|Component in Soy Products Causes Reproductive Problems in
Genistein, a major component of soy, was found to disrupt the development of
the ovaries in newborn female mice that were given the product. This study adds
to a growing body of literature demonstrating the potentially adverse consequences
of genistein on the reproductive system.
“Although we are not entirely certain about how these animal studies on genistein
translate to the human population, there is some reason to be cautious,” said
Dr. David A. Schwartz, Director of the National Institute of Environmental Health
Sciences (NIEHS). “More clinical studies are needed to determine how exposure
during critical windows of development can impact human health.”
Genistein is the primary naturally occurring estrogen in plants (called phytoestrogens)
and can mimic the effects of estrogen in the body. Genistein can be found in
foods containing soy such as soy-based infant formulas as well as over-the-counter
The results of this study conducted by researchers at the National Institute
of Environmental Health Sciences (NIEHS), part of the National Institutes of
Health, in collaboration with an investigator at Syracuse University, are published
in the January issue of Biology of Reproduction.
The NIEHS researchers previously showed that mice given genistein immediately
after birth had irregular menstrual cycles, problems with ovulation, and problems
with fertility as they reached adulthood. The new study looks at the direct effects
of genistein on the ovaries during early development.
“We knew genistein was linked to reproductive problems later in life, but we
wanted to find out when the damage occurs,” said Retha R. Newbold, MS, a developmental
endocrinologist at NIEHS and an author on the study. “The study showed that genistein
caused alterations to the ovaries during early development, which is partly responsible
for the reproductive problems found in adult mice.”
Female mice were injected with three different doses of genistein during their
first five days of life. The genistein given to the mice was comparable to what
human infants might receive in a soy-based formula, which is approximately 6-9
mg/kg per day. The researchers examined the effects on days 2 through 6.
The researchers found effects at all levels. Mice treated with the high dose
(Gen 50 mg/kg) were infertile and mice treated with lower doses were subfertile,
meaning they had fewer pups in each litter, and fewer pregnancies. Mice receiving
the highest level of genistein, 50 mg/kg per day, had a high percentage of egg
cells that remain in clusters, unable to separate and therefore develop abnormally.
The researchers explain that oocytes that remain in clusters are less likely
to become fertilized based on previous research. The largest difference between
the genistein treated and normal mice was found at six days of age where 57 percent
of the egg cells in the non-treated ovaries were single or unclustered; and only
36 percent in the genistein treated group were single.
We think genistein inhibits the oocytes or egg cells from separating apart,” said
Wendy Jefferson, Ph.D. of NIEHS and lead researcher on the paper. “Since there
are many egg cells in the same follicle instead of just one, the resources from
the surrounding cells are spread too thin and they can’t get the support they
need to become a mature functioning egg cell.”
“You need at least one good healthy single oocyte that is ovulated and fertilized
by a sperm to get a healthy baby. Genistein seems to have a way of making this
task very difficult,” said Newbold.
“I don’t think we can dismiss the possibility that these phytoestrogens are
having an effect on the human population,” said Dr. Jefferson. “They may not
show their effects or be detected until later in life, but chances are they are
having an effect.”
Note: The National Toxicology Program, Center for the Evaluation
of Risks to Human Reproduction (CERHR) will hold an independent expert panel
meeting on “Genistein and Soy Formula” on March 15-17, 2006, at the Radisson
Hotel Old Town, Alexandria, VA. http://ntp.niehs.nih.gov/files/GenisteinSoyMtg.pdf.
The NTP is an interagency program headquartered at NIEHS.
NIEHS, a component of the National Institutes of Health, supports research
to understand the effects of the environment on human health. For more information
on environmental health topics, please visit our website at http://www.niehs.nih.gov/home.htm.
The National Institutes of Health (NIH) — The Nation's Medical Research
Agency — includes 27 Institutes and Centers and is a component of
the U. S. Department of Health and Human Services. It is the primary Federal
agency for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.