When to Supplement with Vitamin B12

National food surveys indicate that the median daily intake of vitamin B12 in the United States by men is approximately 5 micrograms and the median intake for women is about 3.5 micrograms - well within the recommended levels. The RDA is 2.4 micrograms for adults. B12 is not recommended for children.

There are, however, a number of conditions that may cause deficiencies of vitamin B12. Individuals who suffer from malabsorption syndrome of any cause will most likely need extra vitamin B12. Disease/conditions requiring supplementation under a doctor's care include post stomach surgery, pernicious anemia, post-gastric bypass surgery, Crohon's disease, and HIV patients with chronic diarrhea. Treatment usually involves monthly intramuscular injections of 100 micrograms of vitamin B12. In addition, the National Academy of Sciences has recommended that individuals over the age of 50 meet their RDA mainly by consuming foods fortified with the synthetic form of B12 or a supplement containing vitamin B12.

Vitamin B12 and Pernicious Anemia
Vitamin B12 requires an intrinsic factor - a compound made inside the body - for absorption from the intestinal tract into the bloodstream. This intrinsic factor is made in the stomach, where it attaches itself to the vitamin and carries it to the small intestine to be absorbed.

Certain people have a defective gene for intrinsic factor in their genetic makeup, so they can't make it in their bodies. This defect usually becomes evident in midlife. If the intrinsic factor is missing, vitamin B12 cannot be absorbed from the diet and deficiency occurs. When this happens, or when the stomach has been injured and cannot produce enough intrinsic factor, B12 must be provided via injections in order to bypass the stomach.