But the story isn't quite that simple. As early as the 1940s, the legendary oncologist Sydney Farber found that when he gave folic acid to patients with acute leukemia, their cancers proliferated wildly. (Indeed, the first highly effective cancer chemotherapy, which Farber helped to discover, worked by counteracting folate. The anti-cancer drugs methotrexate and fluorouracil do the same.) More recently, a paper in the Journal of the American Medical Associationfound that people who had a history of adenomas, or benign tumors that may become malignant, and who took folic acid supplements did not protect themselves from a recurrence, as some researchers had hypothesized. In fact, they might have increased their risk. The data are still messy, but there's enough evidence to feel nervous about overdoing it on the supplements, as this review points out. One theory is that once abnormal cells are present, folic acid may aid and abet their spiraling growth.
This is worrisome for the general population, because abnormal cells creep up on us with age. A large number of older people have precancerous growths in their colons. Similarly, large numbers of older men have abnormalities in their prostates. Many of these will never progress or do any harm. But if it's true that excess intake of folic acid goads them on, it might actually cause more cases of colon and prostate cancer, says Joel Mason, a professor of medicine and nutrition at Tufts. This would be especially true for older people taking high levels of the vitamin in supplement form. (There's no evidence that naturally occurring folates in food are harmful, by the way, so kale is not a culprit.)
So what about those pregnant women downing all that extra folic acid? Young women do tend to have lower cancer risk than older people. Certainly their unborn children have had far less time to accumulate precancerous abnormalities than their soon-to-be grandparents. So for those fetuses, we might expect folic acid to reduce the risk of cancer—perhaps by stabilizing the DNA, as researchers think might be the case in healthy adults, as well. This is why a recent study of pregnant mice comes as such a surprise. Canadian researcher Young-In Kim and his colleagues gave pregnant and nursing mice the mouse equivalent of roughly 800 to 1,000 micrograms of folic acid (an amount that many pregnant women today are ingesting daily, above and beyond naturally-occurring sources in food). They then exposed the pups to a chemical known to induce breast cancer. But instead of being more protected, the pups whose moms got folic acid were actually substantially more likely to develop tumors. No one really knows why this happened—or whether the results will mean anything in people (plenty of cancer studies in mice don't). But the researchers suggest that folic acid might be altering DNA expression, quieting genes that would otherwise have been protective.
Speculation about folic acid has evolved into its own mini-genre, well beyond these cancer studies. One paper even suggests that too much folic acid for mom might increase the risk of asthma for her offspring in early childhood (though the link remains uncertain). The point is not to fan fears that too much of the vitamin is cataclysmic. But a little caution would not be a bad idea. So pregnant women, yes—by all means, take a multivitamin with the recommended amount of folic acid. But don't also buy fortified drinks and stand-alone folic acid supplements. Spend the money instead on something you could really use: an extra body pillow, perhaps, or a big bottle of Tums.