The Lancet recently published an invited comment by Dr. Steinman on dietary influences on twinning in the journal's May 6 issue.
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.
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.