Emerging evidence suggesting a link between high consumption of folic acid and increased cancer risk is igniting debate among factions of the medical community who fear that either giving the issue attention, or ignoring it completely, will lead to public harm.
Folic acid, the synthetic form of folate, which is found naturally in food, is an important B vitamin that can reduce a mother's chance of giving birth to a child with neural tube defects such as spina bifida.
It's so important, in fact, that folic acid has been added to white flour, enriched pasta and cornmeal products in Canada since 1998.
The fortification program is credited with greatly reducing the rate of neural tube defects in Canada. Members of the country's obstetrics community say it's a critical public health measure and are even pushing for the government to more than double the amount added to food.
But a new wrinkle in that success story is prompting concern. Mounting evidence from research shows there may be a connection between consuming high levels of folic acid and the development or progression of some cancers.
A study by Toronto researchers in this month's Cancer Research journal found high levels of folic acid could cause cancer in a rat's offspring. A 2009 U.S. study published by the Journal of the National Cancer Institute found men who took folic acid supplements had a higher risk of developing prostate cancer over a 10-year period than those who took a placebo.
Although the issue isn't fully understood, experts say that because folic acid plays an important role in cell division and DNA maintenance, consuming too much of it may fuel growth of cancerous or pre-cancerous cells. Researchers say it's unclear what unsafe levels of folic acid may be, but some suggest that anything over 1 milligram a day over long periods could be harmful.
Many scientists agree that the findings are worrying enough to warrant a thorough investigation to determine if fortification and the rising consumption of supplements and multivitamins containing folic acid could be putting people at risk.
"I think we need to talk about this," said Young-In Kim, professor of medicine and nutritional sciences at the University of Toronto and staff gastroenterologist at St. Michael's Hospital, who is considered a leading researcher in the field. "We're trying to help reduce the risk of birth defects, but we don't want to put any other people at risk of developing other problems."
However, some obstetricians oppose the idea that folic acid may be linked to cancer, or are cautious about making that connection.
Gideon Koren, director of the Motherisk program at Toronto's Hospital for Sick Children, says the recent studies have been poorly designed. He worries they could cause women not to take folic acid, leading to more neural tube defects.
The Society of Obstetricians and Gynaecologists of Canada, on the other hand, has become more accepting of the findings.
"It's not something we should turn our head to," said Douglas Wilson, head of obstetrics and the department of medical genetics at the University of Calgary. "We should put money into surveillance."
The SOGC published guidelines in 2007 pushing for increased folic acid fortification and calling for women with certain medical conditions to take up to 5 milligrams a day.
Dr. Wilson, who is chairman of the SOGC's genetics committee, is rewriting the guidelines. The revisions will emphasize ensuring that vulnerable or high-risk women get enough folic acid and remind other women to consume the correct dose, not more. The new guidelines will be made public later this year, Dr. Wilson said.
Joel Mason, professor of medicine and nutrition at Tufts University in Boston, said attitudes in the obstetrics community have begun to shift.
"It's hard to argue with objective scientific evidence," Dr. Mason said.
The Public Health Agency of Canada advises women of childbearing age to take a multivitamin containing 0.4 milligrams of folic acid daily to guard against neural tube defects. But because those defects typically occur before many women realize they're pregnant, the federal government decided to add the vitamin to foods to ensure women get enough.
A 2007 Canadian study published in the New England Journal of Medicine found the rate of neural tube defects dropped from 0.86 per 1,000 live births to 0.4 per 1,000 after mandatory fortification began.
A study published in the Canadian Medical Association Journal last December found that less than 1 per cent of the population has a folic-acid deficiency, but that 22 per cent of women of childbearing age are not consuming enough. (During pregnancy, women must have substantial levels of folic acid in their blood to protect the fetus against neural tube defects).
The gap helps explain why groups such as Motherisk want folic acid added to foods in Canada.
But others are focused on getting to the bottom of the relationship between folic acid and cancer.
"My point is that, regardless of the strength of the existing evidence right now, if … this [cancer]promoting effect of excessive folate intake really exists, we can't afford to overlook the possibility because the ramifications if we choose to ignore it are tremendous, if indeed it exists," Dr. Mason said.