In L-5-MTHF, the “L” signifies that it is an all-natural [6S] isomer, and this is the active circulating form of folate found in the body and one of the several forms found in food. In nature, this is found only as this [6S] isomer, which is why Designs for Health uses L-5-MTHF, the natural isomer, rather than the mixed (or racemic) form. This product uses the patented folate derivative, Quatrefolic®. This innovative form of folate has demonstrated high bioavailability, solubility, and long-lasting stability. Quatrefolic® L-5-MTHF has been shown to help increase blood folate levels much better than folic acid.