I think dbkita mentioned a possible defect in the potassium channels. Cells allow sodium and potassium in and out via channels, and if those are defective, then the balance is disrupted. I also read that potassium channels need magnesium to close, so if there is a deficiency, the channels can't close and you lose potassium. It's unusual that so many people ON HERE have this issue with potassium so my guess is that there is an over-arching reason why that encompasses both the susceptibility to ME/CFS and potassium deficiency. Or else it's new RBCs. Or both.