The great news is, fasting is slow, painful slow, but delivers quickly through the ordeal. How nature done planned it.
I have been brain dead cast into the empiric mold of western medical training (not always a bad thing), but cling to a couple perspectives from the more vague holistic side of the fence. Soft stuff without much hard core data to back up the claim.
The swami guru type dudes claim that the human body runs on about the energy to fire up a 25 watt light bulb. This energy remains somewhat constant depending on a lot of things. Something that takes up more watts has to leech energy from something else.
Non science idea that one pathway is shared by assimilation versus elimination drains on available energy in out current mess. Modern civilized cannibals are gorging our guts 3 times a day, if we are in a country where we can eat or afford processed corpses. Only time we fast is when we are asleep. Thus (according to the model) we are mainly in the assimilation mode, taking energy from the elimination mode that is trying to rid the body of deadly toxins. Ancestors had little reliable food and had to go many meals or days with nothing. Fasting imposed by the Mother to clean us out.
Now we get fat, toxic, wretched diseases until we are mercifully taken out of the gene pool.
Fasting turns on the elimination nukes. How much hell to pay to restore balance depends on how toxic we are from our past dietary sins when we start the process.
John