Folic acid in the food supply reduces birth defects, but may cause extra cancer

For 20 years, the United States has been fortifying some foods with folic acid (the synthetic form of folate, a B vitamin) as part of a public health effort to prevent neural tube defects in newborns. The effort is paying off: the rate of neural tube defects is down by 30%. But there’s also evidence that the added folic acid is contributing to colorectal cancers, reports the March 2008 issue of Harvard Women’s Health Watch.

In the early years after folic acid fortification was mandated, the average level of folate in the blood of Americans nearly doubled. During that same period, a 15-year decline in colorectal cancers suddenly reversed. Although this link is not proof of cause and effect, there are biological reasons why extra folic acid may be to blame.

Only long-term monitoring can reveal whether fortifying foods with folic acid is safe—and clarify who is more (or less) likely to benefit. For now, unless you’re pregnant, lactating, or have a recognized folate deficiency, the daily recommended intake of folic acid is 400 mcg—the amount found in a multivitamin. The maximum safe amount from fortified foods and supplements is 1,000 mcg per day. With all the folic acid in fortified foods, you may exceed that limit, especially if you are taking a multivitamin. So it’s a good idea to check nutrition labels and make sure your daily folic acid intake is within bounds.

Harvard Women’s Health Watch notes there’s no known health risk from foods naturally high in folate, so try to get much of your daily requirement from a diet rich in fruits and vegetables.