Fortifying flour and bread with folic acid has helped to reduce the incidence of birth defects in Canada, but now, some Canadian doctors are worried we may be getting too much of the vitamin.

Folic acid, the synthetic form of the B vitamin folate, is known to reduce the risk of neural tube defects, such as spina bifida, congenital heart problems and cleft palates.

In fact, since fortification of white flour and other selected grains became mandatory in Canada in 1998, there has been a 46 per cent reduction in the prevalence of neural tube birth defects.

But a number of studies in recent years have raised worries about the effects of too much folic acid. Several studies have suggested that the vitamin may raise the risk of some types of cancer in men, such as colorectal cancer and prostate cancer.

Other research has suggested that because folate allows the synthesis of DNA, the vitamin may actually help cancer cells proliferate in people with the very earliest stages of cancer.

Now, a study in the Canadian Medical Association Journal finds that very few Canadians have folate deficiencies. In fact, more than a third of Canadians have levels that are considered quite high.

This study, conducted by researchers at Children's Hospital of Eastern Ontario Research Institute and The Hospital for Sick Children, is the first in more than three decades to examine the folate status of Canadians.

Researchers looked at 5,248 Canadians aged 6 to 79 years and found that less than 1 per cent showed folate deficiencies. Yet 40 per cent showed high folate concentrations.

However, in a subset of women of childbearing age – for whom adequate levels of folate is most important – 22 per cent had blood levels below the concentration considered safe to guard against neural tube defects.

The study raises worries that those who need folate the most are not getting enough, even with grain fortification, while others are getting high doses.

The study's lead author, Cynthia Colapinto, of the CHEO Research Institute, notes that folic acid fortification was designed to target women of childbearing age, but it may also be exposing the entire population to high levels of the synthetic vitamin.

"Given the absence of folate deficiency in the general population and the apparent shift toward Canadians having high serum folate concentrations, there appears to be little rationale for doubling folic acid levels in the Canadian food supply," she said in a news release.

The authors note that improved folate status, in part through fortification, has led to positive health outcomes such as the dramatic reduction in neural tube defects.

"However, given speculations about the possible adverse effects associated with high levels of folic acid, including increased risk of certain cancers in those with pre-existing neoplasms, further attempts to improve the folate status of Canadian women of childbearing age by increasing fortification levels should be approached cautiously," they write.

The authors add that ongoing monitoring of the folate status of Canadians and the relationship between folic acid and health outcomes is needed.

Because neural tube defects occur in the first weeks of a pregnancy -- often before women realize they are pregnant -- Health Canada recommends all women of childbearing age take 0.4 mg of folic acid a day.

Health Canada also advises that taking more than 1 mg a day of folic acid without the advice of a doctor is not recommended.