Twins, race and diet

Christina Verni:

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 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.

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