Well for me it was a prolonged illness that overran my antioxidant system which lowered my glutathione. Which coincidentally helps recycle vitamin c and e. It also is the body's primary detoxifier. Once my level of oxidative stress was too high for my antioxidant system to handle then the my antioxidant system started to weaken. My glutathione became low and I stopped detoxifying properly. The toxins interfere with the b12 metabolism and slow the production of methyl groups which essentially less and less glutathione to be produced. If you could research Richard Van Konynenburg's work on chronic fatigue syndrome then you will find all the technical factors and data that support that. For me it was my test results for glutathione. 3 years after I got a really bad infection my reduced to oxidized ratio was half of what it really should be. The problem is the condition is essentially chronic once problems develop where your glutathione lowers. It will never be fixed until the problem is addressed. Unfortunately doctors simply aren't aware of it or don't know where to look. So, I was able to learn that from the researcher I mentioned and forums.
However, some parasites do take up b12. So, it's possible that could be a factor. Also genetics have came into it with me too.. So, ya know.. yeah.. it's a mess..
There is a methylation protocol that Richard Konynenburg devised to help address people with low glutathione. Or hopefully bypass gene's that prevent the full production of the various cycles related to it's production without the expensive genetic testing.
Yeah. He saved my life. except now I have parsites.