One of the concerns most often raised by anti-soy groups is that because soy contains high levels of isoflavones, which mimic the female sex hormone estrogen, chronic exposure to high levels of soy in early life can lead to more feminine and less fertile males. However a new study, published in the journal Environmental Health Perspectives last month, has found that early life exposure to soy products does not result in more feminine behaviour in younger children.
The study, conducted by researchers at the Gillings School of Global Public Health, University of North Carolina (Chapel Hill), involved 3,664 boys and 3,412 girls who were 3.5 years of age. The researchers evaluated the childrens PSAI scores, which is a measure of the type of gender-linked activities a child participates in (higher scores indicate more masculine play while lower scores indicate more feminine play).
The researchers found that neither early soy exposure (exposure to soy milk or formula at <4 months age and/or sustained use at 6 months), or late soy exposure (soy milk or formula use between 4-15 months of age), were associated with PSAI scores in males. Interestingly, early soy exposure was associated with slightly higher (more masculine) PSAI scores in girls although scores were still in the normal range. Late soy exposure did no significantly influence PSAI scores in girls.
Other factors associated with higher PSAI scores in both boys and girls included: smoking during pregnancy, presence of an older brother, and advanced maternal age. Higher levels of maternal education were associated with lower PSAI scores in boys and higher PSAI scores in girls. Children, particularly boys, who had been primarily breast fed during infancy tended to have sightly lower PSAI scores than formula fed children.
The research comes on the back of several other studies published in the last few years that have looked at the effect of soy foods on males. One well known study, published in 2008 by researchers at the Harvard School for Public Health, found that those who consumed soy based foods on a regular basis had lower sperm concentrations than those who consumed little or no soy. The findings were controversial however in that the relationship was only significant in overweight and obese men, and only for those at the highest end for soy food consumption.
Other studies have found no relationship between soy intake and sperm quality. In 2008, Canadian researchers looked at the effects of soy protein isolate and milk protein isolate on sperm quality in 32 healthy males. No significant differences were found in sperm count, motility, or morphology, between the soy and milk protein groups. The researchers concluded that “Consumption of soy protein of low or high isoflavone content does not adversely affect semen quality in a sample of healthy adult men.”
Further evidence against a relationship between soy intake and male fertility can be found by the observation that Asian males tend to have similar levels of fertility to Caucasian men, despite consuming much more soy, although any differences in fertility due to soy consumption may be masked by the fact that Asian men are less likely to be overweight or obese than their Caucasian counterparts (obesity is a fairly strong risk factor for low fertility).
In any event, scientific evidence to date does not support a strong link between soy intake and either femininity, or lower levels of fertility in males and parents should not be unduly concerned about introducing soy products to their children for these reasons.