to think of what horrors lie down the road when evidence of *synergistic* effects come to light. In ourselves and in other species.


    Yes, I wonder about synergies too. We really do have so little information about so many of the materials we introduce into your systems. Because I’m a toxicology writer, I’ve tended to take an increasingly pragmatic approach. Be as aware as possible (hence my story on the underreported effects of soy) and protect yourself against the risks that seem most worrisome, which for me would be things like lead or mercury, arsenic, cadmium, some of the long-lasting compounds like early-generation flame retardants. And not make yourself too crazy:) It’s a chemical world out there and we just need to learn how to be as smart as we can in navigating it. Hope that makes sense!

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