If you have poor or non-existant intrinsic factor, you cannot absorb B12 from food.

If you have the DNA error (10-30%) in US have it, you cannot activate B12 (or folic acid) to their methyl forms which are the active type to work in the body.

If you take acid blocking drugs regularly for GERD or heartburn, you will not absorb B12 from food reliably or at all. Acid is required in the stomach to break the protein down enough for intrinsic factor to work.

If you take metformin or long term antibiotics you won't absorb adequate B12.

If you a vegan you risk low B12.

New work is showing some people have poor transcobalamin levels. This peptide is the carrier molecule for B12. If this is broken then high serum levels are necessary to compensate for that. Food sources when all things work normally only provide about 2-4 micrograms of B12 a day. Studies show 1000mcg taken orally can provide 13 mcg a day. The rest is lost.

The supplements are especially high in B12 orally to bypass these situations. The B12 can be absorbed passively without intrinsic factor if it is taken on an empty stomach.

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