A mechanism was traced for the first time in July 2026
Dr Clare Craig
There is only one trial of ordinary women given a lower dose of folic acid in pregnancy and it showed nine pregnancy losses for each prevented neural tube defect. For high risk women, thirty years after trials showed that folic acid at a drug dose could lower the number of affected pregnancies, no one had shown how it did so.
In July 2026 one of the mechanisms was demonstrated. In these models folic acid was not working by simply replacing missing folate. It acted on a signalling pathway changing the levels not of folate but of retinoic acid.
What had gone wrong in the embryo
The experiments used a frog model in which the pax3 gene is disabled, causing neural tube defects. Giving folic acid prevented the defect but so did retinoic acid, which the body makes from vitamin A. There is a separate model in which folic acid corrects defects caused by alcohol and in that model too retinoic acid was protective.
Where retinoic acid levels were too low, the cells destined to become the nervous system multiplied too quickly. The neural plate expanded abnormally and its edges ended up too far apart to meet.
So where did folic acid come in?
The rescue in simple terms
Folic acid increased the amount of an enzyme called ALDH1L1. That enzyme converts a vitamin A derivative into retinoic acid, so more of it meant more retinoic acid. Restoring retinoic acid restored normal closure.
vitamin A → retinaldehyde →(ALDH1L1)→ retinoic acid → closure
Retinaldehyde is derived from vitamin A and is the raw material the enzyme uses. Folic acid enhances the enzyme that converts it. Ordinary folate metabolism continues in parallel. When the researchers also blocked the enzyme that converts folic acid into tetrahydrofolate, using methotrexate, folic acid could no longer rescue the defect. Folic acid itself has to be converted to have an effect on the enzyme. That raises an obvious question the experiments did not answer: could natural folate, which does not need to be reduced by the slow DHFR enzyme, do the same job? Only the synthetic form was tested.
How they showed the enzyme was essential
When the researchers removed the gene for the ALDH1L1 enzyme, giving folic acid no longer rescued the defect. They then put the human version of the enzyme back into those frog embryos but that was insufficient to result in healthy development. Only when retinaldehyde was supplied as well was neural tube closure successful. The enzyme and vitamin A are both required. Separately, in mouse cells, folic acid also raised the enzyme and increased retinoic acid signalling.
Why this is surprising
ALDH1L1 is not even the enzyme normally responsible for making retinoic acid. Other closely related enzymes usually do that job (ALDH1A1, ALDH1A2 and ALDH1A3). ALDH1L1 main function is as part of folate metabolism, and making retinoic acid is a secondary activity.
Other models with no mechanism
The initiating defects in these models were not caused by folate deficiency. One was caused by disabling pax3, the other by alcohol. Folic acid rescued both. Its effect on this separate pathway may explain that.
What is still not explained is how folic acid switches up ALDH1L1. It raised the amount by about half as much again and concentrated it in the tissue that goes on to form the neural plate. The share there rose with the neighbouring region falling by the same amount. The gene can be regulated through methylation of its promoter in other settings, but whether folic acid altered its methylation here, or switched it up by some other route, was not tested.
Too much retinoic acid is also harmful
The required amount and timing of retinoic acid is key for neural development. Too little and the neural plate expands so that its edges fail to meet. Too much is itself a potent teratogen. One mouse model produces spina bifida in 79 percent of fetuses by giving excess retinoic acid during the critical window and a rat model of myelomeningocele is made the same way, by mouth on the tenth day of gestation. The right amount of retinoic acid, at the right time, is what matters.
What does this mean for high risk women?
If the protective effect depends on an enzyme converting a vitamin A derivative, then the availability of vitamin A matters. The researchers tested this directly. At low doses, vitamin A alone did not prevent the defects and folic acid alone barely did. Given together, the two rescued closure efficiently. The authors conclude that in women at high risk of another neural tube defect, optimising vitamin A status might improve the preventive effect of folic acid and they suggest their findings may explain why folic acid fails in some pregnancies. Vitamin A was not considered in the British modelling for folic acid fortification.
