Does Soy Formula Affect Hormones? Unpacking the Science

November 25, 2025 •

5 min read

According to the American Academy of Pediatrics, soy-based formula can be a safe alternative for term infants with certain indications like galactosemia or a vegetarian diet preference. However, the question, "Does soy formula affect hormones?" continues to be a top concern for parents due to the presence of phytoestrogens.

Quick Summary

This article examines the scientific evidence regarding soy formula's effects on infant hormones, focusing on the role of phytoestrogens and isoflavones. It reviews findings from clinical and animal studies concerning reproductive development, growth, and thyroid function, addressing common parental anxieties.

Key Points

Phytoestrogens Present: Soy formula contains naturally occurring isoflavones, which are plant-based compounds with a structure similar to human estrogen.
No Significant Long-Term Effects: Numerous clinical studies, including those following individuals into adulthood, have found no significant adverse effects on growth or reproductive health from infant soy formula consumption.
Subtle, Transient Differences: Some studies have observed minor, temporary differences in certain estrogen-responsive tissues in girls, but these have not been linked to lasting health problems.
Thyroid Concerns for Specific Infants: Soy can interfere with medication absorption in infants with congenital hypothyroidism. These infants require close monitoring and possible dosage adjustments, but healthy infants are not typically affected.
High Isoflavone Exposure is Different: While isoflavone levels in soy-fed infants are high relative to body weight, their hormonal effects are much weaker and more complex than human estrogen, and do not necessarily lead to adverse health outcomes.
Animal vs. Human Metabolism: Adverse effects reported in some animal studies are not directly applicable to human infants due to significant differences in metabolism and exposure to purified compounds versus whole formula.
AAP Recommendations: The American Academy of Pediatrics considers soy formula safe for term infants with specific conditions like galactosemia or for vegetarian families, but does not recommend it as a routine first choice over cow's milk formula.

The Phytoestrogen Connection: A Closer Look at Soy's Hormonal Activity

Soy formula is formulated with soy protein isolate, which contains compounds called isoflavones. These isoflavones, including genistein and daidzein, are classified as phytoestrogens due to their structural similarity to the human hormone estrogen. This similarity is the root of most concerns about whether soy formula affects hormones. While these compounds can bind to estrogen receptors (ERs) in the body, their effects are generally much weaker and more complex than those of endogenous estrogen. This is because isoflavones primarily interact with estrogen receptor beta (ER-β), which is distributed differently throughout the body than ER-alpha (ER-α). The ultimate effect depends on numerous factors, including the dosage, timing of exposure, and individual metabolism.

Isoflavone Exposure in Infants

Infants fed soy formula typically have significantly higher plasma concentrations of isoflavones than breastfed infants or adults consuming soy foods, when scaled for body weight. In fact, one study found soy-fed infants had circulating isoflavone concentrations thousands of times higher than their own estradiol levels. This high exposure during a critical developmental window, sometimes called 'minipuberty,' has fueled intense research and debate. Despite the high levels, clinical studies have repeatedly shown that these biochemical differences do not translate into significant adverse clinical effects for most infants.

Reproductive and Sexual Development: Clinical Findings

Long-term clinical studies are crucial for understanding the impact of early-life nutrition on reproductive health. Several key human studies have compared the long-term outcomes of infants fed soy formula versus cow's milk formula.

A University of Iowa study followed subjects fed either soy or cow's milk formula for up to 30 years. Researchers evaluated over 30 health outcomes, including reproductive development and cancer. The study found no statistically significant differences in health outcomes between the groups, though some minor differences in menstrual periods were noted in women.
The 'Beginnings Study' from the Arkansas Children's Nutrition Center used ultrasound to examine the reproductive organs of infants fed different diets at 4 months and 5 years of age. No evidence was found that soy formula exerted any short- or long-term estrogenic effects on the reproductive organs of boys or girls.
A longitudinal study by the NIEHS and Children's Hospital of Philadelphia observed subtle, transient differences in some estrogen-responsive tissues, like slower uterine volume decreases and higher vaginal cell indices in girls fed soy formula compared to those fed cow's milk formula. However, no differences were found in breast-bud diameter or hormone concentrations. The long-term significance of these subtle changes is still under investigation.

Overall, the evidence does not support significant, lasting adverse effects on reproductive development in healthy, term infants.

Potential Effects on Thyroid Function

The link between soy isoflavones and thyroid function is a long-standing area of research. In adults with adequate iodine intake, soy consumption has not been shown to significantly affect thyroid function. However, for infants, especially those with congenital hypothyroidism, the picture is more complex.

Soy can interfere with the absorption of thyroid hormone medication (L-thyroxine). Studies have shown that infants with congenital hypothyroidism who were fed soy formula experienced prolonged increases in Thyroid Stimulating Hormone (TSH) levels, potentially requiring higher medication doses. This is primarily a drug-absorption issue, not a direct endocrine disruption by isoflavones in healthy infants, and is managed by monitoring and adjusting medication. Modern soy formulas are also fortified with iodine to prevent goitrogenic effects.

Soy Formula vs. Cow's Milk Formula: A Comparison

To better understand the hormonal and developmental context, it is helpful to compare the effects of soy and cow's milk-based formulas.


Feature	Soy Formula	Cow's Milk Formula	Notes
Hormonal Content	Contains phytoestrogens (isoflavones) which are plant-based, weak estrogen mimics.	Does not contain phytoestrogens, but may contain naturally occurring animal hormones, though in clinically insignificant amounts.	The phytoestrogen levels in soy formula are far higher than estrogen levels in human breast milk or cow's milk formula, but their effects are weaker.
Effect on Reproductive Development	Some studies show subtle, transient effects on estrogen-responsive tissues in girls, but no significant long-term impact on growth or reproductive function is typically observed.	No significant hormonal effects related to the formula's composition have been noted on infant development.	Long-term human studies generally find no major differences in reproductive outcomes between the two formula types.
Impact on Thyroid	Can impair the absorption of L-thyroxine in hypothyroid infants, requiring monitoring and dosage adjustments. Modern versions are iodine-fortified.	No reported absorption issues with thyroid medication.	All infants with congenital hypothyroidism should be closely monitored regardless of formula type.
Primary Use Case	Used for infants with galactosemia, hereditary lactase deficiency, or whose parents prefer a vegetarian/vegan diet.	The most common type of formula, suitable for most healthy infants.	The American Academy of Pediatrics states few indications exist for choosing soy over cow's milk formula for healthy term infants.
Protein	Isolated soy protein.	Cow's milk protein.	Cow's milk-allergic infants may also be sensitive to soy protein, in which case a hydrolyzed formula is recommended.

The Role of Animal Studies and Human Metabolism

Much of the concern regarding soy formula stems from older animal studies that reported adverse effects from exposure to high doses of isolated isoflavones. It is important to remember that these findings cannot be directly extrapolated to humans for several reasons:

Different Metabolism: Animals, particularly rodents, metabolize isoflavones differently than humans.
Higher Doses: Animal studies often use pharmacological doses of purified isoflavones, whereas infants are exposed to a whole-food product.
Metabolite Differences: While some animals convert daidzein into the more potent estrogenic compound equol, most human infants do not become equol producers until later in childhood.
Species Differences: The effects and sensitivity to isoflavones vary widely across species, meaning what is observed in a rodent model may not occur in humans.

Clinical studies on human infants and adults have not replicated the significant adverse hormonal outcomes seen in some animal models, reinforcing that direct conclusions from animal research are inappropriate.

Conclusion

While soy formula contains phytoestrogens that can affect hormone-responsive tissues, the available scientific evidence does not indicate significant or lasting adverse hormonal effects on the long-term health and development of most healthy, term infants. Concerns related to reproductive and sexual development in otherwise healthy children have not been substantiated by robust human data, and large-scale, long-term studies are largely reassuring. For infants with congenital hypothyroidism, the main issue is potential interference with medication absorption, which requires careful medical supervision, not a fundamental hormonal disruption by the soy. Major health organizations, including the American Academy of Pediatrics, endorse soy formula for specific situations, such as managing certain metabolic disorders or accommodating vegetarian diets, but still recommend cow's milk-based formula for most healthy term infants when breastfeeding is not an option. Parents should consult a pediatrician to determine the most appropriate formula for their baby's specific health needs.

For more detailed information on a wide range of nutritional topics, you may visit the Nutrition Source at Harvard T.H. Chan School of Public Health.

Frequently Asked Questions

No, clinical studies have not found a link between soy formula consumption and the early onset of puberty in children.

Clinical and animal studies have largely found no significant adverse effects on male reproductive development, testosterone levels, or fertility resulting from infant soy formula consumption.

Yes, soy formula is considered safe for healthy, term girls. While some studies observed subtle differences in vaginal cells and uterine volume, these were transient and not associated with significant long-term health issues.

For most healthy children, there is no concern. The issue primarily affects infants with congenital hypothyroidism who are on thyroid medication, as soy can interfere with its absorption.

Decades of use and research have shown soy formula to be a safe and nutritionally complete option for healthy, term infants. Concerns raised by some older studies have largely been addressed and not supported by broader evidence.

The differences observed are often subtle and not clinically meaningful. Variances in study methodology, population differences, and the complex nature of isoflavone interactions likely account for the differing results.

Soy formula is not recommended for preterm infants, infants with a cow's milk allergy who may also be sensitive to soy, or infants with congenital hypothyroidism unless closely monitored by a physician.