I couldn't find the post where someone mentioned that their brainfog was resolved by B1 (thiamine). Thanks to them I started taking 200 mcg / day and I had instant improvement. Since then I have been on a research binge.
I'm not saying this will fix it for others, but I'm thinking that it will for me and that I not only have a b12 deficiency but a B1 and who knows what deficiencies in other b vitamines.
I liked this video that talks about why diabetics and in my case I have insulin resistance dump Thiamine and how critical it is to glucosis:
youtube.com/watch?v=m3DopqT...
I went back an looked at my labs and only b12 was tested.
I will be getting my b1, b2 and other vitamins tested now to confirm this.