Nobody ran the trial that would settle the everyday dose
The recurrence trial that anchors this advice gave 4mg of folic acid daily, ten times what people are now told to take. The 0.4 to 0.8mg range was inferred, not tested against it.
What the study found
The 400 microgram dose is a policy choice, not a trial result
The recurrence trial anchoring this advice gave 4mg of folic acid daily, ten times what people are now told to take. The 0.4 to 0.8mg range was inferred, not tested against it. Cochrane compared 400 micrograms with higher doses and found no difference, but was not powered to find one.
The caveat · Flour already fortified since 1998
None of this means 400 micrograms is the wrong number. It means nobody ran that trial. The Task Force reaffirmed its position rather than reviewing the evidence from scratch, and it gives a range without saying where in the range to sit. Its advice also addresses a country whose flour has been fortified since 1998, so the benefit of a tablet on top of that is not the benefit the old trials measured.