An important point was made. Sometimes what we DON 'T eat is every bit as important as what we do. I can't heal my gut while eating milk or cheese. On the other hand too many veggies overwhelm me with unusable and even methylfolate blocking veggie folates. They are every bit as bad as folic acid for me.
And then there is that problem that appears to be a defining characteristic, problem getting cobalamins into the CSF past the brain blood barrier. It might be a nutritional factor. Methylfolate is very important as it helps us retain B12 longer. However, eating animal based foods with 3-10mcg of b12 a day, never did it for me. I just couldn't get enough b12. When I changed to vegetarian, I could feel the difference after one day without meat. My vegetarian friends all told me various anti-meat BS. However, as time did tell, it wasn't "toxins" coming out of my system, it was the life force, shear vitality, that was leaking out at 6mcg per day and nearly killed me. Cyanocobalamin didn't do the trick. Sure it kept my MCV from exploding past 100, but it didn't stop most B12 deficiency symptoms from developing. It didn't stop brain and cord degeneration as in Sub Acute Combined Degeneration.
So, for those of us with CFS, FMS, MS, Parkinson's, Supra Nuclear Palsy, and less directly autism and Alzheimer's, to get MeCbl and AdoCbl past BBB into the brain and cord in large enough quantities (several micrograms daily likely) to allow brain functioning without demyelination or other damage has become the challenge if we want to live. No possible vegetarian diet can do this for us. The "normal" amount of active b12s leaves us in the cold. Only animal based foods have MeCbl and AdoCbl and they don't have enough for this group to live a healthy life. Also, and perhaps this has something to do with the BBB situation, most of us have difficulty with folic acid, folinic acid and veggie folate to various degrees.