Scientists have known for years that women who take folic acid before and during pregnancy can prevent neural tube defects like spina bifida in their babies. And indeed, since the United States and Canada started fortifying enriched-grain products with folic acid in the mid-1990s, cases of neural tube defects have decreased miraculously, by as much as 50 percent, according to some studies.

Fortifying cereals with folic acid to aid pre-natal health began in the mid-1990s, but may have had an unintended consequence. PHOTO: iSTOCK

But this public health success story may have a downside. Dr. Joel Mason, director of the Vitamin and Carcinogenesis Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging, suspects that fortification may be the cause of approximately 16,500 cases of colorectal cancer in North America each year.

His study, published in Cancer Epidemiology Biomarkers & Prevention, finds a correlation between when companies began fortifying enriched-grain products such as flour, cereal, rice and pasta with folic acid in 1996 (it became mandatory in 1998) and an increase in the rates of colorectal cancer. Colon cancer had been on the decline for 15 years, most likely because more people began undergoing colonoscopies to screen for and remove precancerous polyps. But since fortification, national cancer registries have counted four to six additional cases of colorectal cancer annually for every 100,000 people. That translates to about 15,000 more cases in the United States and 1,500 more in Canada each year than that downward trend would have predicted.

Biologically, the association could make sense. Folic acid is the synthetic form of the nutrient folate, which is a crucial element in the function of a cell, and seems to protect against cancer in normal, healthy cells. If cells have begun to malfunction, however, and turn precancerous or cancerous, folic acid has the opposite effect—it helps the cancer cells replicate, speeding up the growth of the cancer.

Older adults may be particularly at risk from too much folic acid in their diets, Mason says, because as many as 35 to 50 percent of people over age 50 already have precancerous polyps in their colons, but don’t know it. “The addition of substantial quantities of folic acid into the food stream may have facilitated the transformation of benign growths into cancers, or small cancers into larger ones,” says Mason, who is also an associate professor at the Friedman School of Nutrition Science and Policy.

The victory of folic acid fortification over neural tube defects has prompted some groups to call for even more folic acid to be added to the American food supply, while other countries are discussing following suit.

Mason urges caution, debate and more research. In the meantime, older Americans who eat enriched-grain products and also take multivitamins should watch their total intake of folic acid. But they shouldn’t worry about getting their folate naturally from beans, fruits and leafy green vegetables, which can help keep people of all ages healthy.

This story ran in the December 2007 Tufts Journal.