More questions are being raised about the safety of folic acid supplementation after new research has found links between the B vitamin and increased cancer risk.

Researchers in Norway found that heart-disease patients treated with a combination of folic acid and vitamin B12 had an increased risk of cancer and death compared to patients who didn't receive the vitamins as treatment.

Unlike Canada and the United States, Norway doesn't require folic acid to be added to any food. The market for vitamin supplements is also relatively small and study participants were discouraged from taking them, which gave researchers a unique ability to assess the effect folic acid could have on a group who receive it in high doses. The study, appearing Nov. 18 in the Journal of the American Medical Association, fuels fears that mandatory fortification of the food supply with folic acid could yield unintended consequences.

"Folic acid fortification and supplementation may not necessarily be as safe as previously assumed," Marta Ebbing, the study's lead author and a physician at Haukeland University Hospital, said in an interview yesterday.

The issue has come under increasing scrutiny and debate in the medical community in recent years as a growing number of studies have suggested that high amounts of folic acid can potentially speed up the progression of cancer in genetically predisposed individuals.

The debate is complicated by the fact that folic acid, when taken by expectant mothers, significantly reduces the risk of children being born with neural tube defects, such as spina bifida.

Researchers caution that much more work needs to be done to understand the potential risks and whether any changes in public health policy are needed.

Folic acid is the synthetic form of folate, a vitamin found naturally in leafy greens and other fruits and vegetables. The federal government has required food manufacturers to add folic acid to white flour, enriched pasta and cornmeal products since 1998 as a way of ensuring women receive enough of the vitamin to curb the incidence of neural tube defects in the population. But some food makers may also add folic acid to other items, such as cereals, on a voluntary basis.

Although the amounts added to food aren't very high, some researchers are worried Canadians who also consume multivitamins or supplements containing folic acid may be getting too much.

"We are concerned about folic acid supplementation actually promoting existing cancer," said Young-In Kim, professor of medicine and nutritional sciences at the University of Toronto and gastroenterologist at St. Michael's Hospital. "[But] we need to be careful because fortification did wonderful things."

The new research combines two studies of more than 6,000 heart disease patients who received some combination of folic acid and B vitamins or patients who received a placebo. Patients who took vitamins received 0.8 milligrams of folic acid, 0.4 milligrams of vitamin B12 and 40 milligrams of vitamin B6 a day. While some patients received a combination of vitamins, others took vitamin B6 or folic acid alone.

After more than six years of follow up, the researchers found a heightened incidence of cancer and death among those who received folic acid and vitamin B12. Vitamin B6 wasn't associated with any increased risk of health problems.

Dr. Ebbing said the findings suggest that folic acid – and not vitamin B12 – is likely responsible for any increased cancer risk because it was present in higher concentrations.