Breast development in the first 2 years of life: an association with soy-based infant formulas.

OBJECTIVE: To evaluate the estrogenic effect of soy-based formulas in female infants. These formulas contain significant amounts of phytoestrogens, compounds with structural similarity to estradiol. PATIENTS AND METHODS: A cross-sectional study consisting of 694 female infants ages 3 to 24 months that consecutively attended 10 general pediatric clinics, none of them having been referred for breast development. The presence of breast buds served as a marker for the endocrine effect of soy-derived phytoestrogens.
RESULTS: Of the participants, 92 had consumed soy formulas for more than 3 months. Breast tissue was more prevalent in the second year of life in infants fed soy-based formula vs those that were breast-fed and those fed dairy-based formula (22.0% vs 10.3%; P = 0.02) with an odds ratio of 2.45 (95% confidence interval 1.11-5.39). No differences in breast bud prevalence were observed during the first year of life. Unlike infants on dairy-based formulas and breast-feeding, infants fed a soy-based formula did not demonstrate a decline in the prevalence of breast during the second year of life.
CONCLUSIONS: We suggest that phytoestrogens impose a preserving effect on breast tissue that is evolved in early infancy, leading eventually to a slower waning of infantile breast tissue.