I should have figured it out sooner ... the past couple of weeks I have had extra unexplained fatigue, sometimes severe - yesterday it was very bad. I also had had slight congestion and figured it was a special bug that made me very tired with only slight symptoms. Yesterday I finally began to realize that something else was going on - low phosphorous came immediatleyi to mind and sure enough that was the problem.. Low phosphorous can cause horrible fatigue - from Wikipedia: I took 2 doses of my phosphate supplement yesterday and drank a glass of kefir this morning, and suddenly now I have energy. Yesterday I barely managed to wash my hair and that was all I was able do. Ordinarily I can do more, if not crashed etc. Hypophosphatemia can occur in people with ME/CFS in at least 2 ways: refeeding syndrome and phosphate diabetes, though there very well could be others. Hypophosphatemia is the hallmark of refeeding syndrome (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440847/) I first experienced it after taking B1. The B1 initially boosted my energy markedly and then a day or 2 later severe fatigue hit. It felt different than the fatigue I get with low potassium - no muscle twitching, it just felt different. I’d read about hypophosphatemia before in connection with refeeding syndrome so I guessed that’s what happened. I read about natural sources of phosphorous, drank 2 or 3 glasses of kefir and within a few hours the horrible fatigue started to lift. Since that time I’ve drunk kefir a few times a week (also good for my gut) but I’ve slacked off a lot for several weeks. I also eat a lot of sunflower seeds which are high in phosphorous. And I have a monosodium phosphate supplement I got from Swanson’s which I use from time to time in small doses, while trying to get the phosphate I need from food. I am beginning to think I might need extra phosphate every day, just as I do with potassium. Phosphate diabetes (unrelated to diabetes types 1 and 2) occurs in 10% of ME/CFS patients according to this study: But there is so little research on phosphate diabetes and ME/CFS, I would not be at all surprised if it was much more common. We have so many other weird things happen to us, why not that? This article says that hypophosphatemia rarely shows up in the blood because and that Which means that it will not be on your doctor’s radar screen. My alkaline phosphates level (part of comprehensive metabolic panel) a few years ago was low, but never flagged by anyone, and low alkaline phosphates can be caused by hypophosphatemia, as well as other things. However, my level the last 2 years was normal, albeit on the low end. But I get symptoms of hypophosphatemia, perhaps as explained by the Oregon State article above - that the body compensates to keep normal serum levels but you can still be deficient. Perhaps this is similar to how people with ME/CFS can have normal serum potassium levels but low intracellular potassium: https://forums.phoenixrising.me/ind...ded-in-methylation-treatmt.18670/#post-291422 Or high levels of serum B12 but low intracellular. Standard blood tests alone just don’t seem to work that well for people with ME/CFS. Everything you read about low phosphorous will talk about how rare it is and how you practically have to be starving to have it or some rare genetic defect. I don’t know of any doctors who will look at this. So, as almost always, we have to do our own research on this.