Common manifestations of vitamin B12 deficiency include weakness, numbness and tingling, fatigue, dizziness, swelling and irritation of the mouth and tongue, and irritability. Anemia can develop, as noted, but a high intake of folate can compensate for a deficiency of vitamin B12 and prevent anemia.
The most serious manifestation of B12 deficiency is impaired brain function, due to the effects of B12 on nerve cells. Advanced B12 deficiency causes dementia severe enough to resemble Alzheimer’s disease. Generally, though, the dementia caused by B12 deficiency is completely reversible with supplementation. While folate can prevent B12-deficient anemia, it cannot prevent the dementia – only B12 itself can do that job.
Vitamin B12 is found in animal foods (and fortified cereals), so vegans are vulnerable to deficiency. But the most important cause of deficiency has to do with the unique way B12 is absorbed. To get into the bloodstream, B12 must be escorted by a protein called intrinsic factor, produced by cells of the stomach. Many disorders of the stomach, such as gastritis, particularly common after age 50, can interfere with the production of intrinsic factor. This condition is called pernicious anemia. Medications that affect the stomach – such as aspirin, antacids and proton-pump inhibitors – can also interfere with intrinsic factor production and result in B12 deficiency. The commonly used diabetes medication, metformin (Glucophage), can do so as well.