Eating milk and other dairy products could increase a woman's chance of having twins, a US doctor is proposing, based on a study of vegan women.
The rate of twin births in the United States rose by more than 75% between 1980 and 2003. Some of this can be explained by the use of fertility treatments, which ups the risk of multiple births.
But that can't explain all of the jump, researchers say. Bearing twins is more risky for both mother and child than having a single baby, so scientists want to know what's causing the rise.
Gary Steinman of the Long Island Jewish Medical Center in New Hyde Park, New York, carried out a simple comparison: he gathered together childbearing records for more than 1,000 vegan women who do not eat any animal products. He calculated that vegans were around five times less likely to bear twins than omnivorous women or vegetarians who eat dairy food, reports Nature.
The study is published in the May 2006 issue of the Journal of Reproductive Medicine available on 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 reason 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.
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, informs CNN-IBN.O.Ch.
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