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The Merck Manual--Second Home Edition logo
 
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Chapter 154. Vitamins
Topics: Introduction | Vitamin A | Vitamin D | Vitamin E | Vitamin K | Vitamin B1 | Vitamin B2 | Niacin | Vitamin B6 | Vitamin B12 | Folic Acid | Vitamin C
 
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Folic Acid

Folic acid (folate), with vitamin B12, is necessary for the formation of normal red blood cells and the synthesis of DNA, the genetic material of cells.

Folic Acid Deficiency

Because the body stores only a small amount of folic acid (folate), a diet lacking in folic acid leads to a deficiency within a few months. Folic acid deficiency is common because many people do not eat enough raw leafy vegetables or citrus fruits. Undernutrition associated with alcoholism is a common cause. Also, alcohol consumed in large amounts interferes with the absorption and processing (metabolism) of folic acid. Certain anticonvulsants (such as phenytoin and phenobarbital) and drugs used to treat ulcerative colitis (such as sulfasalazine) decrease the absorption of this vitamin. Methotrexate (used to treat cancer and rheumatoid arthritis) and trimethoprim-sulfamethoxazole (an antibiotic) interfere with the metabolism of folic acid.

Women who are pregnant or breastfeeding and people undergoing dialysis may develop this deficiency, because their need for folic acid is increased.

Symptoms, Diagnosis, and Treatment

People who have folic acid deficiency develop anemia similar to that due to vitamin B12 deficiency.

Fatigue may be the first symptom. In addition to the general symptoms of anemia (such as paleness, irritability, shortness of breath, and dizziness), folic acid deficiency may cause a red and sore tongue, a reduced sense of taste, weight loss, and diarrhea. If a pregnant woman has folic acid deficiency, the baby may have a birth defect of the spinal cord (neural tube defect).

If a blood test detects large red blood cells in a person who has anemia or who is undernourished, doctors measure the folic acid level in a blood sample. A low level indicates this deficiency.

Treatment consists of taking daily doses of a folic acid supplement. People who are taking drugs that interfere with the absorption or metabolism of folic acid should take a folic acid supplement as a preventive measure. Women who are pregnant or who are planning to become pregnant should take higher doses to reduce the risk of having a baby with a birth defect.

Folic Acid Excess

Folic acid is generally not toxic. Very high doses may worsen nerve damage in people who have vitamin B12 deficiency.

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