"Patients with a history of intestinal surgery, strictures, or blind loops may have bacterial overgrowth that can compete for dietary vitamin B12 in the small bowel, as can infestation with tapeworms or other intestinal parasites....

"Use of methylmalonic acid and homocysteine levels in the diagnosis of vitamin B12 deficiency has led to some surprising findings..."

http://www.aafp.org/afp/2003/0301/p979.html