That's a reason, but likely not the only reason. There are likely reasons relating to regulation of other digestion processes that slow down to a halt while fas

That's a reason, but likely not the only reason. There are likely reasons relating to regulation of other digestion processes that slow down to a halt while fasting. Coming up from a fast and eating a lot of food quickly may cause refeeding syndrome - regardless of whether it is carbs or protein/fat. Switching back to carbs from pure fat or pure protein is not dangerous in the same way, AFAIK (but I'm not a doctor).