Vitamin B₉ (folic acid and folate inclusive) is essential to numerous bodily functions. The human body needs folate to synthesize DNA, repair DNA, and methylate DNA as well as to act as a cofactor in biological reactions involving folate.^[7] It is especially important in aiding rapid cell division and growth, such as in infancy and pregnancy. Children and adults both require folic acid to produce healthy red blood cells and prevent anemia.^[8]

Folate and folic acid derive their names from the Latin word folium (which means "leaf"). Leafy vegetables are a principal source, although in Western diets fortified cereals and bread may be a larger dietary source.^{[citation needed]}

A lack of dietary folic acid leads to folate deficiency, which is uncommon in normal Western diets.^{[citation needed]} A complete lack of dietary folate takes months before deficiency develops as normal individuals have about 500–20,000 µg^[9] of folate in body stores.^[10] This deficiency can result in many health problems, the most notable one being neural tube defects in developing embryos. Common symptoms of folate deficiency include diarrhea, macrocytic anemia with weakness or shortness of breath, nerve damage with weakness and limb numbness (peripheral neuropathy)^{[citation needed]}, pregnancy complications, mental confusion, forgetfulness or other cognitive declines, mental depression, sore or swollen tongue, peptic or mouth ulcers, headaches, heart palpitations, irritability, and behavioral disorders. Low levels of folate can also lead to homocysteine accumulation.^[7] DNA synthesis and repair are impaired and this could lead to cancer development.[