This is a common condition in the elderly which comes from the inability to secrete sufficient stomach acid to kill bacteria.  Ingested bacteria can thus survive and reside in the stomach and the upper part of the small bowel.  This problem affects approximately 20% of people between 60 and 69 years of age, and 40% of people over 80.

These bacteria, which are not killed by the normal stomach acid, interfere with the absorption of vitamin B12.  Putting these patients on antibiotics to reduce the number of bacteria returns their B12 levels to normal.  The deficiency of vitamin B12 may not be severe enough to show up in the blood, but it can be associated with psychiatric dementias. These deficiencies may be disguised as Alzheimer's disease or senile dementia.