Soy, Pregnancy and Pubertal Development
By Mark Messina, PhD, Executive Director, Soy Nutrition Institute
The age at which puberty occurs is dropping throughout the world, especially for girls. Factors such as maternal diet during pregnancy are thought to contribute to the trend. A new study from Great Britain sought to understand the relationship between soyfood intake during pregnancy and puberty by looking at urinary isoflavone levels in pregnancy and the age of puberty onset in female offspring.1 This first of its kind study raises a number of interesting questions.
Urinary levels of O-desmethylangolensin (ODMA), an intestinally-derived metabolite of the soybean isoflavone daidzein, was associated with an increased likelihood of girls entering into puberty before the age of 11.5 years. In contrast, no relationship was found between puberty and urinary levels of genistein, daidzein or the daidzein metabolite equol. The findings are somewhat surprising since ODMA is thought to be the least estrogen-like of all these compounds.
One possibility raised by the authors is that earlier puberty may simply be the result of a particular phenotype which is also characterized by the ability to make large amounts of ODMA. That is, both the ability to make ODMA and early puberty are consequences of the phenotype and are not otherwise related.
A limitation of the study is that soy consumption is generally extremely low among British women. Therefore, it’s unlikely that even among the women consuming the most soy intake was sufficient to exert a physiological effect.2
The study does point to questions, however, about whether or not isoflavones affect the developing fetus. In Asia, where soyfoods are commonly consumed, no epidemiologic studies have looked at this relationship. While it is known that isoflavones cross the placental barrier, the likelihood of an impact is questionable.3,4 This is because the human fetus is exposed to very high concentrations of endogenously produced estrogen.5 Proportionally, any impact of isoflavones, even in mothers who have a high consumption of soyfoods, would be negligible. This is in contrast to rodents, for example, who have a much lower concentration of uterine estrogen, making them a poor model for studying these kinds of questions.
Research has looked at the relationship between soy consumption in infancy or childhood and puberty onset, but the findings have been mixed, showing both early6 and delayed7,8 puberty onset. Korean researchers found higher serum isoflavone levels in children with precocious puberty but these studies had a number of important weaknesses.9,10 In contrast, Loma Linda University researchers found that childhood soy intake was unrelated to age of menses onset in Seventh-day Adventist girls.11 This homogenous population is a good one for studying soy since many Adventists consume large amounts of soy.
One final observation worth noting is that soy consumption in childhood and adolescence has been linked to lower lifetime risk for breast cancer.12,13 That’s a finding that would not be compatible with early puberty onset which appears to be linked to higher breast cancer risk.14