According to StatPearls, synthetic folic acid — as an artificial dietary supplement — needs to be converted into the active form tetrahydrofolate (THF) by dihydrofolate reductase.
In the cells, folic acid is reduced to THF, a biologically active form, in a two-step process that requires two molecules of NADPH and the enzyme dihydrofolate reductase (DHFR).
But naturally occurring folates — as natural constituents of the diet or derived from intestinal flora — are derivatives of tetrahydrofolate, (the active form), so the human body doesn’t need to convert them using dihydrofolate reductase.
If this is the case, why do humans have the enzyme dihydrofolate reductase?