What is Vitamin B12?

Vitamin B12 (cobalamin) is an essential water-soluble vitamin that plays a key role in red blood cell formation, nerve function, and DNA synthesis. The body cannot produce B12, so it must come from diet or supplements.

Why is it Important?

Vitamin B12 is essential for:

Producing healthy red blood cells
Maintaining nervous system function
DNA synthesis and cell division
Energy production (metabolism)
Preventing megaloblastic anemia

Deficiency can cause irreversible neurological damage if left untreated.

Normal Ranges

Classification	Range	Unit
Normal	200 – 900	pg/mL
Borderline Low	150 – 200	pg/mL
Deficient	< 150	pg/mL
Optimal (functional medicine)	> 400	pg/mL

Note: Some people have symptoms of deficiency even with "normal" levels (200-400 pg/mL). Additional tests (methylmalonic acid, homocysteine) can confirm functional deficiency.

What Do Abnormal Results Mean?

Low Vitamin B12 (Deficiency)

Common causes include:

Pernicious anemia (autoimmune condition preventing B12 absorption)
Vegan or strict vegetarian diet (B12 only in animal products)
Malabsorption disorders (Crohn's, celiac disease)
Stomach surgery (gastrectomy, gastric bypass)
Medications (metformin, PPIs, H2 blockers)
Atrophic gastritis (reduced stomach acid)
Aging (reduced absorption capacity)
Tapeworm infection (Diphyllobothrium latum)

Symptoms of B12 Deficiency

Hematological:

Macrocytic anemia (large red blood cells)
Fatigue and weakness
Pale or jaundiced skin

Neurological (can be irreversible):

Numbness or tingling in hands and feet (paresthesia)
Balance problems and difficulty walking
Memory problems and confusion
Depression or mood changes
Vision problems

Other:

Glossitis (sore, inflamed tongue)
Mouth ulcers
Shortness of breath

High Vitamin B12

Less common and usually not harmful, but can indicate:

Excessive B12 supplementation
Liver disease (B12 stored in liver)
Myeloproliferative disorders (blood cancers)
Kidney disease

Confirming B12 Deficiency

If serum B12 is borderline or symptoms persist despite normal levels, additional tests include:

Methylmalonic acid (MMA): Elevated in B12 deficiency
Homocysteine: Elevated in B12 or folate deficiency
Holotranscobalamin (active B12): More sensitive marker

Dietary Sources of B12

Animal products: Meat, poultry, fish, eggs, dairy
Fortified foods: Plant-based milk, cereals, nutritional yeast
Supplements: Oral tablets, sublingual drops, or injections

Vegans and vegetarians: Must supplement or eat B12-fortified foods.

Treatment of Deficiency

Oral supplements: 1000-2000 µg daily (effective for most people)
B12 injections: For pernicious anemia or severe malabsorption (1000 µg intramuscularly)
Sublingual or nasal sprays: Alternative absorption routes

When Should You Get Tested?

Vitamin B12 testing may be ordered if you have:

Macrocytic anemia (high MCV)
Neurological symptoms (numbness, tingling, balance problems)
Chronic fatigue or weakness
Vegan or vegetarian diet
History of stomach surgery or GI disorders
Long-term use of metformin or PPIs
Unexplained cognitive decline