Folic acid is a water-soluble B vitamin that takes its name from the Latin word for leaf, folium, because it was first isolated from spinach leaves. Biochemically, folic acid (or folate) functions as a methyl donor after being enzymatically reduced to tetrahydrofolate by the enzyme dihydrofolate reductase. This biochemical reaction is the target of a number of chemotherapeutic antimetabolites such as methotrexate that bind to the enzyme and prevent the reduction. Folic acid is found in brewer's yeast, liver, fruits, leafy vegetables, oranges, rice, soybeans, and wheat. Clinically, folic acid promotes normal red blood cell formation, helps to maintain the central nervous system, and promotes normal growth and development. Deficiencies in folic acid cause conditions such as anemia, weakness, lack of energy, paleness, mental confusion, and headaches.