Babies fed soy-based formula may have 'subtle changes' in reproductive tissues, warn researchers who say long-term follow-ups are needed.
The US-based team found that infants fed a soy-based formula from birth had subtly but potentially important differences in a number of reproductive tissues when compared to those breast-fed or raised on cow’s-milk formula.
Published in The Journal of Clinical Endocrinology & Metabolism, the investigation found higher vaginal cell maturational index (MI) and a slower decrease in uterine volume in female infants after 36 weeks, while minor changes in the development of breast-bud diameter were also found in boys.
The changes seen are characteristic of oestrogen-like responses arising from the presence of the phytoestrogen compound genistein in soy, said the team - which was both funded and led by the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health.
"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 senior author Dr Virginia Stallings, Director of the Nutrition Centre at the Children’s Hospital of Philadelphia (CHOP).
"We don't know whether the effects we found have long-term consequences for health and development, but the question merits further study," she added.
Long-term effects unknown
Soy formula has long been used by non-breastfeeding mothers concerned with lactose intolerance, milk allergies, or through vegan diet preference. Nevertheless, soy protein contains high amounts of genistein, an oestrogen-like compound.
"Modern soy formula has been used safely for decades. However, our observational study found subtle effects in oestrogen-responsive tissues in soy-fed infants,” commented first author Professor Margaret Adgent from Vanderbilt University Medical Centre, Nashville.
In previous animal studies, genistein caused abnormal reproductive development and function in rodents. However, little is known about its effects on infants. Thus, genistein may alter the body's endocrine system and potentially interfere with normal hormonal development, cautioned the researchers.
Although the changes seen in this study were subtle and no cause for alarm, their long-term health implications are not known and therefore require additional follow-up, the researchers emphasised.
Further work should not only include replication studies, but also long-term follow-up of the children in this study into childhood and adolescence, the researchers advocated.