We ask the question is soy formula safe for vegan babies? We evaluate the evidence on hormones, weight gain and reproductive health when children consume soy based products.
I’ve written about my own struggles with breastfeeding and as a result, I am a BIG proponent of the idea that fed is best. There are 1001 reasons why infant formula is necessary, and it’s critically important never to shame moms who opt to supplement or exclusively bottle feed. But one of the questions I’ve consistently seen come up in my mom groups is about what kind of formula is best.
Currently, the majority of formulas on the market are cow’s milk based. However, with the rise in plant-based diets, and cow protein allergy in infants (you can read about dairy and colic here), soy formula has become a popular alternative. In fact, the global soy formula market is expected to grow by $278 million by 2023, and currently, it represents around 12% of the US formula market. While numerous health benefits of soy and other plant-based proteins have been uncovered, there is still a fair degree of suspicion regarding soy, particularly in the case of infant formulas. So is soy formula safe for vegan babies? Let’s take a look at some possible concerns, and what the evidence has to say.
Reasons for Choosing Soy Formula Over Cow’s Milk Baby Formula
When soy formula first appeared on the market in 1909, it was advertised as a way to address diarrhea in infants. Over the following years, it proved a good alternative for babies who had a cow’s milk allergies, or for infants whose parents were vegan. Another great use for soy formula is with children who have galactosemia, a genetic condition that affects around 1 in 30,000 babies, and prevents them from properly metabolizing the sugar galactose. We’ve all heard of lactose, the sugar in milk that can cause digestive distress in many adults. Lactose is actually made up of two smaller sugars: glucose and galactose. That means that babies who can’t metabolize galactose are in trouble, whether they are drinking cow’s milk formula or good ole-fashioned breast milk. That’s where soy formula can swoop in to save the day, offering a full-spectrum protein source without the lactose!
Allergies & Colic
Parents may also turn to soy formula to try to address digestive issues like colic. However, this review of 12 scientific studies found insufficient evidence that switching from cow’s milk formula to soy formula was effective in reducing symptoms of colic. Another, older review from 2013 concluded that soy formula may help reduce colic, but then went on to advise against its use, due to concerns surrounding soy allergies in children. It’s important to note that it is the protein in cow’s milk, not the lactose, that researchers believe is behind issues with cow’s milk formulas. Outright allergies to cow’s milk affect around 3% of children, and while the prevalence of soy allergies is actually lower than that, 10-14% of children with cow’s milk allergies are also allergic to soy, so switching to soy formula may be a gamble. Instead, the authors suggest the use of extensively hydrolyzed cow’s milk formula. Hydrolyzed cow’s milk products have been greatly processed to break apart any of the proteins that might otherwise cause allergies in their full and natural forms. We have a full article all about colic and dairy to check out here.
Parents may choose soy formula due to concerns over lactose in cow’s milk. It turns out, however, that there’s loads more lactose in human breast milk than cow, sheep and goat’s milk! Almost all babies are built to consume lactose, and it’s only later in life, at around 5 years of age, that many of us slow down production of lactase, the enzyme that allows us to digest lactose. We’ve written a piece on lactose and babies, which is a whole story unto itself, and well worth the read!
Alright, so we’ve covered the possible rationale for using soy baby formula, but what exactly are the concerns?
Is Soy Formula Safe for Vegan Babies?
Isoflavones in Soy Products and Baby Formula
The controversy around soy products for babies (and honestly everyone else) boils down to a class of compounds in soy known as isoflavones. At this point, things might get a bit confusing, so bear with me! Isoflavones are actually part of a much greater class of compounds (there’s over 6000 of them!) found in plants, known as flavonoids. All those articles about how chocolate, coffee and red wine are actually healthy? You can thank flavonoids for that! Green tea, cocoa, blueberries, citrus fruit, and red wine all have various different flavanoids, and they exert anti-inflammatory, anti-oxidant and even anti-microbial effects. This has garnered them a lot of attention from researchers for their potential in lowering the risk of heart disease and cancer, and isoflavones have been no exception. Isoflavones are found in all legumes, but can be found in their highest concentrations in soybeans. And while the high isoflavone content of soy may be helpful in lowering the risk of chronic diseases, isoflavones have a darker side: they can act as hormone disruptors known as phytoestrogens.
Phytoestrogens in Soy Products and Baby Formula
Phytoestrogens are chemicals found in plants that mimic the action of estrogen in our bodies. We all know estrogen as the female sex hormone (although it’s found in small amounts in men too), and it’s one of the many chemical messengers used in our bodies. Hormones carry different “messages,” and after they are released, they travel through the blood until they reach specific receptors. Once they bind to those receptors, they initiate all kinds of changes, from opening channels in our cell membranes to altering the expression of our genes. Phytoestrogens are similar enough in shape to the “real” estrogen produced in our bodies, so they can bind to, and activate estrogen receptors in our cells. In fact, soy isoflavones have been investigated as a means of reducing hot flashes in menopausal women (they actually did perform better than a placebo!), and may have a protective effect against breast cancer.
There are three main isoflavones in soy:
Genistein (you may also see this one referred to as “genistin,” which is a slightly different chemical form)
Genistein has been the most widely researched of the three, so we’ll focus on it in particular here.
Genistein Exposure in Infants from Soy Formula
Just how big a dose of genistein are soy-formula-fed babies getting? Let’s break it down. Japan leads the world for consumption of soy products, with a 2005 National Nutritional Survey showing an average intake of 57.7 grams per day, resulting in up to 50mg of soy isoflavones daily (compare that to US estimates of only 3mg daily)! A study looking at serum concentrations (the amount of a certain substance circulating in the blood) found that Japanese men over 40 had an average of 493nmol/L of genistein in their blood. Compare that to another study in 4-month-old, soy-formula-fed babies, which found an average serum concentration of genistein that was more than five times that! To put that another way, babies fed soy formula seem to be getting a dose of genistein that is five times higher than that of the world’s most prolific soy-eating adults! Babies fed soy formula also had 200 times the circulating levels of genistein when compared to babies that were breastfed or fed cow’s milk formula. It’s important piece to note is that this study measuring serum genistein in babies only included 7 infants consuming soy formula, so it’s wise to take these results with a pinch of salt. Other research has shown that most of these phytoestrogens are broken down into forms that aren’t biologically active. However, it’s still a reason to consider the possibility that babies on soy formula are being exposed to very high levels of hormone-mimicking compounds, early in life.
There’s still no clear indication of just what effects early-life exposure to soy isoflavones like genistein might have. A major reason scientists have yet to come to a consensus regarding the long-term safety of soy formula is that much of the research around phytoestrogen exposure in early life has been done in animals, specifically, rats. This is an issue throughout all nutrition research: animal studies are great, because they allow us to conduct research without endangering humans, but at the end of the day, we aren’t rats! So we must remember that whatever conclusions are drawn from animal studies may not always apply to humans.
That being said, what have animal studies shown?
Dangers of Genistein from Soy Baby Formula in Animal Studies
A 2003 study exposed pregnant and then lactating rats to genistein, at levels comparable to those found in a human diet relatively high in soy. The researchers then measured testicular growth, sperm count and testicular gene expression in the male offspring of the soy-exposed rats. The study found no significant effects on any of those measures as a result of exposure to genistein. So far, so good!
But another study looked at female rats given high levels of genistein after birth and found some potentially alarming effects. The researchers mimicked (to the best of their abilities) an infancy on soy formula, by injecting female baby rats with genistein for the first 5 days of their lives. The researchers then examined these rats as adults at 18 months, and found their uterine weight to be twice that of rats fed a normal diet during infancy. These rats also had a significantly higher incidence of uterine adenocarcinoma (a type of cancer), and a much higher rate of oviduct abnormalities, when compared to rats not treated with genistein. These results are shocking, especially when you consider that the rats in this experiment were given levels of genistein that were comparable and proportional to those that would be ingested by a baby on soy formula. However, these rats were given genistein via injection and not in their diet, and once again, this was an animal study so there’s a lot of questions unanswered. Frequently, animal studies involve much longer than normal exposure to the hormones being investigated, and involve methods of administering these hormones that aren’t relevant to humans (i.e. injecting phytoestrogens versus consuming them in food).
Dangers of Genistein in Soy Products in Human Studies
So what about human studies? Between 1965 and 1977, 811 infants participated in a controlled feeding study at the University of Iowa, and were given either cow’s milk or soy formula. As adults, they then participated in a 1999 study, in which researchers measured over 30 different health outcomes in this group, including age of puberty, menstrual history, height and weight, level of education attained, and reproductive health. The study found that there were were no differences in any of these measures between the 248 adults who had been given soy formula as babies, and the remaining 563, who had been fed cow’s milk formula. The study concluded that this large sample size provided further reassurance of the safety of soy formula. However, women from the soy formula group reported slightly longer menstrual periods and greater discomfort. (Here, it’s worth mentioning this separate study done on African American women, which found that women who had been fed soy formula as babies had heavier menstrual bleeding as adults).
Ok, so maybe soy isoflavones can mess around the development of a rat’s uterus, but soy formula still seemed relatively safe in human babies. That is, until a recent study in the Journal of Clinical Endocrinology and Metabolism was published. Researchers enrolled 102 babies being fed only soy formula, 111 babies being fed only cow’s milk formula, and 70 babies who were being breastfed exclusively, and tracked these babies from birth up until the age of 36 weeks. The researchers were particularly interested in this window of time at the very beginning of life, because this is a period during which estrogen-responsive development occurs. During gestation, babies are exposed to the estrogen produced by their mothers. Their genitals and breast tissue go through “estrogenization,” a process that occurs in both boys and girls. But after birth, there’s no longer a constant flow of estrogen from Mom, and the baby’s breast and uterine tissue actually shrinks somewhat. And that’s where soy formula comes into play. In this study, the girls fed soy formula showed a higher average vaginal cell maturation index, and slower decreases in uterine volume (both measures of response to estrogen), when compared to girls fed cow’s milk formula or those who were breastfed. In other words, while their breast and uterine tissue should have started to shrink after birth, the isoflavones from soy formula slowed down this natural decrease in size. And what about the boys in the study? Well, those boys fed soy formula had larger breast-bud diameter than those who were breastfed, but there was no difference when they were compared to boys on cow’s milk formula. It’s important not to over-sensationalize these findings: boys fed soy formula aren’t going to grow up needing a bra! This study lasted only 9 months, the differences seen were small, and this says nothing about permanent, long-term effects of soy during infancy.
Bottom Line: Is Soy Formula Safe for Vegan Babies?
OK, so what’s the verdict on soy formula for vegan babies or babies with cow’s milk allergies? The existing long-term studies in humans show that, aside from the potential for heavier periods in women, soy formula during infancy doesn’t lead to any negative consequences to health during adulthood. Soy formula has been in use for well over a century, without any obvious health impacts. It may not be equivalent in its hormonal activity to breast milk but at this point, it does not appear to have any long term implications. The Canadian Pediatric Society recently released an article on soy formula that would be good reading for anyone still wondering about its possible risks and benefits. The organization concludes that soy formula has not been associated with any harm, or any measurable long-term health risks, but advises doctors to limit their recommendation for soy formula to infants with galactosemia, or those whose parents have religious or cultural reasons for avoiding cow’s milk. The good news is that if you’re not comfortable with soy, there are a lot of other options on the market including goat’s milk formula, hydrolyzed formulas and donor breast milk, so choose an option that you’re comfortable with. The bottom line is, babies are exclusively fed formula for such a short period of time, and more emphasis should be on encouraging a balanced diet throughout the life cycle than fixating on this one very short period of it.
Updated on January 28th, 2020
Abbey Sharp is a Registered Dietitian (RD), regulated by the Ontario College of Dietitians. She is a mom, YouTuber, Blogger, award winning cookbook author, media coach specializing in food and nutrition influencers, and a frequent contributor to national publications like Healthline and on national broadcast TV shows.