Medical Uses of Vitamin B12 (Cobalamin)

• Pernicious anemia (autoimmune loss of intrinsic factor, needed for B12 absorption)
• Malabsorption states: atrophic gastritis, gastrectomy/bariatric surgery, Crohn's disease, celiac/sprue, small bowel bacterial overgrowth, fish tapeworm infestation
• Pancreatic insufficiency
• Dietary deficiency (strict vegan diets)
• Drug-induced deficiency (metformin, proton pump inhibitors, H2 blockers, colchicine)
• It's essential to rule out B12 deficiency before treating megaloblastic anemia with folate alone, since folate can correct the anemia while allowing neurologic damage to progress unchecked (Robbins, Cotran & Kumar Pathologic Basis of Disease).

• Subacute combined degeneration of the spinal cord (dorsal column and corticospinal tract demyelination) - classic complication of untreated deficiency
• Peripheral neuropathy
• Cognitive impairment/dementia workup - B12 is checked in reversible causes of cognitive decline
• Psychiatric manifestations including depression, and case reports of hallucinations/psychosis linked to deficiency

• Schilling test - historically used a flushing IM dose of B12 to assess absorption capacity in suspected pernicious anemia (largely replaced by modern lab markers like methylmalonic acid and serum transcobalamin II).

• Pregnancy: B12 supplementation to prevent fetal neural tube-related and metabolic issues in deficient mothers.
• Elderly patients with atrophic gastritis (common cause of food-bound B12 malabsorption).
• Patients on long-term metformin (Type 2 diabetics) - regularly screened and supplemented per systematic review evidence.

• Oral (high-dose, e.g. 1000-2000 mcg/day) works even in pernicious anemia in many patients because a small fraction is absorbed passively without intrinsic factor.
• Intramuscular/parenteral - traditional route for pernicious anemia, severe deficiency, or malabsorption, especially with neurologic involvement.
• Sublingual/intranasal - alternative routes with evidence of comparable efficacy to oral in some studies.

• "Shotgun" vitamin therapy that includes folic acid without confirming B12 status is dangerous - folate can mask/worsen the hematologic picture while neurologic damage progresses (Goodman & Gilman's Pharmacological Basis of Therapeutics).
• Rapid correction of severe megaloblastic anemia can cause hypokalemia and, rarely, sudden death; potassium should be monitored.
• Anaphylaxis has been reported with parenteral B12 - intradermal test dosing is recommended in patients with suspected sensitivity.
• Patients with Leber's hereditary optic atrophy should avoid cyanocobalamin specifically, as it can cause severe, rapid optic nerve damage.

Medical Uses of Vitamin B12 (Cobalamin)

• Pernicious anemia (autoimmune loss of intrinsic factor, needed for B12 absorption)
• Malabsorption states: atrophic gastritis, gastrectomy/bariatric surgery, Crohn's disease, celiac/sprue, small bowel bacterial overgrowth, fish tapeworm infestation
• Pancreatic insufficiency
• Dietary deficiency (strict vegan diets)
• Drug-induced deficiency (metformin, proton pump inhibitors, H2 blockers, colchicine)
• Essential to rule out B12 deficiency before treating megaloblastic anemia with folate alone, since folate can correct the anemia while allowing neurologic damage to progress unchecked (Robbins, Cotran & Kumar, Pathologic Basis of Disease).

• Subacute combined degeneration of the spinal cord (dorsal column and corticospinal tract demyelination) - a classic complication of untreated deficiency
• Peripheral neuropathy
• Cognitive impairment/dementia workup - B12 is checked as a reversible cause of cognitive decline
• Psychiatric manifestations including depression, and case reports of hallucinations/psychosis linked to deficiency

• Schilling test - historically used a flushing IM dose of B12 to assess absorption capacity in suspected pernicious anemia (largely replaced by modern lab markers like methylmalonic acid and serum transcobalamin II).

• Pregnancy: supplementation in deficient mothers to prevent fetal neurologic/metabolic complications.
• Elderly patients with atrophic gastritis (common cause of food-bound B12 malabsorption).
• Patients on long-term metformin (Type 2 diabetics) - regularly screened and supplemented.

• Oral (high-dose, e.g. 1000-2000 mcg/day) - works even in pernicious anemia in many patients because a small fraction is absorbed passively without intrinsic factor.
• Intramuscular/parenteral - traditional route for pernicious anemia, severe deficiency, or malabsorption, especially with neurologic involvement.
• Sublingual/intranasal - alternative routes with comparable efficacy to oral in some studies.

• "Shotgun" vitamin therapy including folic acid without confirming B12 status is dangerous - folate can mask/worsen the hematologic picture while neurologic damage progresses (Goodman & Gilman's Pharmacological Basis of Therapeutics).
• Rapid correction of severe megaloblastic anemia can cause hypokalemia and, rarely, sudden death; potassium should be monitored.
• Anaphylaxis has been reported with parenteral B12 - an intradermal test dose is recommended in patients with suspected sensitivity.
• Patients with Leber's hereditary optic atrophy should avoid cyanocobalamin specifically, as it can cause severe, rapid optic nerve damage.

Normal Physiological Function of Vitamin B12 in a Healthy Body

1. Cofactor for methionine synthase (cytoplasm)

• Converts homocysteine to methionine, using methylcobalamin as the cofactor and methyltetrahydrofolate as the methyl donor.
• Methionine is then used to generate S-adenosylmethionine (SAM), the body's principal methyl donor used in methylation of DNA, RNA, proteins, lipids, and neurotransmitters (Yamada's Textbook of Gastroenterology; Harper's Illustrated Biochemistry, 32nd Ed).
• This reaction also regenerates tetrahydrofolate (THF) from the "trapped" methyl-THF form, which is required to keep the folate cycle running for purine and thymidine synthesis, and therefore DNA synthesis. This is why B12 and folate metabolism are tightly linked - without B12, folate gets stuck in the methyl-THF form ("methyl-folate trap"), starving cells of usable folate even if folate intake is normal (Katzung's Basic and Clinical Pharmacology, 16th Ed).

2. Cofactor for L-methylmalonyl-CoA mutase (mitochondria)

• Uses adenosylcobalamin as the cofactor to convert L-methylmalonyl-CoA to succinyl-CoA.
• This step feeds into the citric acid cycle and is essential for normal catabolism of odd-chain fatty acids, cholesterol, and certain amino acids (valine, isoleucine, methionine, threonine) (Tietz Textbook of Laboratory Medicine, 7th Ed).

Downstream physiological effects of these two reactions

• Erythropoiesis: Adequate DNA synthesis (via the folate-dependent thymidine pathway) is required for normal maturation and division of red blood cell precursors in the bone marrow. This is why B12 is described as "required for erythropoiesis" (Tietz Textbook of Laboratory Medicine).
• Nervous system maintenance: B12 is needed for normal myelin synthesis and maintenance, likely through SAM-dependent methylation reactions that affect myelin basic protein and phospholipid methylation. This underlies its role in keeping peripheral nerves, the spinal cord dorsal/lateral columns, and CNS function intact.
• Homocysteine regulation: Because it drives homocysteine remethylation to methionine, adequate B12 keeps plasma homocysteine levels low, a normal physiological function relevant to vascular health.
• General cell division: Rapidly dividing tissues (bone marrow, GI epithelium) are most sensitive to disruptions in the DNA synthesis pathway, which depends indirectly on B12 through folate regeneration.

Absorption and handling in a healthy body (relevant to how the vitamin normally functions/circulates)

• Dietary B12 is released from food protein by gastric acid and pepsin, binds R-proteins, then is transferred to intrinsic factor (secreted by gastric parietal cells) in the duodenum.
• The B12-intrinsic factor complex is absorbed via specific receptors in the terminal ileum.
• In circulation, B12 is carried mainly by transcobalamin II (the biologically active "holo-transcobalamin" fraction delivered to cells) and haptocorrin/transcobalamin I (a larger, longer-lived storage/buffering pool).
• The liver stores 1-6 mg of B12 (mostly as adenosylcobalamin), enough to supply the body for months to a few years even with zero intake, which is part of why deficiency develops slowly.