Healthnotes Newswire — Consumption of soy-based formula during infancy does not adversely affect overall health or reproductive function in early adulthood, according to a study published recently in the Journal of the American Medical Association.¹ This finding should reduce concerns that the estrogen-like compounds (known as phytoestrogens) naturally present in soy might pose a risk to developing infants.

Animal studies, some conflicting, have linked the consumption of phytoestrogens to abnormal sexual development and reduced sperm counts. Similar problems could theoretically occur in humans who consume relatively large amounts of phytoestrogens during an important stage of development. Researchers therefore performed a long-term follow-up study of 811 men and women (ages 20 to 34 years) who had been fed either soy formula or cow’s milk formula during infancy. No significant differences were found between the soy and cow’s milk groups for 30 different outcomes, including height, weight, age of onset of puberty, breast size, or proportion of women who had had at least one pregnancy.

There were a few minor differences between groups; for example, the duration of menstruation was slightly longer and discomfort with menstruation was more common in women fed soy formula than in those fed cow’s milk. However, according to one method of analyzing the data, these differences were not statistically significant.

Several factors should be considered before soy formula is pronounced free of any potential adverse effects. First, only 42.2% of the women fed soy formula during infancy had had at least one pregnancy, compared with 47.8% of the women fed cow’s milk formula. Although this difference was not statistically significant, it is consistent with the possibility that consuming soy formula may lead to infertility in a small proportion of women. Longer follow-up periods with larger groups of women would be needed to investigate that possibility more fully.

Second, some of the authors of this report have financial ties to one or both of the manufacturers of two of the most widely distributed infant soy formulas, Isomil® and ProSobee®. While such conflicts of interest do not necessarily invalidate the data, they should alert the reader to the potential for conscious or unconscious bias in the design or interpretation of the study. For example, depending on one’s point of view, the reduced pregnancy rate described above could be passed off as not statistically significant, or flagged as a potential concern that warrants additional study. Finally, virtually all doctors and scientists agree that human breast milk, not soy or cow’s milk formulas, is the optimal food for infants. Despite these caveats, it is reassuring that no obvious health problems were identified in this group of young adults who received soy formula as infants.