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Friday 26th May, 2006
Dietary changes were found to affect a
woman's chances of having twins - The Journal of
NEW HYDE PARK, NY ? An obstetrician well known for his care
of and research into multiple-birth pregnancies has found that
dietary changes can affect a woman's chances of having twins,
and that her overall chance is determined by a combination of
diet and heredity.
By comparing the twinning rate of vegan women, who consume no
animal products, with that of women who do eat animal products,
Gary Steinman, MD, PhD, an attending physician at Long Island
Jewish (LIJ) Medical Center in New Hyde Park, NY, found that the
women who consume animal products, specifically dairy, are five
times more likely to have twins. The study is published in the
May 2006 issue of the Journal of Reproductive Medicine,
available May 20.
The Lancet recently published an invited comment by Dr.
Steinman on dietary influences on twinning in the journal's May
The culprit may be insulin-like growth factor (IGF), a
protein that is released from the liver of animals -- including
humans -- in response to growth hormone, circulates in the blood
and makes its way into the animal's milk. IGF increases the
sensitivity of the ovaries to follicle stimulating hormone,
thereby increasing ovulation. Some studies also suggest that IGF
may help embryos survive in the early stages of development. The
concentration of IGF in the blood is about 13 percent lower in
vegan women than in women who consume dairy.
The twinning rate in the United States has increased
significantly since 1975, about the time assisted reproductive
technologies (ART) were introduced. The intentional delay of
childbearing has also contributed to the increase of
multiple-birth pregnancies, since older women are more likely to
have twins even without ART.
"The continuing increase in the twinning rate into the
1990's, however, may also be a consequence of the introduction
of growth-hormone treatment of cows to enhance their milk and
beef production," said Dr. Steinman.
In the current study, when Dr. Steinman compared the twinning
rates of women who ate a regular diet, vegetarian diet with
dairy, and vegan diet, he found that the vegan women had twins
at only one-fifth the rate of women who commonly do not exclude
milk from their diets.
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In addition to a dietary influence on IGF levels, there is a
genetic link in numerous species of animals, including humans.
In cattle, regions of the genetic code that control the rate of
twinning have been detected in close proximity to the IGF gene.
Researchers have found through large population studies of
African American, Caucasian and Asian women that blood IGF
levels are greatest among African Americans and lowest in
Asians. Some women are just genetically programmed to make more
IGF than others. Twinning rates in these demographic groups
parallel the IGF levels.
"This study shows for the first time that the chance of
having twins is affected by both heredity and environment, or in
other words, by both nature and nurture," said Dr. Steinman.
These findings are similar to those observed in cows by other
researchers, namely that a woman's chance of having twins
appears to correlate directly with her blood level of
insulin-like growth factor.
"Because multiple gestations are more prone to complications
such as premature delivery, congenital defects and
pregnancy-induced hypertension in the mother than singleton
pregnancies, the findings of this study suggest that women
contemplating pregnancy might consider substituting meat and
dairy products with other protein sources, especially in
countries that allow growth hormone administration to cattle,"
said Dr. Steinman.
Dr. Steinman has been studying factors that cause or
contribute to twinning ever since he delivered a rare set of
identical quadruplets in 1997 at LIJ Medical Center. His most
recent study published in this month's Journal of Reproductive
Medicine on fraternal, or dizygotic, twinning is the seventh in
a series. The other six studies, published in the same journal,
focused on identical, or monozygotic, twinning. Some of his
findings are summarized below.
Previous twinning studies
Dr. Steinman found that women who become pregnant while
breastfeeding are nine times more likely to conceive twins than
women who are not breastfeeding at the time of conception. He
also confirmed findings by others that identical twin sets are
more often female than male, especially in conjoined twin sets,
and that monozygotic twin sets are more likely to miscarry than
dizygotic sets. Dr. Steinman also found evidence through
fingerprint analysis that as the number of fetuses in a
monozygotic set increases, so does the level of physical
diversity among them. In his most recent study of the mechanisms
of twinning prior to the new study, Dr. Steinman confirmed that
use of in vitro fertilization (IVF) methods increases the
incidence of monozygotic twinning -- where the transfer and/or
implantation of two embryos results in three infants -- and he
proposed that adding more calcium or reducing the chelating
agent ethylenediamine tetraacetic acid (EDTA) in the IVF
incubation media might decrease the unwanted complication.