I think it is like a slow adaptive reservoir model. A normal person does not need an infusion of much b12 in their diet. They absorb at a relatively low rate provided they have intrinsic factor. Everything is fine. If they have antibodies to intrinsic factor as per pernicious anemia then they have problems.
From wikipedia:
"Patients experiencing an insufficiency in their intrinsic factor levels cannot benefit from a low dose oral
vitamin B12 supplement, because it will not absorb through the wall of the small intestine. Historically, the disease was thought untreatable before the discovery that it could be managed with regular injections of vitamin B12, thus bypassing the digestive tract. More recently, Swedish researchers discovered that sufficiently large doses of B12 can also be absorbed
sublingually, removing the necessity for injectable B12.
[2] However, no standards have been set for treatment by the sublingual route yet, and injections of B12 are the only reliable method of treatment."
I would ignore the last line only in that the medical community can take geological epochs to develop standards for things that are not its main focus like cancer. CV, etc.
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The complications come in when we talk active vs inactive forms. Mankind never got cyanob12 until modern supplements. Before that if we ate the liver of a cow we got relatively more active b12. If we ate only vegetables all the time we got small amounts of it. The body uses intrinisc factor to help balance the absorption. If you eat a source with a lot of natural b12s then your body will cap it with a sublinear profile vs amount ingested. Even if you really hammer at those clams or oysters you start to essentially saturate at some level. But if your diet is low in B12 it will probably up the absorption coefficient some in order to try to balance things out. The body is pretty picky about this. Let's ignore people with intestinal damage due to Crohn's, Celiac's, etc. as those would hurt absorption directly I warrant.
The reason I used above the term "reservoir" is a normal person does not huge amounts of active B12. On the other hand it won't hurt them as they will excrete what they don't need. Big deal. But if methylation processing is weakened via genetics or over time by epigenetic stresses the "reservoir" starts to run dry. Especially of useful active forms. Or at least the distribution of active forms and other intermediates are skewed due to poor ATP, lack of methyl donors, etc.
I agree with Freddd the travesty is the medical community tries to refill that "reservoir" with cyanob12 which is really wrong. It should be refilled with the forms that we would have consumed. That being said methylation of the B12 (or adding adenosyl groups) is a dynamic process. If you consume methylb12, ORALLY I don't think it makes it through the gut lining as mb12 almost at all. It probably goes to hydroxy form and gets methylated on the other side in the liver and then once used by cells may get stored in glutathionyl form until it is recycled or removed. Sublingual is different. You drive a diffusion gradient through the cell layers in the oral cavity and push it into your bloostream though at still only a marginal rate compared to injectables.
Personally if someone wants to take ORALLY mb12 or hb12 I am not sure there is a huge difference only because of the conversion needed. I don't think the methyl group makes it easily into the portal vein to the liver so there is no guarantee that the added methyl group just does not swim around in the gut for a bit then get bound by something else and excreted.
But there is a huge difference imo between SUBLINGUAL or INJECTABLE mb12 and hb12. Now the dosing is very different with oral. And I am not sure the body is used to seeing that much mb12 or hb12 at once. In the former case that is what we want, where the amount is based on the individual of course and how bad the active "reservoir" is in trouble. In the latter case that means a lot of work to convert to active b12s. Now maybe one's genetics (like MTR, MTRR, etc.) do a lot to answer the question of how a subllingual or injectable hb12 will impact a person
When I take sublingual hb12, even on the 1st day it is like someone sucked the energy right out of me. I also get a weird sore throat. I do pass out early and sleep but I feel kind of oddly ill (this is for 2000 mcg hb12 lozenges) I think with my MTRR mutations and low lithium levels, that the 2500 mcg of Enzymatic Therapy mb12 I take is probably only keeping pace with the reservoir. Certainly not overfilling it. Some on here take really high doses to force mb12 into the CNS, and trust me in the periphery they are overflowing that reservoir with intent, since that is the only way to get even satisfactory concentrations in the CNS.
So like I said an adaptive reservoir. I think for those of one these forums upping Intrinsic factor is like shooting at a male African elephant with a BB gun. That is my personal opinion of course.
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