Because something funny happens to organism on fast and it starts conserving absolutely everything. So what is recommended amount during "normal operation" is p

Because something funny happens to organism on fast and it starts conserving absolutely everything. So what is recommended amount during "normal operation" is probably too much while your organism is in emergency preservation mode. Now, fast is no magic, the organism still uses up stores of various chemicals, just slower, so it still can "run out" of some things even if it still has a bunch of fat available for energy. One way organism can preserve chemicals is the simple fact it no longer has to digest stuff and then expell a lot of waste products. All of this is normally huge drag on your body, mostly on your liver. When on fast, that process is no longer necessary (mostly) and your liver gets much more efficient at the smaller number and volume of metabolic pathways. So when you are on fast and drop a vitamin, your body has not much else to do and will be very efficient at incorporating those vitamins into your body when normally most of these vitamins would not be used because the pathways necessary would be busy doing something else. It is also really funny to observe you can be not eating anything but still poop for at least a weak.