Vitamin B12 Deficiency

Summary

The liver has at most 5-10 years and as little as 1-2 years worth of vitamin B12 reserves. The most common cause of B12 deficiency is pernicious anemia from autoantibodies against intrinsic factor that binds with food bound B12 and allows absorption into ilium. The most common drugs affecting absorption are metformin and gastric acid reducers (PPI/H2 blockers). Patients on strict vegan diets are at risk for B12 deficiency and should be counseled to take B12 supplements. A lack of B12 limits proper DNA/RNA synthesis, gene methylation, and neuronal myelination. Treatment is with oral or intramuscular B12 supplementation.

Key Terms

Cobalamin

A water-soluble vitamin, also known as vitamin B12, essential for red blood cell formation, neurological function, and DNA synthesis.

Cyanocobalamin

A synthetic form of vitamin B12 used to treat and prevent vitamin B12 deficiency.

Hydroxocobalamin

A natural form of vitamin B12 used to treat vitamin B12 deficiency and cyanide poisoning.

Methionine Cycle

A biochemical pathway that involves the conversion of homocysteine to methionine, requiring vitamin B12 as a cofactor.

Folate Cycle

Epidemiology

Etiology

MCC: Pernicious Anemia

• Decreased Intake
  • Strict Vegan Diet
• Malabsorption
  • Gastrectomy / Bariatric Surgery
  • Celiac Disease
  • Ileum resection
• Autoimmune
  • Intrinsic Factor autoantibodies
  • Parietal Cell Autoantibodies
• Inflammation
  • Crohn's Disease
  • Chronic Alcoholism
• Infection
  • H. Pylori
  • Tapeworm (fish tapeworm Dibothriocephalus latus)
• Medications
  • Metformin
  • PPI/H2 Blockers/Antacids (reduce gastric acid)

Physiology

SynthesisVitamin B12 and folate cannot be synthesized by humans.

Stomach:stomach parietal cells produce intrinsic factor that binds with B12.

Ilium:the B12 intrinic factor complex is absorbed by mucosal receptors.

Blood:transcobalamin proteins transport B12 to cells that upatake with endocytosis.

Cells:intracellular B12 is metabolized into methylcobalamin or adenosylcobalamin.

DNA Synthesis:lack og B12 blocks cofactor for recycling of 5-methyl-tetrahydrofolate to tetrahydrofolate.

Total Body Stores: approximately 2-5mg are stored in the body. Half of the store is in the liver.

HPI

Physical Exam

• Mild
• Moderate
• Severe

Diagnostics

• Serum B12

Treatment

• Food Sources
  • Highest in clams and liver ¹
• Oral Formulations
  • OTC availible
  • 1000-2000 mcg/day

Patient Education

• Strict Vegan diets should be supplemented with B12

Referrals

Knowledge Check

Case Study

References