Fortification of flour with folic acid, associated with reducing the occurrence of neural tube defects (NTD) like spina bifida and anencephaly, was made compulsory in the US in 1998, and has led to the number of birth defects falling by 25 per cent.
The new study, published in the American Journal of Obstetrics and Gynecology (Vol. 194, pp. 520-526), could have implications for Europe where folic acid fortification is not mandatory, but is being seriously considered by some countries like the UK and Ireland, where experts predict its introduction is a question of 'when' and not 'if'.
The new cross-sectional study assessed the folic acid levels in the blood of 9421 women in southern California with different ethnicity/race, vitamin use, weight and age.
The researchers found that pregnant black and Hispanic women had lower serum folic acid levels, despite the fact that all cereals and grain are fortified in the US. These groups were associated with lowest vitamin use: about 25 per cent of both groups were regular supplement users, compared to 44 per cent of non-Hispanic whites (NHW).
"Our study shows that, even after food fortification with folic acid in the United States, taking vitamin supplements regularly has a large effect on serum folate levels," wrote lead author Jean Lawrence from Kaiser Permanente Southern California.
The average serum folate level for all the women surveyed was 19.8 ng per mL, with black women associated with an average of 17.9 ng per mL, while NHW had average levels of 22.1 ng per mL.
The optimum serum level of folate that protects against NTD is not known.
"The relation between folate status and NTD risk may be an inverse linear one rather than a threshold effect, which suggests that lower NTD rates are possible."
"Until the optimum level of serum folate is defined, women of childbearing age should be encouraged to take a vitamin that contains folic acid daily to ensure adequate folic acid status," wrote Lawrence.
A limitation of the study that should be noted is that the researchers did not collect any information about dietary folate intake and merely related multivitamin or folic acid supplements to serum folate levels measured by a blood test.
European strategy has been to recommend and prescribe folic acid supplements. However, a recent study (British Medical Journal, Vol 330, pp 571) reported that the advice did not result in women actually taking the supplements.
The new study implies that folic acid fortification of cereals and grains may also not be sufficient, suggesting that a policy combining the two should be pursued to reduce the incidence of NTD.
Another possibility, suggested by some campaigners, is to raise the level of folic acid fortification in the US from its current level of 140 micrograms per 100 grams of grain.
The Scientific Advisory Committee on Nutrition (SACN), a group of independent experts that advises the UK's food authorities and health department, said recently: "mandatory fortification of flour with folate should be introduced" as long as proper consideration is given to a number of related issues, including the level of fortification and the form of folate to be used.
Studies have shown that folic acid is more easily absorbed from fortified foods (85 per cent) and supplements (100 per cent) than the folate found naturally in foods (50 per cent).