This vitamin has been shown to provide cardioprotective effects, most importantly of which is its ability to decrease blood levels of the amino acid homocysteine-elevated homocysteine levels are a risk factor for coronary artery disease and heart attack. Data suggests that this vitamin may be the most important single nutrient in providing protection against homocysteine-researchers estimate that a minimum of 13,500 deaths from cardiovascular disease could be prevented each year by increasing the intake of this vitamin. It is important to note that deficiencies of this vitamin are a common consequence of the conventional treatments for congestive heart failure (i.e. high-dose diuretic therapy, etc.). Furthermore, a deficiency of this vitamin during pregnancy can result in miscarriage or brain or neural tube defects, causing children to be born mentally retarded. What is this vitamin?