Yeah, from reading this forum, the causes of brain fog and fatigue could be near infinity.
Typically, the experts say that once one begins B12 supps., they should try to keep levels above 1000.
When last checked, mine were above 3000; I was forced to take enough to stop the neurological damage that was unremitting with lower dosages.
Recently, I've been trying to pinpoint the bacteria that steal our B12 for their own consumption. B12 deficiency from SIBO makes more sense to me than anything now. ( I've scoured the genetics of it, but it doesn't explain enough, and I don't have pernicious anaemia.)
As with everything I research, there tends to be no clear answer. Bacteroides fragilis, for instance, is a commensal bacteria, but it can be overgrown in SIBO. B. fragilis appears to have a great affinity for B12.
I forget the bacteria that are found in the infection called Tropical Sprue, but there are several that cause B12 deficiency. ( These bacteria are also found in SIBO.)
In recent months, I've been able to reduce the B12 dose, but some days ago, I began to have vertigo again, along with fatigue, neuropathy, optic neuritis,and fog. I've had to increase the dose again.
I think there's a chance for great improvement by treating the SIBO. This will take more time, the modified paleo diet, is showing some promise. I hope to starve those buggers into submission, but not eradication.