Soy formula safety study puts (some) fears to rest

Remember back in the old days when I used to grab whatever infant feeding study was in the news and dissect it just for the hell of it? That was fun. So much more fun than the mommy war stupidity. Statistical methods and confounding factors are way easier to understand than the intricacies of parenting-blog politics.

Let’s get back to those stress-free basics for a minute, by examining a study in this month’s issue of Pediatrics (Developmental Status of 1-Year-Old Infants Fed Breast Milk, Cow’s Milk Formula, or Soy Formula, Pediatrics, May 28, 2012) which compares the mental, motor and language status of infants fed with soy formula, infants fed with milk-based formula, and breastfed infants. Contrary to the typically sensationalist, breast-is-best portrayal the media gave of this study, it actually seemed to come from a place of real concern for infant development and feeding choice. There’s been a fair bit of scaremongering regarding soy formula in recent years; the concern is that feeding a tiny human so much soy (soy contains isoflavins, which according to the study authors, can “raise the possibility of potential estrogenic effects”) will affect that baby’s development in a negative way. It’s not clear, exactly, what these negative effects may be – man boobs? Reproductive problems? Poor neurological outcomes? Two heads? Zombie behavior? (Okay, that was probably uncouth, considering this past weekend’s bizarre news out of Miami…) Regardless, parents have been warned not to use soy formula unless their babies cannot tolerate dairy-based formula or they are vegan (which, I’m sad to report, is misleading – there is no such thing as a vegan commercial formula – there are animal-derived ingredients in every formula on the market). The authors of this study (one of whom does belong to an organization called the Soy Nutrition Institute, so there is some conflict of interest – a fact which is clearly stated at the top of the article, for the record) hypothesized that the milk-based formula-fed babies and the soy formula-fed babies would have similar development, and that the breastfed babies would score slightly higher.

Women were recruited to the Arkansas-based study during pregnancy, and all had decided prior to birth whether to feed their babies breastmilk, milk-based formula, or soy formula. The researchers followed these women and their offspring for one year, testing the babies with developmental and language assessments at 3, 6, 9 and 12 months. Their findings matched the original hypothesis – all three groups had similar scores, with the breastfed babies scoring slightly higher.

Now, the point of this study was to see if soy formula supported development as well as milk-based formula, so I’m a little confused about the inclusion of breastfed babies. I suppose one could argue that since breastfeeding is considered the biological norm, it is important to have a baseline for which to compare the different formulas… but it’s not that simple. The researchers controlled for maternal IQ and SES (socioeconomic status), as well as maternal age. However,  this is just the tip of a very large and foreboding iceberg when it comes to the factors which confound infant feeding studies. There’s a table in the Pediatrics article showing the mean IQ and SES of the three groups, and these are significantly different. The mean IQ of breastfeeding mothers was 109.8; milk-based formula moms scored 106, and soy formula mamas scored the lowest at 103.8. As for SES, the moms were assigned a “score” based on highest level of education and occupation; for this category, the breastfeeding, milk-based, and soy-based mothers had scores of 49.8, 45.6, and 45.9, respectively.

These factors were controlled for, of course, meaning that the scores of the babies were “weighted” in ways which (theoretically) factored in the impact of IQ and socioeconomic status. But even within those parameters, there are important caveats: what about paternal IQ? What about childcare status: home with mom or dad, private nanny, grandparent, or daycare? What about maternal emotional state, or parenting style? What was it that made some of these women choose breastfeeding, some choose soy formula, and others choose milk-based formula?

Obviously, researchers can’t control for everything, and in the case of this particular study, in the end it didn’t really matter. The study authors explain:

In our cohort, standardized mental, psychomotor, and language development scores were very similar among the 3 feeding groups, with averages falling within the clinically normal limits….BF infants scored slightly better than formula-fed infants…These results are consistent with a large body of literature demonstrating advantages of breastfeeding on cognitive function later in life.  However, it is important to point out that developmental test scores of all 3 diets groups were within the standardized norms, and differences between BF infants and formula-fed effects were quite small in magnitude and thus difficult to interpret in terms of potential clinical relevance. Previous studies have shown lesser advantages when taking into consideration confounding factors or segregating for variables such as being small for gestational age. Here, results were similar after adjusting for confounding factors and controlling for other variables of concern. Thus, our results demonstrate a potential beneficial effect of breastfeeding on cognitive function.

To reiterate, the researchers are acknowledging that the differences were statistically significant, but nothing to get overly excited about. It did seem that the breastfed babies had a slight advantage, but it’s hard to know if this benefit will carry over to later childhood, or if it was even due to the breastfeeding itself.
But, um.. did the study prove all that much about soy formula, either? The confounding factors should be less of an issue when comparing the two groups of formula fed infants (although it’s interesting to note that the mean IQ of the moms who chose soy formula was significantly lower than that of the milk-based group), but the study focused on such a limited period of time, albeit one of tremendous growth. Like the majority of infant feeding studies, we only have a hyper-focused snapshot of these babies for one year of their lives. I’d be very interested to see the development of these three groups over time (provided the authors could work a little on their controls, and get a bit more nuance-y with their bad selves). How do the different types of formula affect psychosocial development at the preschool level? Age of onset for puberty? Fertility issues later in life? If the concerns over soy revolve around estrogenic effects, we’re talking about irregularity in menstrual cycles, reproductive problems, and abnormal breast cells, as well as the aforementioned vague worries about brain development. (Please note: I do not believe that soy formulas are inherently unsafe. I think, like every other decision in infant feeding, every family needs to perform a risk/benefit assessment. If your baby can only tolerate soy, feed them soy. There have been enough studies searching for reasons to avoid soy formula, and only one which found a long-term negative outcome, which was longer menstrual periods – not the biggest deal in the world, although don’t tell your daughter I said that in about 13 years. My point is only that this particular study does not comprehensively address the concerns which make the internet peanut gallery skeptical about the safety of soy formula.) 
My final assessment? The study gives decent evidence that babies fed on either soy formula, milk-based formula, or breastmilk will thrive and develop normally, within similar ranges. In other words, it doesn’t seem to matter all that much how your baby is fed, at least in terms of motor and mental development for the first year. 
You know, come to think of it, I kind of like this study…

Suzanne Barston is a blogger and author of BOTTLED UP. Fearless Formula Feeder is a blog – and community – dedicated to infant feeding choice, and committed to providing non-judgmental support for all new parents. It exists to protect women from misleading or misrepresented “facts”; essentialist ideals about what mothers should think, feel, or do; government and health authorities who form policy statements based on ambivalent research; and the insidious beast known as Internetus Trolliamus, Mommy Blog Varietal.

Suzanne Barston – who has written posts on Fearless Formula Feeder.


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8 thoughts on “Soy formula safety study puts (some) fears to rest

  1. When I started my daughter on soy formula, I had no idea that there were any health or safety concerns with it. Her doctor agreed it was a good choice since she's lactose intolerant and I had no reason to doubt him. I was a soy baby and I turned out just fun. Although interesting that you point out the longer menstrual periods. Prior to having kids, it wasn't unusual for me to have a 9 day period. I just thought it was normal since it occurred that way every single month.

    As far as motor skills go, my girls developed in the normal ranges except for walking. One was 13 months, the other was 17 months, but I honestly believe that she was just afraid to walk. She had a ton of other fears and would express herself that she was afraid of dogs, thunder, the world, etc. Yes, she could speak in sentences before she could walk. I don't worry about her cognitive development just because she was a formula fed infant and still drank a formula mix up until she was 2 1/2.

  2. First of all, I laughed at your zombie comment. My brother told me about the Miami thing on Monday and we made a pact to take the neccesary steps should one of us turn into a zombie. You need to arrange these things in advance.

    Anyway, my son used soy formula for awhile because my milk had dried up and he was allergic to milk protein. He's turning 8 soon, so this was before the “soy is evil” stuff. He really did great on it. My one concern is that, as a person who has autoimmune thyroid disease, that I've read use of soy formula can predispose him to developing the disease later in life. I personally avoid most versions of soy because it does affect how I feel. Not like I can take it back, so we'll just have to wait and see.

  3. If the study was about soy formula, why did they conclude with the whole “breast is best” stuff again? Other than that, I agree this was an interesting and relevant thing to study, and as you say, it basically says: “As long as you feed your baby something meant to be fed to babies (breastmilk or some kind of formula), your baby will be ok.”

  4. Ha – This is what I thought when I read this!

    There must be some sort of infant feeding law that says that regardless of the outcome of a study, you have to put a “breast is best” sentence in there somewhere. Why couldn't they have said something like what AmyM said – or “this should provide comfort to formula feeding moms that their babies won't be raving idiots because you didn't breastfeed them.” Instead, we get another breast is best conclusion – even though the actual otucome of the study doesn't really show this.

  5. I am in no way an expert in publishing scientific papers, but I have been an author on a few…and the key thing I learned from my bosses are that to get a paper taken seriously, it has to be novel. The soy study seems to be, but the breastmilk part clearly is not.

    Plus, there is a sloppy conclusion which strays from the purpose of the study. It should say something like: “Our results show that there is no difference in development over the first year, between babies fed soy formula and babies fed milk-based formula. Also, when comparing babies fed either kind of formula to breastfed babies, there is very little difference between them as well. Soy formula will not stunt/inhibit/impede an infant's growth and development from birth to one year and it will provide adequate nutrition.”

    Who is the intended audience? Do only pediatricians read Pediatrics, so they can pass along current info to the parents of their patients? If so, this article can be used as a great tool—doctors can reassure the anxious parents that formula (at least milk-based and soy based, FDA approved formula) is an adequate diet for their babies. The average US citizen has no contact with scientific journals at all so I am doubting this info was meant for the parents to read directly. (not that they shouldn't, but most either don't care about the issue, and/or can't get hold of journal articles.)

  6. We put our first baby on soy formula because of her milk protein allergy but she broke out in acne ALL over her body. Thank goodness for Nutramigen!

  7. Question: What would lead a mom-to-be to choose soy formula before birth? I don't know anything really about formula, so my ignorance may be showing, but wouldn't most pregnant women who are choosing formula feeding go with regular formula? They don't have a lactose intolerant baby yet so that can't be the reason. Just curious. Is it less expensive?

  8. I fed my son (now 15) soy after milk was what i thought to be causing colic. I didn’t know that soy was a problem, in fact at the time i was thinking that maybe it was better because of hormones in cows milk.

    Anyhow fast forward, he was a beautiful toddler and child grew healthy and well but in the 6th grade we realized he had cognitive issues and ‘executive function’ issues (not too pronounced and many boys have this) and he went into puberty on the young (but not abnormal) side at age 11. He stopped growing at 14 and is only 5’4 but then i am 4’10 and my husband 5’8. Its hard to know if these problems are related to being fed soy, but had i known the issues with soy i would not do it again! (my 1st son was breastfed and grew to 5’8 but he also genetically takes after my husband more in looks as well my younger son takes after me more all around)

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