Folate

A B vitamin needed to build DNA, so every tissue that divides quickly depends on it.

Deficiency enlarges red cells in the same way low B12 does, which is exactly why the two are measured together.

Treating a folate deficiency while a B12 deficiency is missed can let nerve damage progress unnoticed, so a low folate is a reason to check B12 rather than to supplement alone. The serum value reflects recent intake, so a meal or a supplement before the draw lifts it.

Worth discussing a low value together with B12.