The evidence came from national flour supplies, not from a trial
Across 1.9 million Canadian births, neural tube defects fell from 1.58 to 0.86 per 1000 after fortified flour arrived in 1998. Chile fell 55% and Costa Rica about 60%.
What the study found
Canada fortified its flour and neural tube defects fell 46%
No randomised trial ever settled whether folic acid prevents a first neural tube defect. Whole countries did. Across 1.9 million Canadian births, defects fell from 1.58 to 0.86 per 1000 after fortified flour arrived in 1998; Chile fell 55% and Costa Rica about 60%.
The caveat · Screening and campaigns changed too
None of this was randomised. These are before-and-after comparisons, so anything else that changed over the same years counts too: prenatal screening improved, supplement campaigns ran alongside the fortified flour, and registries differed in what they counted. What this design cannot do is separate the flour from the campaign that arrived with it.