|Classification and external resources|
Folic acid (B9)
In infants and children, folate deficiency can slow growth rate. Women with folate deficiency who become pregnant are more likely to give birth to low birth weight and premature infants, and infants with neural tube defects.
Late studies suggested an involvement in tumorogenesis (especially in colon) through demethylation/hypomethylation of fast replicating tissues.
Some of the symptoms can also result from a variety of medical conditions other than folate deficiency. It is important to have a physician evaluate these symptoms so that appropriate medical care can be given.
Studies suggest that folate and vitamin B12 status may play a role in depression. The role of vitamin B12 and folate in depression is due to their role in transmethylation reactions, which are crucial for the formation of neurotransmitters (e.g. serotonin, epinephrine, nicotinamides, purines, phospholipids).
Low levels of folate or vitamin B12 can disrupt transmethylation reaction, leading to an accumulation of homocysteine (hyperhomocisteinemia) and to impaired metabolism of neurotransmitters (especially the hydroxylation of dopamine and serotonin from tyrosine and tryptophan), phospholipids, myelin, and receptors. Hyperhomocysteinemia could also lead to vascular injuries by oxidative mechanisms which can contribute to cerebral dysfunction. All of these can lead to the development of various disorders, including depression.
Low plasma B12 and low plasma folate has been found in studies of depressive patients. Furthermore, some studies have shown that low folate levels are linked to a poor response of antidepressant treatment, and other studies also suggest that a high vitamin B12 status may be associated with better treatment outcomes. Therefore, not only does adequate consumption of these two vitamins help decrease the risks of developing depression, but they can also help in the treatment of depression when antidepressant drugs are used. 
A deficiency of folate can occur when the body's need for folate is increased, when dietary intake of folate is inadequate, or when the body excretes (or loses) more folate than usual. Medications that interfere with the body's ability to use folate may also increase the need for this vitamin. Some research indicates that exposure to ultraviolet light, including the use of tanning beds, can lead to a folic acid deficiency. 
Additionally, a defect in homocysteine methyltransferase or a deficiency of B-12 may lead to a so-called "methyl-trap" of tetrahydrofolate (THF), in which THF is converted to a reservoir of methyl-THF which thereafter has no way of being metabolized, and serves as a sink of THF that causes a subsequent deficiency in folate. Thus, a deficiency in B-12 can generate a large pool of methyl-THF that is unable to undergo reactions and will mimic folate deficiency.
Some situations that increase the need for folate include:
- certain anemias
- kidney dialysis
- liver disease
- malabsorption, including celiac disease
- pregnancy and lactation (breastfeeding)
- tobacco smoking
- alcohol consumption
Medications can interfere with folate utilization, including:
- anticonvulsant medications (such as phenytoin, and primidone)
- metformin (sometimes prescribed to control blood sugar in type 2 diabetes)
- methotrexate, an anti-cancer drug also used to control inflammation associated with Crohn's disease, ulcerative colitis and rheumatoid arthritis.
- sulfasalazine (used to control inflammation associated with Crohn's disease, ulcerative colitis and rheumatoid arthritis)
- triamterene (a diuretic)
- oral contraceptives
When sulfasalazine is prescribed, folic acid supplements are normally given with the sulfasalazine. The purpose of methotrexate is to inhibit dihydrofolate reductase and thereby reduce the rate de novo purine and pyrimidine synthesis and cell division. It may therefore be counter-productive to take a folic acid supplement with methotrexate. Although the folic acid inhibition of sulfasalazine is normally seen as a side effect, it is possible that it is a part of the therapeutic effect of the drug, given that methotrexate, a frank folic acid inhibitor, is often given if sulfasalazine fails. It would therefore be wise to consult with a physician before taking a folic acid supplement along with sulfasalazine or methotrexate.
Prevention and treatment 
Folate deficiency during human pregnancy has been associated with an increased risk of infant neural tube defects. Such deficiency during the first four weeks of gestation can result in structural and developmental problems. NIH guidelines recommend oral B vitamin supplements to decrease these risks near the time of conception and during the first month of pregnancy.
- Huether, Sue; McCance, Kathryn (2004). "20". Understanding Pathophysiology (3rd ed.). Mosby. p. 543. ISBN 0-323-02368-1.
- Haslam N and Probert CS. (1998). "An audit of the investigation and treatment of folic acid deficiency". Journal of the Royal Society of Medicine 91 (2): 72–3. PMC 1296488. PMID 9602741.
- Coppen A, Bolander-Gouaille C. (2005). "Treatment of depression: time to consider folic acid and vitamin B12.". J Psychopharmacol 19 (1): 59–65. doi:10.1177/0269881105048899. PMID 15671130.
- Karakuła H, Opolska A, Kowal A, Domański M, Płotka A, Perzyński J. (2009). "Does diet affect our mood? The significance of folic acid and homocysteine". Pol Merkur Lekarski 26 (152): 136–41. PMID 19388520.
- Oakley GP Jr, Adams MJ, Dickinson CM (1996). "More folic acid for everyone, now". Journal of Nutrition 126 (3): 751S–755S. PMID 8598560.
- McNulty H (1995). "Folate requirements for health in different population groups". British Journal of Biomedical Science 52 (2): 110–9. PMID 8520248.
- Stolzenberg R (1994). "Possible folate deficiency with postsurgical infection". Nutrition in Clinical Practice 9 (6): 247–50. doi:10.1177/0115426594009006247. PMID 7476802.
- Pietrzik KF and Thorand B (1997). "Folate economy in pregnancy". Nutrition 13 (11–12): 975–7. doi:10.1016/S0899-9007(97)00340-7. PMID 9433714.
- Kelly GS (1998). "Folates: Supplemental forms and therapeutic applications". Altern Med Rev 3 (3): 208–20. PMID 9630738.
- Cravo ML, Gloria LM, Selhub J, Nadeau MR, Camilo ME, Resende MP, Cardoso JN, Leitao CN, Mira FC (1996). "Hyperhomocysteinemia in chronic alcoholism: correlation with folate, vitamin B-12, and vitamin B-6 status". The American journal of clinical nutrition 63 (2): 220–4. PMID 8561063.
- Hoffbrand, AV; Weir, DG (2001). "The history of folic acid". Br J Haematol 113 (3): 579–589. doi:10.1046/j.1365-2141.2001.02822.x. PMID 11380441.
- NIH Dietary Supplement Fact Sheet: Folate url=http://ods.od.nih.gov/factsheets/folate.asp