U01.01.070 Vitamin B9

Vitamin B9, also known as folate, is a vital nutrient with significant roles in DNA and RNA synthesis, and its deficiency has critical clinical implications. Understanding its function, sources, and associated disorders is crucial for USMLE Step 1 preparation.

Function of Vitamin B9

Key Aspects	Details
Converted to	Tetrahydrofolic acid (THF), a coenzyme for 1-carbon transfer/methylation reactions.
Role	Essential for synthesizing nitrogenous bases in DNA and RNA.
Source	Found in leafy green vegetables (“folate from foliage”).
Absorption Site	Jejunum.
Storage	Small reserve pool stored mainly in the liver.

Deficiency of Vitamin B9

Symptom/Sign	Details
Anemia	Macrocytic, megaloblastic anemia.
Polymorphonuclear Cells (PMNs)	Hypersegmented PMNs.
Oral Symptoms	Glossitis (inflammation of the tongue).
Neurologic Symptoms	Absent (differentiating it from vitamin B12 deficiency).
Laboratory Findings	Elevated homocysteine, normal methylmalonic acid levels.
Common Causes	Seen in alcoholism and pregnancy; can be caused by drugs like phenytoin, sulfonamides, and methotrexate.

Clinical Application

Maternal Supplementation

When to Start: At least 1 month before conception and during early pregnancy.
Purpose: Reduces the risk of neural tube defects (e.g., spina bifida, anencephaly).
Mnemonic: “Give vitamin B9 for the 9 months of pregnancy.”

Key Points to Remember

Folate is crucial for DNA and RNA synthesis, making it indispensable in rapidly dividing cells.
Deficiency leads to macrocytic anemia with no neurologic symptoms, unlike vitamin B12 deficiency.
Lab findings include elevated homocysteine but normal methylmalonic acid levels.
Supplementation during pregnancy is vital to prevent neural tube defects.

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