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  3. For mothers-to-be, doctors worldwide advise taking a folic acid supplement. That’s because pregnant women with a deficiency of t

For mothers-to-be, doctors worldwide advise taking a folic acid supplement. That’s because pregnant women with a deficiency of t

admin2012-06-02  37
问题     For mothers-to-be, doctors worldwide advise taking a folic acid supplement. That’s because pregnant women with a deficiency of this vitamin have an increased chance of giving birth to a baby with serious birth defects. Yet a new mouse study shows that folic acid supplementation can itself sometimes increase the risk of birth defects or even cause the death of embryos. Experts caution, however, that the unexpected rodent results are too preliminary to require an immediate change in medical practices until more is known about how the vitamin influences development.
    People typically obtain folic acid, from consuming leafy vegetables, but not everyone gets enough from their diet, particularly pregnant women. Evidence from randomized clinical trials has shown that babies born to women who double their recommended daily dose of folic acid are between 40% and 50% less likely to have birth defects of the spine, skull, and brain. As a result, the United States has fortified most of its grains with this vitamin since 1998, and a handful of other countries have followed suit.     But just how the vitamin influences embryonic growth remains a mystery. So developmental biologist Lee Niswander of the University of Colorado, Denver, set out to investigate folic acid supplementation in mice genetically predisposed to giving birth to embryos with neural tube defects. She and her colleagues fed five such strains of mutant mice food containing either 2 or 10 mg of folic acid per kilogram, which created a range of blood levels of the vitamin equivalent to that in the U. S. population.
    To the surprise of the researchers, in three of the five strains, the extra folic acid seemed to worsen the severity of birth defects rather than remedy them. In one of the mutant lines, eating the higher folic acid diet long term increased the chance that young were born with neural tube defects from 20 % to 60 % . And for another strain, many of whose embryos don’t naturally survive until birth due to their genetic problems, eating the higher folic acid food significantly increased the percentage of lost embryos.
    Niswander says it is clear that folic acid is good for human fetuses, but the new study makes her wonder whether high levels of the vitamin may be harmful in some circumstances. Still, she stresses that more data are needed before any serious reconsideration of how much folic acid to recommend for impending mothers.
    Roy Pitkin, a retired researcher of University of California, Los Angeles, who specialized in pregnant women’s nutrition and chaired an Institute of Medicine panel that in 2000 reviewed folic acid’s health effects also cautions against a rush to judgment: "It would really be throwing the baby out with the bath water to say that because of this one mouse study, we are going to question the food fortification. "
Based on the research, Niswander cautions that
选项 A、the practice of recommending folic acid for the pregnant needs reconsidering.
B、it is still controversial whether folic acid is good for human fetuses.
C、it is too early to judge the effects of folic acid.
D、pregnant mothers cannot take an overdose of folic acid.
答案C
解析 推理判断题。第五段Still一句发生转折，虽然Niswander怀疑高含量的叶酸在一些情况下对人有害，但是她仍强调，在重新考虑推荐准妈妈们服用多少叶酸之前需要收集更多的数据(即仍需做更多的研究工作)。据此判断，Niswander认为该项研究还不足以说明叶酸对人的影响，[C]“评估叶酸的影响为时尚早”正确。[A]与[B]与文意相反。[D]文中未提及。
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