A soy protein, genistein, long known to affect fertility can change how pregnancies start and progress in female mice treated with it as newborns. The changes make it harder for fertilized eggs to implant and grow, possibly contributing to infertility. The effects were observed at levels comparable to those experienced by human infants feeding on soy formula.
The researchers then retrieved either unfertilized eggs or fertilized embryos from the female mice to assess egg health and reproductive organ development .They set up a series of experiments to ask several questions:
First, could unfertilized eggs grow to early development?
Second, could naturally fertilized embryos grow properly in a normal, untreated mouse?
Third, could fertilized embryos taken from untreated mice grow normally in the treated female mice?
What did they find?
Genistein impaired the implantation process in the female mice that were treated as newborns and mated after sexual maturity. After mating, the number of embryos retrieved from genistein-treated mice was about half the number of those collected from control mice.
Fewer implantation sites were present in the uterus of genistein-treated mice compared to control.
Together, these findings suggest that the reproductive tract environment in the genistein-treated mice is likely not optimal for embryonic development.
Tested in vitro, treated and control eggs were equally capable of being fertilized. However, embryos from genistein-treated females developed more slowly than controls. Over time this difference disappeared.
What does it mean?
Genistein affects the uterus and the reproductive tract -- not the egg quality -- of the adult female mice that were treated with the soy phytoestrogen while in the womb. Fertilized embryos developing in the treated females did not attach and thrive as well as embryos in the control animals, even though eggs from genistein-treated females were as healthy as those from their unexposed counterparts.
The genistein-treated mice also lost more embryos early in their development. Embryo death means fewer births and higher rates of infertility.
Together, the results suggest that the uterus may be an important factor in genistein-induced infertility.
These findings pinpoint one actual cause of the observed infertility of early life exposure to genistein. They also highlight the need for a much better understanding of how soy infant formulas and other products fed to newborns and infants may influence a developing baby's reproductive life.
Egg quality was not affected by newborn genistein treatment. The immature eggs from genistein-treated mice developed normally and produced fertile female mice just like in the controls.
These findings add to a growing body of evidence that implies that newborns that eat soy-based products may be predisposed to lower reproductive success as adults.