Low vitamin B12 = elevated homocysteine. Elevated levels of homocysteine in the blood may also increase the tendency to excessive blood clotting. Elevated homocysteine is a significant risk factor for heart attack, stroke and many other diseases. A good book on the subject is "The H Factor Solution: Homocysteine, the Best Single Indicator of Whether You Are Likely to Live Long or Die Young" by James Braly and Patrick Holford.
Causes of vitamin B12 deficiency:
Decreased stomach acid
Atrophic gastritis
Autoimmune pernicious anaemia
Helicobacter pylori
Gastrectomy, intestinal resection
Gastric bypass surgery
Malabsorption syndromes
Crohn’s disease
Celiac disease (gluten enteropathy)
Chronic pancreatitis
Bacterial overgrowth (small bowel)
Fish tapeworm
Alcoholism
Malnutrition - eating disorders
Vegetarianism
Advanced liver disease
Transcobalamin II deficiency
Inborn errors of B12 metabolism
Certain drugs - eg: nexium, metformin, antibiotics
Nitrous oxide
Another good book to check out is: Could It Be B12?: An Epidemic of Misdiagnoses: Sally M. Pacholok RN and Jeffrey J. Stuart DO.
I think that if you've had low B 12 in the past you meed to be extra vigilant about it in the present and future. The symptoms can be neurological and can be mild or very severe. It can be confused with Alzheimer's, MS, stroke and some other serous stuff.
In a healthy normal person they get B12 from their diet and then this special area of your intestines make something called "intrinsic factor" that makes the B12 work.
You might be lacking that intrinsic factor (or have probs with that part of your intestine where it is made). Unfortunately the only way that you can get the activated form of B12 is from shots, so it sounds like you will need to make sure that you get them regularly now.
You could talk to you doc about this and either go in for shots or maybe get the prescription to do your own shots.
Does this help?
Good luck
Horselip