This might be one of the most fascinating papers I've read on B12 deficiency and treatment, with several sections exploring how symptoms relate back to the particular biochemistry processes that go wrong in deficiency. The paper was produced in 2021 for the B12 Institute, Netherlands, a specialist B12 deficiency treatment center.
It is highly technical but worth the effort IMO and should be mostly accessible to many members with some experience in reading about B12 deficiency. It also explores possible treatments/supplements other than B12 that might help the recovery process. These supplements include Glycine, NAC(N-Acetyl-Cysteine), CDP-Choline, Phosphatidylserine, Betaine (Tri-methylglycine), Vitamin C, Alpha-Lipoic Acid, Antibiotics and Probiotics.
The section on choline is quite interesting:
"Phosphatidylcholine, the major constituent of myelin sheath, is biosynthesized via two pathways:
• The CDP-choline pathway (Kennedy pathway) using choline as an essential precursor [133]
• By phosphatidylethanolamine methylation by SAM [134]
During cobalamin deficiency/inactivity, the methylation of PE into PC is inhibited due to the lack of SAM. Therefore, while the methionine cycle is slowly repaired by cobalamin supplementation, choline may act in two ways to accelerate the repair of myelin sheaths:
• choline supplementation can help to accelerate the synthesis of PC via the Kennedy pathway,
• choline is also the precursor of betaine, the coenzyme of BHMT for the alternative methylation
of homocysteine; thus choline could increase the synthesize SAM during MS slow running"
There is also a fascinating hypothesis on a possible link from formaldehyde production to the often reported "early treatment decline" that often happens when treatment begins, as follows:
"it is expected that an excessive folic acid consumption without a proper flow of folate cycle may exacerbate oxidative stress by generating more formaldehyde. However, further investigation on the flux of formaldehyde from folate oxidation must be carried out to confirm the significance. In addition, it was suggested that the oxidative damage of the unstable folates can be enzymatically repaired in mammalian cells.
...
the endogenous formaldehyde from folate oxidation might also explain the worsening of symptoms in patients at the early period of treatment. Patients with a prolonged MS inactivity may accumulate 5-MTHF. Once given a high dose of cobalamin, MS is activated and a burst of THF may occur. In the case of an elevated oxidative stress, the pool of THF may be oxidized into formaldehyde, which create more damages to the body.
We expect that it is a challenge to prove this hypothesis, as it will be difficult to simultaneously capture the moment of 5-MTHF trap release, THF oxidation, and formaldehyde production throughout the metabolism."
There is a lot more in here but I would end up excerpting the whole paper so I'll leave it there.
The paper: Design of a treatment protocol to improve the health of B12 deficient patients