Soy formula’s estrogen levels may affect baby girls’ reproductive system, study suggests

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14 Mar 2018 --- Baby girls who were fed soy-based formula have subtle changes in the cells and tissues of their reproductive system when compared with infants fed with breastmilk or cow’s milk. This is according to the researchers at the Children’s Hospital of Philadelphia, who stress that although the differences are subtle and not a cause for alarm, they do point to the need for further research. The study investigated the postnatal development of estrogen-responsive tissues, along with specific hormone levels, according to the infant feeding practices. 

Many mothers who do not breastfeed choose soy-based formula, often due to concerns about lactose intolerance or other feeding difficulties.

“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,” says Virginia A. Stallings, MD, director of the Nutrition Center at Children's Hospital of Philadelphia.

The study followed 283 pairs of infants and their mothers. Of those, 102 infants were exclusively fed soy formula, 111 on cows-milk formula and 70 on breast milk. “This was an observational study, not a randomized trial,” says Stallings. “All of the mothers had decided on their feeding preferences before we enrolled them in the study.”

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Virginia A. Stallings, MD, Director of the Nutrition
Center at Children's Hospital of Philadelphia.

The study team assessed three sets of outcomes: a maturational index 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 most substantial findings were amongst the female infants who consumed soy-based formulas. In comparison with the girls fed cows-milk or breast milk, they had internal developments consistent with estrogen exposure. For example, the vaginal cell maturational index was higher, and uterine volume decreased slower.

“We don't know whether the effects we found have long-term consequences for health and development, but the question merits further study,” says Stallings. In addition to replication studies by other researchers, she added that ideally the children in this cohort should be followed later into childhood and adolescence.

Stallings further goes on to recommend to expectant mothers the strong support of breastfeeding by the American Academy of Pediatrics. However, for mothers who turn to formula, soy formula is best used for infants with hereditary disorders that render them unable to properly digest milk – such as galactosemia and the rare condition hereditary lactase deficiency.

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