Folate is a B vitamin found in many foods. With B12 and methionine it is used to make methyl groups, it is used to regenerate BH4, and it is involved in purine synthesis.
It isn't shelf stable, so most supplements are folic acid. Folic acid can be converted into folate in the body, involving the MTHFR enzyme. Depending on genetics, some people are more sensitive to their folate status, require more, and do better with food forms as opposed to folic acid.
Reasons to Suspect
If you have MTHFR polymorphisms, high homocysteine, history of stroke or cancer, midline defects, food sensitivities... basically if you're reading this, you should suspect folate.
Folate is in many foods. Some of the richest sources are lentils, chicken liver, legumes and leafy greens. The form in plants is more susceptable to heat damage than the form in animal products.
Folinic acid is another naturally occuring form of folate that is in many supplements. It is used in another part of folate metabolism, and does shortcut some of the processing folic acid requires. It still needs conversion by the MTHFR enzyme, though, so will not be the magic bullet for those with certain polymorphisms.
Folic acid is in almost all multivitamins and B complexes. It is not a great form for some people, and can compete in the body, making the active folate less accessible. 5-MTHF and methyl folate are names for the active form. It can be bought as a supplement called FolaPro and may also be available in Thorne supplements.
folate deficiency associated with allergies: