Is It Folly to Take Folic Acid?

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, such as spina bifida and anencephaly. 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, or folate, from consuming leafy vegetables, but not everyone gets enough from their diet, particularly pregnant women. The vitamin plays a key role in the development of the neural tube, the embryonic region that gives rise to the spinal cord and brain.

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.

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