The researchers who discovered that too little folic acid causes birth defects haven’t gotten a Nobel Prize (and probably never will) but they should, as this article explains:
After 3 decades of epidemiologic research reporting an association between neural tube defects and maternal use of folic acid, public health organizations developed recommendations and supported interventions to increase folic acid intake among women of reproductive age. In 1992, the US Public Health Service recommended that all women of childbearing age who are capable of becoming pregnant should consume 400 µg of folic acid daily.
. . . In 2005, after the National Campaign and mandatory fortification, approximately 33% of women reported taking a daily supplement of folic acid, only a modest increase from the 25% reported in 1995. However, median blood folate levels among women of childbearing age increased from 4.8 to 13.0 ng/mL between 1994 and 2000, with a more recent study reporting median blood folate levels at least 2 times the levels prior to fortification.
To evaluate the impact of this public health intervention, 4 study groups have conducted time trend analyses among the US population, and all have reported a decline of neural tube defects after the introduction of mandatory folic acid fortification. Specifically, these studies reported an 11%–20% reduction in occurrence of anencephaly and a 21%–34% reduction in occurrence of spina bifida when comparing pre- versus postfortification rates. Similarly, the occurrence of anencephaly and spina bifida was observed to reduce 38% and 53%, respectively, in Canada and 46% and 51%, respectively, in Chile following folic acid fortification.
Here is the first article on the subject. As the dean of a school of public health put it, this discovery by itself justifies all the money ever spent on schools of public health.