@Sidny - respiratory depression and low cardiac output can be signs of hypophosphatemia (see
here), as well as fatigue, weakness, etc. I've experienced this and it makes me very tired. There's a subgroup of ME/CFS patients who have
phosphate diabetes (unrelated to "regular" diabetes). I don't know if I have this, but I do have ongoing problems with low phosphorous and very often have to take a supplement. Low phosphorous also can be caused by refeeding syndrome (see
https://en.wikipedia.org/wiki/Refeeding_syndrome). When I first started taking extra thiamine, it initially boosted my energy very nicely and then a day or 2 later I got hit with severe fatigue, due to hypophosphatemia.