Vitamin B12

Vitamin B12, or cobalamin, is a water-soluble vitamin that is important in enzymatic reactions, normal blood formation, neural function, and metabolism of proteins, fats, and carbohydrates. Absorption of vitamin B12 from foods requires many steps. In the stomach, animal protein releases cobalamin in the presence of gastric acid and pepsin. This then binds with a salivary R protein that gets digested in the small intestine by pancreatic enzymes. This then binds intrinsic factor and is absorbed in the terminal ileum. Any disease, condition, or medication that interrupts any of these steps will affect or prevent the absorption of vitamin B12. Cobalamin is stored in the liver, kidney, heart, spleen, and brain. Food sources of vitamin B12 are animal products like meats, seafood, dairy products, eggs, liver, kidney, and fish.

What are the Symptoms of Vitamin B12 Deficiency?

Indications for investigating vitamin B12 deficiency are: unexplained neurological symptoms (paresthesias, numbness, or cognitive changes), tiredness, memory loss, severe depression, or unexplained elevations in homocysteine levels. Prolonged vitamin B12 deficiency can potentiate mental illness, damage neurons, and aggravate multiple sclerosis. Elevated homocysteine levels are a risk factor for cardiovascular and cerebrovascular disease. Low vitamin B12 levels may initially be suspected with specific abnormal CBC (complete blood count) levels. This diagnosis can be further confirmed using serum vitamin B12 or Schillings tests. False negatives (elevated vitamin B12 in the presence of deficiency) can occur in people with autoimmune disease, liver disease, or lymphoma. Excessive vitamin C intake, pregnancy, folate deficiency, and multiple myeloma can contribute to false positives (low vitamin B12 levels in the absence of deficiency).

What is the Recommended Treatment for Deficiency?

  1. If the underlying cause is poor intake of vitamin B12-rich foods (excluding vegans), provide adequate sources of cobalamin.
  2. If a condition exists preventing the absorption of vitamin B12, replacement is required. Dharmarajan and Norkus1 recommend:
    1. Initial therapy if 1000 micrograms intramuscularly (IM) daily for the first 3-7 days, then weekly for the following 4 weeks.
    2. Maintenance therapy – your doctor will choose one of the following:
      • 100-1000 micrograms IM monthly
      • 1000 micrograms IM quarterly
      • 500 micrograms intranasally weekly
      • 1000 micrograms orally daily

What Conditions can Impair Dietary Vitamin B12 Absorption?

Condition or State

Causes of Deficiency

Absence of vitamin B12 in the diet Vegan diet (long-term); poor intake of vitamin B12-rich food
Lack of adequate intrinsic factor Gastrectomy, pernicious anemia
Inadequate intestinal absorption Gastric bypass surgery, small bowel surgery (especially in the ileum), disease in the ileum (Crohn’s disease, cancer), short bowel syndrome, fistulas
Poor digestion of food Reduced gastric acid (>65 years of age, gastric cancer, H. pylori infection)
Failure to digest salivary R protein Pancreatic disease, lack of pancreatic enzymes
Other malabsorption syndromes HIV, multiple sclerosis
Congenital disease Transcobalamin deficiency
Receptor or mucosal defects Long-term use of certain drugs like H2 receptor antagonists, proton pump inhibitors, Metformin, antacids
Altered use of intestinal organ Bacterial overgrowth, long-term TPN, tapeworm


Cobalamin (vitamin B12) is an essential vitamin that has numerous roles in the body. For further information regarding vitamin B12 deficiency, consult your physician.


Mary Flesher, Clinical Dietitian,The Richmond Hospital
First published in the Inside Tract® newsletter issue 141 – January/February 2004
1. Dharmarajan, TS, Norkus, EP (2001). Approaches to vitamin B12 deficiency: Early treatment may prevent devastating complications. Postgrad Med, 110(1): 99-105.
2. Kuzminski, AM, Del Giacco, ET, Allen, RH, Stabler, SP, Lindenbaum, J (1998). Effective treatment of cobalamin deficiency with oral cobalamin. Blood, 92(4): 1191-1198.
3. Guidelines and Protocol Advisory Committee (2003). Investigation and management of vitamin B12 and folate deficiencies. Medical Services Commission and B. C. Medical Association.
4. Gottschlich, MM (2001). The science and practice of nutrition support: A case-based core curriculum. ASPEN, U. S. A.
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