Soy-milk formula affects infant reproductive organs

Soy-based infant formulas have been found to cause subtle differences in the reproductive-system cells and tissues of the infants who consumed it, compared to babies who used cow-milk formula or were breastfed, according to a new study.

The researchers say the differences, measured in the months after birth, were subtle and not a cause for alarm, but reflect a need to further investigate the long-term effects of exposure to estrogen-like compounds found in soy-based formulas.

"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 Dr Virginia Stallings, Director of the Nutrition Center at Children's Hospital of Philadelphia (CHOP). Stallings is a senior author of a new study published in March in the Journal of Clinical Endocrinology and Metabolism.

"Modern soy formula has been used safely for decades," said Vanderbilt University Medical Center first author Margaret Adgent. "However, our observational study found subtle effects in oestrogen-responsive tissues in soy-fed infants, and we don't know if these differences are associated with long-term health effects."

Some mothers who don't breastfeed have long used soy formula as an alternative to cow-milk formula, often from concerns about milk allergies, lactose intolerance or other feeding difficulties. However, soy protein contains high amounts of genistein, an estrogen-like compound. Like other estrogen-mimicking chemicals found in the environment, genistein can alter the body's endocrine system and potentially 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 infants.

The current study investigated the postnatal development of oestrogen-responsive tissues, along with specific hormone levels, according to infant feeding practices. The researchers particularly compared infants fed with soy formula to those fed with cow-milk formula and breastfed infants.

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

She added: "For new and expectant mothers deciding on how to feed their infants, as always, we strongly support breastfeeding, as recommended by the American Academy of Pediatrics." For mothers who prefer giving formula, the AAP does not recommend soy formula for preterm infants, but states that soy formula is indicated for infants with hereditary disorders that make them unable to properly digest milk, such as galactosemia and the rare condition hereditary lactase deficiency. It also recommends soy formula "in situations in which a vegetarian diet is preferred".

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