5-MTHF is the naturally occurring, predominant form of folate commonly found in cells and is essential for overall health, as it participates as a cofactor in a reaction that involves the remethylation of homocysteine to methionine. Unlike synthetic folic acid, 5-MTHF can be used directly by the body, without the need for an additional conversion via the enzyme (5, 10-methylenetetrahydrofolate reductase (MTHFR)). In certain populations, the body’s ability to convert folic acid to 5-MTHF by use of this enzyme may be compromised due to genetic differences. Metafolin® contains only the S isomer of 5-MTHF and has been shown to be the only form of folate to be able to cross the blood-brain barrier. Additional vitamin D3 (also known as cholecalciferol) has been added to this formula as numerous scientists now feel that supplementation with vitamin D at levels greater than previously thought necessary is critical to helping maintain overall health.