Soy-Based Formulas Impact Babies’ Reproductive Systems

Soy-Based Formulas Impact Babies’ Reproductive Systems

Babies given soy-based formulas as newborns have differences in some of the tissues of their reproductive systems when compared to those who were breastfed or drank a formula-based cow’s milk, according to a new study from Children’s Hospital of Philadelphia. The differences, say researchers, were subtle responses to estrogens.

“Soy formula contains high concentrations of plant-based estrogen-like compounds, and because this formula is the sole food source for many babies in the first six months of life, it’s important to understand the effects of exposure to such compounds during a critical period in development,” said senior author Dr. Virginia A. Stallings.

“Modern soy formula has been used safely for decades,” said researcher Margaret A. Adgent.  “However, our observational study found subtle effects in estrogen-responsive tissues in soy-fed infants, and we don’t know if these differences are associated with long-term health effects.”

Some mothers use soy formula as an alternative to cow-milk formula, often due to concerns about milk allergies or lactose intolerance. However, soy protein contains high amounts of genistein, an estrogen-like compound.

Genistein, like other chemicals in the environment that mimic estrogen, can alter the body’s endocrine system and possibly interfere with normal hormonal development. In laboratory studies, genistein causes abnormal reproductive development and function in rodents, but little is known about its effects on human infants.

For the new study, researchers performed measurements up to age 28 weeks in the boys and age 36 weeks in the girls. The measurements included a maturational index (MI) based on epithelial cells from the children’s urogenital tissue; ultrasound measurements of uterine, ovarian, and testicular volume, as well as breast-buds; and hormone concentrations seen in blood tests.

“The main differences we found related to different feeding preferences were among the girls,” said Stallings. Compared to girls fed cow-milk formula or to those who were breastfed, babies fed soy formula had developmental trajectories consistent with responses to estrogen exposure.

Although the differences measured were small, researchers say there is a need to investigate the long-term effects of exposure to estrogen-like compounds in soy-based formulas.

The study is published in the Journal of Clinical Endocrinology and Metabolism.

Earlier studies have indicated problems with babies fed soy formulas. A 2011 study of almost 3,000 girls found that those who were fed soy formula had a 25 percent risk of reaching menarche early.

A 2012 study performed on rats found that those fed a soy formula designed to simulate the effects of a soy-based formula on human infants affected their fertility as adults.  

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