Symptoms & Effects of Vitamin B12 Deficiency

In human beings vitamin B12 deficiency leads to pernicious anemia (also called Addisonian anemia) which however is primarily due to a lack of the intrinsic factor which brings about a vitamin B12 deficiency. The mechanism of the action of the intrinsic factor is not fully clear but the following role has been attributed to it.
 
Vitamin B12 is released from food during digestion and is then bound to the intrinsic factors (so named by Castle) which is a glycol-protein with a molecular weight of 50,000. The intrinsic factor is secreted by the gastric mucosa and is present in the highest concentration in the fundus of the stomach. The vitamin B12 intrinsic factor complex is carried to the ileum by peristalsis where the complex dissociates. The released vitamin B12 is absorbed into the portal blood through the ileal cells. In the blood, vitamin B12 is carried in combination with a specific carrier protein called transcobalamin-II which is an α-I globin.
Clinical conditions associated with vitamin B12 deficiency:

1. Pernicious Anemia: Here the primary defect is an absence of the intrinsic factor in the gastric juice. The reason for this is not fully known but it may be an auto-immune phenomenon. This disease is shown by an absolute achlorhydria which does not respond even to the administration of pentagastrin which is a strong stimulant of HCL secretion. Another diagnostic feature that orally administered radio-active vitamin B12 is not absorbed but is completely eliminated in feces. But a simultaneous administration of gastric juice from a normal individual results in a normal B12 absorption.
2. Subjects with total gastric Resection: This operation leads to a lack of intrinsic factor. Partial gastrectomy only  rarely results in this condition because this factor is secreted by the upper part of stomach.
3. Dietary lack: This occurs in strict vegetarian because vitamin B12 is not present in plants foods.
4. Infestation which fish tape worm, Diphyllobothrium latum: This worms utilizes vitamin B12.
5. Blind loop syndrome: In this case the intestine is infested with bacteria having a high affinity for Vitamin B12.
6. Sprue: It is a syndrome showing a  generalized malabsorption.
7. Hereditary Malabsorption: In this condition the absorption of Vitamin B12 is impaired due to some unknown mechanism.

   Schilling Test for B12 Deficiency and Pernicious Anemia

To the fasting subject is given orally 1 µg of Co-labeled vitamin B12 and 1000 µg unlabeled vitamin B12 by an intramuscular injection. The amount excreted in urine in 24 hours is measured and percentage of oral dose is estimated. The results are as follows.

Normal                           = more than 12% excreted
Deficient                       = 5—12% excreted
Pernicious Anemia   = Less than 5% excreted.