Folic acid is a synthetic form of folate. It's found in dietary supplements, and it's used to enrich or fortify some processed foods. They're similar in structure, however your body absorbs folic acid better than folate.
Your body can use either folic acid or folate to make deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which contain the blueprints of all your cells. So, folate or folic acid are important for cell division and growth. Chronic folate deficiency may lead to a type of anemia. Women who are deficient at conception or during the first trimester of pregnancy are more likely to have babies who have neural tube defects, including spina bifida and anencephaly. Taking folic acid every day reduces this risk and since the United States Food and Drug Administration required enrichment of grain and cereal products, the rate of these neural tube defects has dropped significantly. Alcoholics, people with liver disease, and people who take certain medications or undergo kidney dialysis are also more likely to be deficient in folate and may benefit from folic acid fortified foods or supplements.
Folate and folic acid have both been associated with cancer and heart health. Scientists who looked at large population studies found that people who ate folate-rich foods had lower risks of certain cancers and cardiovascular disease. This led to people taking folic acid supplements with the hopes of reducing those risks. It made some sense because folate is important to cell division and damage to DNA can lead to cancer, and folic acid reduces blood levels of a protein called homocysteine. Elevated levels of homocysteine are also associated with higher risk of cardiovascular disease. However, when it comes to nutrition, dietary supplements and health risk, population studies can really only find correlations and not direct causes, so randomized controlled trials were needed to see if folic acid supplements truly had any benefit. Unfortunately, those follow-up studies haven't found that taking folic acid reduces any risk of cancer or cardiovascular disease. So while taking folic acid on a daily basis may correct a folate deficiency, taking amounts above 400 micrograms isn't going to help your heart or prevent cancer.
The Institute of Medicine sets the tolerable upper limit (upper safety level) of folic acid as 1,000 micrograms per day, but there is no upper limit set or natural folate intake from foods - you can eat as much as you'd like. Although folic acid supplements are safe, taking them in large amounts can mask a vitamin B-12 deficiency, which can result in neurological damage if the B12 deficiency is not corrected, so if you should speak with your health care provider before taking folic acid supplements beyond what is found in fortified foods.
American Cancer Society. "Folic Acid." Accessed May 11, 2012. http://www.cancer.org/Treatment/TreatmentsandSideEffects/ComplementaryandAlternativeMedicine/HerbsVitaminsandMinerals/folic-acid.
Harvard School of Public Health, The Nutrition Source. "Three of the B Vitamins" Folate, Vitamin B6, and Vitamin B12." Accessed May 11, 2012. http://ods.od.nih.gov/factsheets/Folate-HealthProfessional/
Office of Dietary Supplements, National Institutes of Health. "Dietary Supplement Fact sheet: Folate." Accessed May 11, 2012. http://ods.od.nih.gov/factsheets/Folate-HealthProfessional/
Zhou YH, Tang JY, Wu MJ, Lu J, Wei X, Qin YY, Wang C, Xu JF, He J. " Effect of folic acid supplementation on cardiovascular outcomes: a systematic review and meta-analysis." PLoS One. 2011;6(9):e25142.