"We have found that, in terms of sexual development, there is very little difference between children who, as infants, were fed cow milk formula and those fed soy formula," said Brian L. Strom, MD, MPH, director of the Penn Department of Biostatistics and Epidemiology. "The biggest concern has been whether or not phytoestrogens in soy had a 'feminizing' effect on children."
Strom is quick to add, however, that the idea is not entirely unfounded. Phytoestrogens, which occur naturally in soy, are analogous to the human female hormone estrogen. Infants fed soy formula receive relatively high doses of phytoestrogen during a time of life when they are rapidly growing. It comes as no surprise, then, that the possibility of altering a child's development had concerned some parents and scientists.
"In this study, we interviewed young adults who had been part of feeding studies as infants," said Strom. "We were looking for abnormalities in their adolescent and sexual development attributable to their early exposure to soy phytoestrogens."
This study has its roots in a landmark study at the University of Iowa, which first started looking at the effects of soy formula on infants beginning in 1965 and lasting until 1978. At the time, soy was still relatively new to the American diet. Then, as now, soy was seen as an excellent source of nutrition for infants, especially for infants who are allergic to - or cannot properly digest - cow's milk.
Strom and his colleagues took up the task of tracking down and interviewing 811 adults - some from almost 30 years after the Iowa study began - and comparing those that had been fed soy formula with those that had been fed milk formula.
The researchers asked the participants a long list of questions regarding the timing and events in their puberty and sexual development that might have been effected by phytoestrogens. For the women, they had an array of questions regarding menstruation, physical development, and fertility. For the men, they inquired about possible delayed sexual maturity and fertility problems. They also asked the participants about other possible effects, such as, hormonal disorders, cancers, sexual orientation, and education.
Despite the large number of comparisons made, the groups were essentially indistinguishable. Menstrual bleeding in the soy-fed group was, on average, about one third of a day longer, although without heavier bleeding. Soy-group participants also seemed to experience slightly more discomfort with menstruation, but they were not more likely to seek medical attention for pain or cramps.
"The differences were borderline and - given the number of questions we asked and the size of the study - were probably due to chance," said Strom. "The differences do, however, make leads for follow-up studies."
Strom believes that it would be interesting to revisit these 'formula infants' as they reach their middle and elder years, and says that work still needs done to understand the effects of long-term exposure to phytoestrogens.
While breast feeding remains the preferred means of sustenance for infants, soy formula has been shown to be an excellent source of nutrition for infants. Nearly 20% of all infants in the United States are fed soy formula sometime during their first year of life.
"The good news is that, for children that need formula, soy formula is as safe as cow milk-based formula," said Strom. "As always, the public should be skeptical of accounts that are based on speculation and case reports, rather than proper science."