Vitamin B9, more commonly known as folate or folic acid, is a water-soluble vitamin essential for human growth and development. It encourages normal nerve and proper brain functioning, and may help prevent heart disease and stroke by reducing blood levels of homocysteine (an amino-acid byproduct that is believed to raise the risk of heart attack, stroke and dementia). Studies suggest it might also help protect against cancers of the lung, colon and cervix, and may help slow memory decline associated with aging.
Pregnant women have an increased need for folic acid: it supports the growth of the placenta and fetus, and it helps to prevent several types of birth defects, especially those of the brain and spine, such as neural tube defects. Pregnant women and women of childbearing age should take extra steps to get enough folic acid.
Spinach, green vegetables and beans are good sources, as are fortified products such as orange juice and cereals. Other natural sources of folate include asparagus, bananas, melons, lemons, legumes, yeast and mushrooms.
Article courtesy of Dr. Andrew Weil's website, found here.