Background: Although a daily supplement of 400 micrograms folic acid has been shown to prevent neural-tube defects (NTD), most women do not take the recommended supplement. Thus, food fortification is to be introduced in the USA and is being considered in the UK. Because of safety concerns, the USA has chosen a level of fortification that will increase the average woman's intake by only 100 micrograms. Such an increase, although safe, may be ineffective; but a trial to assess its efficacy would be unethical. Because women with red-cell folate concentrations above 400 micrograms/L have a very low risk of NTD, we undertook a randomised trial of several folic acid doses to find out how much is needed to reach this protective concentration.
Methods: We screened 323 women. 172 with red-cell folate between 150 micrograms/L and 400 micrograms/L were invited to take part in the trial. 121 women were randomly assigned placebo or 100 micrograms, 200 micrograms, or 400 micrograms daily of additional folic acid. Compliance was monitored by having the women sign a dated sheet when taking the tablet. 95 women completed the 6-month study.
Findings: There were significant increases in red-cell folate in all folic acid groups. The placebo group showed no significant change. The median incremental changes and median post-treatment concentrations were 67 micrograms/L (95% CI 43-120) and 375 micrograms/L (354-444) in the 100 micrograms/day group, 130 micrograms/L (108-184) and 475 micrograms/L (432-503) in the 200 micrograms/day group, and 200 micrograms/L (125-312) and 571 micrograms/L (481-654) in the 400 micrograms/day group.
Interpretation: A fortification programme that delivered 400 micrograms folic acid daily to women would protect against NTD, but at the expense of unnecessarily high exposure for many people. Delivery of 200 micrograms daily is also effective against NTD and safer for the general population. Based on projections from the positive folate balance in the group that received 100 micrograms daily, this dose taken continually, as it will be in fortified food, will also produce an important decrease in NTD.