December 2 (HealthDay News) -- Infants whose mothers take folic acid (folate) during early pregnancy may have an increased risk for respiratory illness, according to a Norwegian study.

Folate is recommended to reduce the risk of birth defects, and many countries fortify their flour with folic acid.

The researchers examined data on more than 32,000 children born between 2000 and 2005 who were part of the Norwegian Mother and Child Cohort Study. The mothers had been surveyed about their dietary habits and intake of supplements (including folate) at several points before and after the birth of their children.

After they adjusted for other factors, the researchers found that infants whose mothers took folate supplements in the first three months of pregnancy were slightly more likely than other infants to have wheezing and/or respiratory infections up to the age of 18 months. These children were also 24 percent more likely to be admitted to hospital for treatment of their respiratory infection.

The study appears in the current issue of the Archives of Disease in Childhood.

The researchers noted that folate and other vitamins affect a biochemical process called methylation, which in turn alters genetic activity. There's been no comprehensive research into the impact of methylation on the immune system and respiratory diseases. However, recent evidence suggests that methylation may play an important role in the development of certain immune T-cells and could influence the development of airway inflammation in early childhood, the Norwegian team suggested.

Studies in mice have shown that high levels of folic acid and other similar substances in early pregnancy increase the risk of allergic asthma in offspring.

SOURCE: BMJ Specialists Journals, Dec. 2, 2008