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The importance of folic acid to the synthesis of DNA and RNA is very great. For that reason concern over possible folic acid toxicity takes a back seat to the very real concerns over its deficiency.

Mild symptoms occur at levels over 15,000 mcg of folic acid intake. The Recommended Dietary Allowance (RDA) for adults is 400 mcg per day, for pregnant women it is between 600 and 800 mcg per day and for lactating women it is 500 mcg per day. An upper limit of 1000 mcg per day has been established for supplements. It is considered to be of little benefit above that daily intake level.

Stomach, sleep and skin problems may occur at level of over 1000 mcg per day and seizures can rarely occur. For the most part folic acid toxicity is very infrequent since folates or vitamin B9 is water soluble. As such most excessive dosages are simply excreted in the urine.

The primary concern over excessive amounts of folic acid intake is that it can cause absorption problems when combined with certain prescription medications.

An anticancer drug used for chemotherapy called methotrexate is known to be less absorbed by patients taking excessive dosages of folic acid. Zinc and estrogen are also likely to combine with excess folic acid and becomes less easily absorbed. It is believed that excessive amounts of folic acid--more than 5000 mcg per day--may mask the symptoms of pernicious anemia. This is a disease in which absorption of vitamin B12 is inhibited.

Add to the list possible absorption problems with anticonvulsant drugs and barbiturates and it becomes ill advised to consider mega-dosing folic acid.

Still, finding that delicate balance between necessary levels of folates and excessive amounts is imperative for human health.

In 1998 folic acid enrichment of all grain products became mandatory in the United States because of the overwhelming evidence that folic acid prevents neuro tube defects (NTD) at least some of the time. NTD are profound birth defects causing spina bifida (opening in the spine), anencephaly (missing part of the brain), as well as cleft palate. Additionally, some limb problems and heart defects could be caused by deficiencies of folic acid.

Since 1998 one study has found at least a 19% decrease in the incidence of NTD's. Since the enrichment of grain has only increased the average person's folic acid intake by about 100 mcg per day it is possible that people still need to further supplement their diets.

Some factors may diminish a person's ability to absorb folates in foods and alcoholism appears to be among them. There is also an important problem with the bioavailability of folates in some foods. One notable food where bioavailability is concerned, is corn. Although corn contains folates they do not occur in a form which is easily absorbed by humans. Certain processing--for instance that involved in the production of corn tortillas--increases corn mill folates' bioavailability.

Dietary sources of folates include green leafy vegetables, chicken liver (1 cup supplies 1000 mcg), spinach and asparagus.

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