No woman who's expecting, or expecting to expect, can avoid the advice, from any doctor or health site worth its salt: Take folic acid. The vitamin deserves its exalted status. When women take it before and during pregnancy, it reduces the risk of devastating neural-tube defects in the fetus. It's one of the only things we can do to improve fetal health that is supported by rigorous evidence.
But these days millions of women (and men) are getting high doses of the vitamin—and not because we're piling our plates with natural sources like spinach and collard greens. Since the late 1990s, the United States has fortified many breads, cereals, and pastas with folic acid. And then there are the supplements: both in regular multivitamins and freestanding folic acid pills. For pregnant women, the government recommends a daily dose of 400 micrograms in addition to any folate, the naturally occurring form of the vitamin. But millions of expectant mothers, as well women who are not yet pregnant, go hog-wild, taking more than 1,000 micrograms in pill form a day, an amount the government deems the "tolerable upper intake level." Somewhere along the line, it seems, folic acid crossed the line from vitamin to talisman.
But folic acid is a powerful drug, even if its exact workings remain something of a mystery. For the population at large, it might increase or decrease the risk of certain cancers, depending—perhaps—on whether abnormal cells are already present. A recent mouse study also hints that supplements during pregnancy may boost the risk of breast cancer in the pups (though this has yet to be confirmed in humans). Much of the science here is embryonic, and no reason to shun the vitamin. But there are enough question marks that pregnant women would be smart to stop popping it so freely and to stick to the recommended dose.
The discovery that folic acid could reduce the risk of neural-tube defects was a triumph that has helped to protect millions of babies. In a randomized clinical trial published in 1991, British epidemiologist Nicholas Wald proved that pregnant women who took folic acid supplements were less likely to have fetuses with malformations like spina bifida, in which the spinal cord doesn't develop properly. Since then, because of the fortification of breads, cereals, pastas, and other grains, as well as the emphasis on supplements during pregnancy, the rate of these birth defects has decreased substantially . In the United States today, about two-thirds of pregnant women take supplements with folic acid. Yet Americans have never been able to resist too much of a good thing, and this vitamin is no exception. According to Cathrine Hoyo, an epidemiologist at Duke, one worry has long been that too much folic acid can mask deficiencies in vitamin B12, which are associated with anemia and may be linked to cognitive impairment.
But the big debate these days is over the vitamin's ambiguous relationship to cancer, both in adults and in fetuses. On one hand, a large body of research suggests that getting adequate amounts of folate protects against colon cancer—and possibly prostate cancer, as well. One theory is that the vitamin helps to maintain the integrity of DNA, preventing the kinds of errors that lead to malignancy.