Neurology: Vitamin B12 Deficiency


Vitamin B12 is one of the so called water soluble vitamins and participates in several prime metabolic activities in the body. The prolonged deficiency of this vitamin will have devastating effect on your health and result in multitudinous medical conditions and even death.

Vitamin B12 is an essential vitamin in the sense it is not synthesized in human body rather it needs to be supplied through external sources. Unfortunately the plant sources of food are deficient in this vitamin and although the meat and animal products like liver are rich with it however not everybody eats non-vegetarian food.

Nevertheless the good news is the body needs very minute amounts of B12 for its daily requirements, which is about 2.5 micrograms for typical adult. The body stores about 2 to 5 mg of this vitamin, so generally only a prolonged decreased intake will result in ill health.

Additional details

Vitamin B12 occurs in several forms and they all contain cobalt, a rare element & and all of them possess vitamin activity.

Cobalmin & hydroxyl-cobalamin are the two natural forms of B12. Cyanocobalamin is a synthetic form and the one generally used as the dietary supplement or pills. Methyl and adenosyl forms are the ultimate active forms of this vitamin which are used in the metabolic functions. Now-a-days methylcobalamin is too available in the market.

The gastric acid can destroy the orally ingested vitamin B12, and this is averted by its combining to a factor in the stomach called as haptocorrin which is a glycoprotein (a complex of sugar and protein molecules). As this compound reaches duodenum, the first part of small intestine, the enzymes secreted by pancreas liberate B12 from this association and the free vitamin complexes with another glycoprotein called IF (intrinsic factor) and then reaches ileum (distal small intestine) where it gets absorbed.

What causes B12 deficiency?

  • Strict vegetarian diet
  • Post stomach surgery
  • Post intestinal surgery
  • Pernicious anemia (PA, see below)
  • Intestinal parasites (fish tapeworm)
  • Medication side effect (e.g. metformin)

Clinical Manifestations

Vitamin B12 deficiency may manifest as;

  • Dementia
  • Cognitive dysfunctions not amounting to dementia
  • Neurotic conditions
  • Psychiatric manifestations
  • Spinal cord damage
  • Neuropathy (nerve damage)
  • Anemia etc

Various neuro-psychiatric manifestations have been ascribed to the deficiency of this vitamin. Neuropathy can cause numbness, pins & needle sensation in the limbs. A special form of spinal cord damage called sub-acute combined degeneration can produce gait unsteadiness, muscle weakness, sensory complaints, bowel, and bladder disturbances.


The clinical manifestations of B12 deficiency are not specific for this condition, several health conditions can mimic it, implies you need laboratory tests to confirm the diagnosis and such tests include;

  • Serum Vitamin B12 level
  • Serum homocystine
  • Serum methyl malonoic acid
  • Schilling test
  • Intrinsic factor (IF)
  • Anti-parietal antibody titers
  • Peripheral smear for anemia
  • Bone marrow biopsy etc

The serum vitamin B12 level is the test to begin with and if borderline B12 values are obtained then additional tests like serum homocystine and methyl malonoic acid levels are usually ordered. Peripheral smear may show megaloblastic anemia (large red blood cells). Diagnosis of pernicious anemia is not always easy and the special tests conducted may include;

  • Titers of antibodies to IF and parietal cells of stomach
  • Gastroscopy
  • Gastric biopsy etc.

Schilling test measures the absorption of B12 from the intestine with or without IF but now-a-days it is rarely performed.


Once B12 deficiency is confirmed then no time should be lost in initiating treatment. The treatment options usually are;

  • Oral B12 supplementation
  • Intramuscular B12 supplementation
  • Supplementation with methyl cobalamin
  • Dietary adjustment
  • Treatment for fish tapeworm

If pernicious anemia is suspected B12 is mostly supplemented as injections. Initially high dose B12 is given more frequently then long term maintenance doses are given but less frequently.

If mild deficiencies are seen and/or no pernicious anemia is suspected then high dose oral supplementation is enough too. There are some studies showing that irrespective of the severity or cause of B12 deficiency mega doses of B12 given orally is sufficient and it obviates the need of injection form. However this concept is not universally accepted by all physicians.

If oral supplementation is done in documented B12 deficiency cases then it is imperative to repeat serum B12 levels, sometime after the treatment was started, to make sure that patient is realty absorbing the vitamin and also the blood levels are adequate.


Vitamin B12 is vital for your normal well being and its deficiency should be identified and treated at the earliest.

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