Women who eat animal products, and specifically dairy, are five times more likely to have twins if they become pregnant, says a new US study. Gary Steinman, an obstetrician and specialist in multiple-birth pregnancies, found the results by comparing the twinning rates of pregnant women on vegan diets and those who ate animal products.

    The study, to be published in this month's Journal of Reproductive Medicine, adds to scientists' understanding of how diet may influence pregnancy.

    "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 Steinman, of the Long Island Jewish Medical Center in New York. The study suggests a protein known as insulin-like growth factor (IGF) may be responsible for the increased twinning rate. It says the amount of IGF in a woman's blood may be directly proportionate to her chance of having twins. IGF is released from the liver of both animals and humans and is found in animal's milk.

    Previous studies have indicated IGF may increase ovulation and even help embryos survive in the early stages of their development.

    Women who consume dairy have 13 per cent more IGF in their blood on average than those on a vegan diet.

    Production of the IGF protein is triggered by the presence of growth hormones. This has led Steinman to suggest the introduction of growth-hormone treatment for cows in the 1990s, to enhance milk and beef production, may have contributed to a rise in the number of twins over the last decade.

    The lifestyle trend towards later child birth is also thought to contribute, as older women are more likely to have twins.

    美國一項最新研究顯示，那些常常食用奶制品的女性，生雙胞胎的可能性是那些不常食用奶制品的女性的5倍。研究人員指出，造成這一現象的原因可能與人們喂養奶牛時添加的生長激素有關。

    據生活網日前報道，此項研究的負責人、美國長島猶太醫療中心的加里斯坦曼說，攝入生長激素后，人和動物的肝臟能分泌一種名為IGF的生長蛋白。

    而IGF會隨著血液循環進入動物的乳汁。IGF能增加排卵作用同時還可能有助于懷孕早期階段胚胎的存活。

    研究顯示，那些秉承絕對素食主義的婦女血液中IGF的濃度要比那些常食用奶制品的婦女低大約13％。絕對素食者拒絕食用一切源于動物的食物。自1975年以來，生育多胞胎的比率明顯上升。

    其中一部分原因要歸咎于更多的大齡婦女懷孕，因為統計數據顯示，大齡婦女更易生育雙胞胎。然而，生育雙胞胎的比率在上世紀90年代仍持續增長則可能是由于人們為了給奶牛催奶或增加牛肉產量使用生長激素的結果。