@Jo86 - I've read that B12 has helped many ME/CFS sufferers with brain fog. I just realized something - I don't have brain fog, though I definitely have ME/CFS. However, my doctor had me taking B12 (and sometimes doing B12 injections) for many years before I came down with ME/CFS. That might be the reason I've never suffered from brain fog, taking B12 for so long.
There's not a one size fits all supplement program for persons with ME/CFS, but here's what has helped me the most - and please only start one new thing at a time. Otherwise you won't know what is doing what to you! Also, if you have some sort of refeeding syndrome reaction as I did with methylfolate and thiamine, if it's mixed in with everything else, you'll have no way of knowing what's going on.
d-ribose - I took this for 12 years or so, it helped noticeably with energy
methylfolate - nice boost in energy with this. However, a few days later it then caused my potassium to drop suddenly, causing severe fatigue, but because I had read about this possibility, I started taking potassium and titrated up to 1000 mg a day over a couple of days, and the severe fatigue went away. I've had to take potassium every day since 2010, but it's worth it because the methylfolate helps me a lot. I had to experiment to get the right dose of methylfolate, we're all different.
B1 (thiamine) - this made a big difference in my energy. I still remember the first time I took it. I had read about people getting a lot of energy with B1 which is why I tried it. However, again, a few days later I was hit with severe fatigue. But this was different than the methylfolate fatigue - potassium did nothing for this fatigue. It took me a long time to figure it out, but finally I realized it was the same process - refeeding syndrome - which caused the fatigue. The B1 was depleting my phosphorous - hypophosphatemia is the hallmark of refeeding syndrome. So I drank several glasses of kefir - high in phosphorous - and sure enough, the awful fatigue then dissipated. I was so glad to discover this, because it meant I could keep taking the B1 which made me feel so good! I thereafter found a phosphorous product, easier than drinking a lot of kefir, and I have to take the phosphorous a couple of times a week.
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B6 - Nutreval testing showed a severe B6 deficiency which shouldn't have existed because I'd been taking a B complex forever, but there it was. So I added in more B6 and it helped my energy a little.
l-carnitine - or acetyl-l-carnitine - at one time this helped my energy a lot, worth an experiment I think
branched chain amino acids - these cut my PEM recovery time by more than half. I've been taking about 4000 mg a day since 2014 (or 2016?), anyways, if I cut back, my PEM recovery time starts to increase again so I just keep taking them. I wouldn't be without them. They also help my energy some. And they've helped several people on the board here with energy and functioning. I've been taking this product:
AmazonSmile: Optimum Nutrition Instantized BCAA Capsules, Keto Friendly Branched Chain Essential Amino Acids, 1000mg, 400 Count
These are the main ones which have helped me with energy but if I were you I'd start with the B12 and see how you do. B12 can cause the same potassium deficiency that methylfolate did with me. If you're deficient in B12 or methylfolate, your body adapts to subpar nutrition and thus uses less potassium to function. When you remedy the deficiency, cells start ramping up their activity, increasing the need for potassium, and thus inducing a potassium deficiency. People with ME/CFS seem prone to refeeding syndrome. Here's an article which explains more about refeeding syndrome:
Refeeding syndrome: what it is, and how to prevent and treat it - PMC (nih.gov)
Member Freddd here has written a lot about potassium and methylation. Also, here's a post which explains how people with ME/CFS can have low intracellular potassium despite normal blood levels:
Why is potassium supplementation needed in methylation treatmt? | Phoenix Rising ME/CFS Forums
fwiw, my functioning today is quite a bit better than 24 years ago when I first got sick - I have more energy, stamina etc. I've done a LOT of experimentation, not all of it with good results of course, but overall I am much improved - and I take a lot more things than listed above, but these might be a good starting point.