Folic acid and neural tube defects: the current evidence and implications for prevention.

The results of the MRC Vitamin Study have established the specific role of folic acid in the prevention of neural tube defects. Folic acid supplementation (4 mg/day) at the time of conception reduced the recurrence rate by about 70%. Evidence from observational studies and the Hungarian randomized trial (that used 0.4-0.8 mg/day supplemental folic acid) indicates that a similar level of prevention can be achieved among women who have not already had an affected pregnancy and that this may be achieved with a lower dose of folic acid. The medical implications of these results are important. (1) Women planning a pregnancy should take folic acid supplements. (2) Because a large proportion of pregnancies are unplanned and many of those that are planned will occur in women who will not have taken folic acid supplements, a complementary general public health strategy is needed, based on dietary advice and food fortification designed to achieve the benefit with adequate safety. (3) Any public health prevention strategy needs to be monitored for efficacy and possible harm. (4) Research is needed on the dose-response relationship between folic acid and neural tube defect prevention and the mechanism of action.