Over the last 30 years, the number of twin births has nearly trebled. This rise seems to have followed the introduction of in vitro fertilization and a preference for having children later in life. But in the mid-1990s, doctors began limiting the number of transferred embryos and still the proportion of twin births rose. Now new research seems to show that growth hormone in the food supply may be responsible.
"The continuing increase in the twinning rate into the 1990s, however, may also be a consequence of the introduction of growth hormone treatment of cows to enhance their milk and beef production," argues physician Gary Steinman of the Long Island Jewish Medical Center.
Steinman and his international colleagues solicited answers on a questionnaire from mothers. He then compared the number of twin births from those who consumed meat and/or milk and those who consumed no animal products at all. The omnivores and vegetarians were five times more likely to have twins than the vegans.
Steinman argues that insulin-like growth factor, a protein released by the liver in response to growth hormone, may be the reason. Studies have shown that the protein increases ovulation and that it persists in the body after entering via digested food, particularly milk. Daily drinking of a glass of milk over a 12-week period raised levels of the protein in the body by 10 percent and vegan women have 13 percent lower concentrations of it in their blood.
There is also a genetic factor at work. Africans--particularly the Yoruba tribe of Nigeria when eating their traditional diet of yams--have the highest number of twin births while the Japanese experience the lowest. Yet, when Japanese people move to California their twinning rate doubles and Yorubas eating a European diet drop to match Caucasian levels of twin births.
The insulin-like growth factor seems to play a key role in explaining the recent trend. Although the twinning rate in the U.K.--where bovine growth hormone is banned--rose by 16 percent between 1992 and 2001, it increased by 32 percent in the U.S., where the substance is not banned, Steinman noted in a comment in the May 6 issue of The Lancet. "This study shows for the first time that the chance of having twins is affected both by heredity and environment or, in other words, by both nature and nurture," Steinman says. The paper presenting the research appears in the current issue of the Journal of Reproductive Medicine. --David Biello