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 embryos transferred in the course of in vitro fertilization and still the proportion of twin births rose. Now new research seems to show that bovine growth hormone in the food supply may be responsible.
Using data obtained from mothers by way of questionnaire, physician Gary Steinman of the Long Island Jewish Medical Center and his colleagues compared the number of twin births from moms who consumed meat and/or milk and those who consumed no animal products at all. They found that the omnivores and vegetarians were five times more likely to have fraternal twins than the vegans.
In a report published in the current issue of the Journal of Reproductive Medicine, 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. Drinking a glass of milk a day over a 12-week period raised levels of the protein in the body by 10 percent. Vegan women, it turns out, have 13 percent lower concentrations of it in their blood.
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Steinman observed in the May 6 issue of The Lancet that 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. Of the new work he says: "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."