It is estimated that the addition of 140 micrograms of folic acid to foods will prevent only about 5 to 20 percent of folic acid-preventable NTDs. The FDA did not require that more than this amount of folic acid be added to these foods because of the concern that high levels of folic acid might mask a vitamin B-12 deficiency. This potentially dangerous condition is called pernicious anemia and is seen mainly in elderly people. Left untreated, the vitamin B-12 deficiency can cause irreversible neurologic damage. Some health organizations, including the March of Dimes, feel that the level of fortification can be safely increased without risk to the public's health.
Research currently is under way to examine the possibility.
How folic acid prevents NTDs is not well understood. Most studies suggest that it may correct a nutritional deficiency , while others suggest that supplemental folic acid helps some people compensate for inborn errors in how the body processes folates . These people have problems breaking down folates found in food to forms of folic acid the body can use, resulting in lower folic acid levels in the blood. However, taking folic acid raises levels of the vitamin in the blood, which can reduce the risk of having an affected baby. This risk may be reduced by taking folic acid throughout pregnancy.