Vitamin B9 (folate, folic acid) works with vitamins B6 and B12 (cobalamin) and other nutrients to control blood levels of the amino acid homocysteine. 

Elevated homocysteine levels in blood are a risk factor for cardiovascular disease. Although B vitamin supplementation has been proven effective to control homocysteine levels, current data from intervention trials have not shown that lowering homocysteine levels decreases cardiovascular disease risk. Researchers are not sure whether homocysteine is a cause of heart disease or just a marker that indicates someone may have heart disease.