Absorption of vitamin B12

• Vitamin B12, also known as cyanocobalamin, is a water-soluble vitamin
• Approximately 56% of a 1 mcg oral dose of vitamin B12 is absorbed
• absorption decreases drastically when the capacity of intrinsic factor is exceeded (at 1–2 mcg of vitamin B12)
• Existing evidence does not suggest any differences among forms with respect to absorption or bioavailability. 
• However the body’s ability to absorb vitamin B12 from dietary supplements is largely limited by the capacity of intrinsic factor- only about 10 mcg of a 500 mcg oral supplement is actually absorbed in healthy people

Individuals who have had gastrointestinal surgery

• removal of all or part of the stomach, often result in a loss of cells that secrete hydrochloric acid and intrinsic factor 
• This reduces the amount of vitamin B12, particularly food-bound vitamin B12 that the body releases and absorbs. 
• Surgical removal of the distal ileum also can result in the inability to absorb vitamin B12. 

Additional Information (combination with Folate)

• Large amounts of folic acid can mask the damaging effects of vitamin B12 deficiency by correcting the megaloblastic anemia caused by vitamin B12 deficiency
• Preliminary evidence suggests that high serum folate levels might not only mask vitamin B12 deficiency, but could also exacerbate the anemia and worsen the cognitive symptoms associated with vitamin B12 deficiency
• Permanent nerve damage can occur if vitamin B12 deficiency is not treated. 
• For these reasons, folic acid intake from fortified food and supplements should not exceed 1,000 mcg daily in healthy adults

Reference:
http://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/