High Quality Content by WIKIPEDIA articles! Levomefolic acid (INN) or metafolin (5-methyltetrahydrofolic acid) is the natural, active form of folic acid used at the cellular level for DNA reproduction, the cysteine cycle and the regulation of homocysteine among other functions. The un-methylated form, folic acid (vitamin B9), is a synthetic form of folate found in nutritional supplements. Synthetic folic acid is metabolized in the body into levomefolic acid. Approximately 10% of the general population (homozygous TT) lack the enzymes needed to receive any benefit from folic acid. Another 40% of the population (heterozygous CT) appear to convert only a limited amount of folic acid into levomefolic acid. They cannot fully process supplemental folic acid at RDA or higher dose levels. The remaining population do not have a known MTHFR polymorphism and can therefore metabolize folic acid more efficiently.