Folic acid supplements up asthma risk

Mums who take folic acid supplements throughout their pregnancy, in a well-intentioned attempt to boost baby's health, are increasing their risk of having a child with asthma, a study has found.

A groundbreaking Australian study, which could explain rising rates of the chronic lung condition, found pregnant women who overuse the supplement were 30 percent more likely to have a child with asthma.

Taking a folic acid supplement is recommended for women who intend to fall pregnant, as it is known to prevent neural tube defects during a foetus' critical first weeks in the womb.

Associate Professor Michael Davies, from the University of Adelaide's Robinson Institute, has uncovered an "additional and unexpected" consequence for women who also take the supplement throughout their pregnancy.

"We see a substantial proportion of women taking these folate supplements throughout pregnancy, and it may be because people think it is entirely benign," Dr Davies told AAP on Wednesday.

"Folate is incredibly important because of its role in preventing neural tube defects (like spina bifida) but because it is so important, and so bioactive, it needs to be treated with some respect as well."

Dr Davies reviewed the cases of more than 550 women who had given birth, assessing their diet and supplement intake before pregnancy and then rates of asthma among their children.

For women who took the supplements according to advice - before conception and not more than several weeks into their pregnancy - there was no increased risk of asthma.

However, Dr Davies found women who took the supplement throughout their pregnancy, or from 16 to 30 weeks, increased their risk of having a child with asthma by about 30 percent.

"Our finding should be reassuring to women who take folate for the purpose of preventing neural tube defects, because we found no evidence of early supplementation (leading to asthma)," Dr Davies said.

"It was really only use in later pregnancy, it was confined to that area alone ... and only for supplemental use."

Dr Davies said having a diet rich in green leafy vegetables, certain nuts and fruits known to be sources of folate (the natural form of folic acid) did not carry an asthma risk.

Childhood asthma rates have been increasing across developed countries and Dr Davies said his research showed how changing diet, and particularly the overuse of a supplement, could play a role.

It also comes after mandatory fortification laws required Australia's bakers to add a small amount of folic acid to their bread products (organic flour excluded).

This move, from October, was designed to address a known folate deficiency across the community.

"There is no evidence to suggest dietary sources of folate, or even folate fortification, increases the risk of asthma," Dr Davies said.

"(But) that is a sensible question for further study".

The research is published in the American Journal of Epidemiology.