I'm in Greg's Facebook group(s) re B12 deficiency. I joined a couple of years ago when I stumbled across them by chance, and was immediately attracted by the Kreb's Cycle of the group's banner! Anyway I have been (attempting to) follow his protocol during that time and still do so.
There's a lot to unpack, really. The group consists of a large mix of people, some have CFS/ME, many have hypo (or occasionally hyper) thyroidism (I am hypo with Hashi's and think I have some degree of CFS, am still investigating that). Others there have autistic children whom they are trying to help or are diagnosed with B12 deficiency but can't get well.
The protocol has a very definite 'order' to it, in terms of the 'cascade of energy' - from Iodine through Selenium, Molybdenum, B2 and B12. The thyroid is crucial to this as it requires both Iodine and Selenium, and Greg monitors the thyroid by using TSH along with total T4 and T3 measurements (rather than the 'frees').
As doctors misuse TSH all the time to assess one's thyroid function (by using it on its own without looking at thyroid hormone levels), I am very resistant to monitoring one's thyroid via TSH (which is a pituitary hormone after all), and aiming to get it to a value of 1.0 in lab tests. Not everyone's TSH 'set point' is the same, plus there is diurnal variation in TSH, plus some folk have problems with their pituitary that causes their TSH to not respond properly. I have never seen any of those points mentioned. I do see people post how their TSH is, say, 1.3 and they can't seem to get it lower, how much more Iodine do they need to take. I feel there can be an over-emphasis on TSH which doesn't help.
The 'functional' deficiencies have been nicely covered by
@Hip above. You may have a very high top of the reference interval value for B12 in your blood but yet be 'paradoxically' deficient - this is because it is in an inactive form and cannot be utilised, yet you will think you are replete in it and so will your doctor. Greg says that sublingual supplementation is ineffective and serves to increase this paradoxical level in the blood, and he invented his B12 oils for transdermal application to overcome this.
I definitely had signs of B12 deficiency - peripheral neuropathy - despite taking B12 sublingually in large doses. I have been using B12 transdermal oils and was just thinking last night how my feet are now free of neuropathy. I also take B2 as it seems to be the cornerstone of so many enzymes - via FAD.
Greg claims that both histamine intolerance and oxalate issues (which I have) can be remediated via fixing B2 deficiency (as it's needed to metabolise these) but as yet I have not seen the back of these unpleasant conditions. Some in the group have done so, and have gone from only being able to eat a few foods to back to a normal diet. I can't speak to how many have been 'cured' of CFS/ME, I'll have to take his word for it.
The OAT is Greg's favourite analytical tool along with thyroid labs, full blood count and iron panel. He has accumulated hundreds of OAT tests from many people who've contacted him. I don't doubt for a minute that he has helped people, but of course everyone is different. He is not a fan of many supplements (e.g. magnesium) preferring that people get their nutrition from food. This of course can be extremely difficult if you have food intolerances.
I think I will pause here or my post will reach epic proportions, if it hasn't done so already! I do recall Greg mentioning Creatine as important in CFS but will put that in another post.