Vitamin B9 (Folate & Folic Acid)

Direct Answer

Vitamin B9 comprises natural dietary folates (polyglutamates) and synthetic folic acid (monoglutamate). Adult RDA is 400 µg DFE, with a Tolerable Upper Limit of 1,000 µg/day synthetic folic acid, essential as a single-carbon donor in thymidylate DNA synthesis and neural tube closure.

Source: NIH ODS / CDC / EFSA · Last reviewed Aug 28, 2026

Specs

Nutrient Type	Water-Soluble Pteroylglutamate Vitamin
Adult RDA / RDI	400 µg DFE / day (600 µg DFE pregnancy / prevents neural tube defects)
Tolerable Upper Limit (UL)	1,000 µg / day (Synthetic folic acid only; masks B12 deficiency neuropathy)
Chemical Forms	(6S)-5-Methyltetrahydrofolate (5-MTHF, active circulating form), Tetrahydrofolate (THF), Folic Acid (Pteroylmonoglutamic acid, synthetic)
Primary Biological Function	1-Carbon transfer reactions, dUMP -> dTMP methylation (Thymidine DNA synthesis), Methionine-Homocysteine cycle, Erythropoiesis
Deficiency Condition	Neural Tube Defects (Spina bifida, anencephaly in fetuses), Megaloblastic Macrocytic Anemia, Elevated Homocysteine
Top Whole Food Sources	Beef Liver, Spinach, Asparagus, Brussels Sprouts, Roman Lentils, Chickpeas, Fortified Flours
MTHFR Genetic Polymorphism	Homozygous MTHFR C677T variant reduces folate reduction activity by ~70%, benefiting from direct 5-MTHF supplementation

Dietary Folate Equivalents (DFE) Formula

1 µg DFE = 1 µg food folate = 0.6 µg synthetic folic acid consumed with food = 0.5 µg synthetic folic acid taken on an empty stomach. Folic acid has ~85% bioavailability vs ~50% for food polyglutamate folates.