Mason said that while his study is not definitive, its conclusions are not theoretical, either. In the 1940s, when leukemia patients got large doses of folic acid, their cancer growth accelerated. This prompted Dr. Sidney Farber and others to try antifolates for the treatment of childhood leukemia - now considered the birth of chemotherapy.
If there's a 10 percent chance that his concerns about folic acid are valid, Mason said, "I don't think we can afford to take that 10 percent chance." He said he doesn't think the U.S. should stop fortification until more evidence is collected, but that the United Kingdom should wait.
Some folic acid researchers are starting to see the nutrient as a double-edged sword. When it's given to someone with no polyps or tumors, it might prevent their formation. But in patients with growths so small that they're undetectable, folic acid could speed up cancer development, said Dr. Cornelia Ulrich, a researcher at the Fred Hutchinson Cancer Research Center in Seattle.
By age 60, she said, about 30 percent of the population has polyps that could potentially grow faster with folic acid. "We've known for a while that cancer actually likes folate," she said. "There may be this dual effect."
Ulrich said she's not worried about current U.S. fortification levels because flour and pasta account for only a small percentage of the folic acid in many bodies.
She is more concerned about supplements taken by older adults and by cancer patients. Fortified health bars and drinks can provide even more folic acid, adding up to potentially problematic levels.
"There can be groups who easily can have 1 milligram of folic acid a day, usually health-conscious people who think they are doing themselves good," she said.
And while research is slim, she added, "there's reason to believe taking a folic acid supplement could do harm."
Dr. Young-In Kim, a nutrition expert at the University of Toronto who has studied folic acid in animals, said fortification places too many people at risk. He wants to see large-scale fortification suspended while officials target young women with messages about getting enough folic acid.
"We need to carefully consider whether we've done the right thing," he said. "This was the largest human experiment ever done, putting the entire population of the U.S. and Canada on high doses of folic acid."
