Mothers, take note! Feeding infants, especially girls, soy-based formula can changes differences in some reproductive-system cells and tissues, a study has found.
Although the differences in the months after birth were subtle and not a cause for alarm, there is a need to investigate the long-term effects of exposure to oestrogen-like compounds found in soy-based formulae, researchers said.
"Soy formula contains high concentrations of plant-based oestrogen-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 Virginia A Stallings, from the Children's Hospital of Philadelphia (CHOP) in the US.
"Modern soy formula has been used safely for decades. 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," said Margaret A Adgent, from Vanderbilt University in the US.
Some mothers who do not 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 oestrogen-like compound.
Like other oestrogen-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 study, published in the Journal of Clinical Endocrinology and Metabolism, 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.
Of 410 infant-mother pairs enrolled, 283 pairs completed the study. Of those, 102 infants exclusively fed on soy formula, 111 on cow-milk formula, and 70 on breast milk.
"This was an observational study, not a randomised trial. All of the mothers had decided on their feeding preferences before we enrolled them in the study," said Stallings.
About half of the babies were girls, and 70 per cent of the infants were African American.
Researchers repeatedly performed measurements up to age 28 weeks in the boys and age 36 weeks in the girls.
The team assessed three sets of outcomes: 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, those fed soy formula had developmental trajectories consistent with responses to oestrogen exposure.
Vaginal cell MI was higher and uterine volume decreased more slowly in soy-fed girls, both of which suggest oestrogen-like responses. The study team found similar patterns in differences between soy-fed girls and breastfed girls.
"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.
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