Hi, adreno.
It's true that in otherwise healthy people who have an absolute B12 deficiency, it is possible to treat either orally or sublingually, so long as a high enough dosage is used. However, if a person has ME/CFS, with glutathione depletion and a functional B12 deficiency, I don't believe that oral dosing will work unless a very large dosage is used. Here's why:
When B12 is taken orally, if a person has normal intrinsic factor production, the limit of absorption by the intrinsic factor mechanism for a single dose is about 2 micrograms. However, if larger dosages are taken orally, about 1% will be absorbed by diffusion, without intrinsic factor involvement. So, for example, if 500 micrograms are taken orally, about 5 micrograms will be absorbed apart from the intrinsic factor mechanism. This is enough to treat an otherwise healthy person who has an absolute B12 deficiency, even if it is due to pernicious anemia, which knocks out the intrinsic factor mechanism.
However, if a person has ME/CFS, the amount of B12 that must be put into the blood is of the order of 1,000 micrograms or more per day. So if oral absorption is to be relied upon, the person would have to take about 100,000 micrograms, or 100 milligrams of B12. This is a very big dosage. However, if it is taken sublingually, the fraction absorbed will be much higher, so that something like 2,000 micrograms will do the job.
As I posted on another thread, I think the reason why such a high dosage is needed to treat ME/CFS is that glutathione is depleted. Normally, glutathione increases the affinity for the CblC complementation group, part of the intracellular B12 metabolic pathway, by a factor of over 100. When glutathione is depleted, the affinity drops, and it is necessary to raise the concentration of B12 significantly in order to achieve a high enough rate in the B12 metabolic pathway. This is based on research done in Korea, published only a few months ago.
Best regards,
Rich
