What is vitamin B9 used for?

Birth defects of the spine and brain

Neural tube defects are birth defects of the spine (spina bifida) and of the skull and brain (anencephaly). The neural tube closes 21 to 28 days after conception, often before a woman knows she is pregnant.^[4] A 2015 Cochrane review pooled 5 trials with 7,391 women. Folic acid, alone or with other vitamins, lowered the risk of a neural tube defect: risk ratio 0.31, from 6,708 births, with high-quality evidence.^[1] The US Preventive Services Task Force recommends 400 to 800 µg of folic acid a day for everyone who plans to or could become pregnant.^[5] This use belongs to the goal fertility. The protocol for neural tube defects lists what trials and health bodies used.

Stroke and the heart

Folic acid lowers homocysteine, an amino acid in the blood that is linked with heart disease.^[4] In 12 trials with 1,114 people, folic acid lowered homocysteine by 25% at typical Western blood levels, with a similar effect from 0.5 to 5 mg a day.^[6] A 2017 Cochrane review of 15 trials with 71,422 people found that B vitamins that lower homocysteine did not prevent heart attacks or deaths. Strokes were slightly fewer: 4.3% against 5.1%.^[3] A 2024 review of 21 trials with 115,559 people found 10% fewer strokes with folic acid (risk ratio 0.90). The benefit was seen in areas where grain is not fortified or only partly fortified with folic acid, and not in areas where it is (risk ratio 1.04).^[2] See the goal cardiovascular and the protocol for stroke.

Memory and thinking

Most trials found no effect of folic acid on memory and thinking or on the risk of dementia, even when it lowered homocysteine.^[4] A 2022 review pooled trials with 6,155 people. B vitamins changed the score of a short memory and thinking test (MMSE) by 0.14 points compared with placebo.^[7] In a Dutch trial, 818 people aged 50 to 70 with high homocysteine took 800 µg of folic acid a day or a placebo for 3 years. Memory and the speed of processing information changed for the better in the folic acid group.^[8] See the goal cognition.

Mood

No trial in a 2018 review compared folate alone with placebo as a treatment for depression. As an add-on to antidepressants, only folic acid below 5 mg a day or L-methylfolate at 15 mg a day helped. The evidence was of low or very low quality.^[9] A 2022 review of L-methylfolate added to antidepressants found a small effect.^[10] Many of the doses used in depression trials are above the upper limit and are taken only under medical care.^[4] See the goal stress and mood.

Anemia and energy

A lack of folate causes megaloblastic anemia. Its symptoms include weakness, tiredness, trouble concentrating and shortness of breath.^[4] A trial gave iron with or without folinic acid, a form of folate, to 371 pregnant women with iron deficiency anemia. In 60 days, hemoglobin rose by 1.42 g/dL with both and by 0.80 g/dL with iron alone.^[11] See the goals energy and essential health.

Arsenic in drinking water

In a 24-week trial in Bangladesh, 622 people exposed to arsenic in their water took folic acid (400 or 800 µg a day), creatine, both, or a placebo. Folic acid helped the body change arsenic into forms that leave the body more easily in urine. The effect faded after people stopped.^[12] See the goal detox.

Side effects of methotrexate

Methotrexate, a medicine for rheumatoid arthritis, works against folate. A 2013 Cochrane review of 6 trials with 624 patients found that folic or folinic acid lowered stomach and gut side effects by 26% and abnormal liver tests by 77%. It did not make methotrexate work less well.^[13]