Sorry I didn't see this earlier....
I've known a number of people who had a discipline of fasting at least one day each week..... and extended fasts once or twice a year. I've never developed the discipline like they have. I have fasted periodically for various resons and lengths. Twice for 7 days.. but never longer. By my experience, after 3 days, one stops being hungry. But I experienced the symptoms of fasting such as dizziness and fatigue etc. In my opinion, it is "dieing to" and praying through those symptoms that bring such value to the fast. Biblical meditation during fasting is very beneficial. God really does some amazing things in the process.
But it is important to remember what scripture says in a few places....
Matthew 6:16 "Moreover when ye fast, be not, as the hypocrites, of a sad countenance; for they disfigure their faces, that they may appear unto men to fast. verily I say unto you, They have their reward. v17 But thou, when thou fastest, anoint thine head, and wash thy face; v18 That thou appear not unto men to fast, but unto thy Father which is in secret; and thy Father, which seeth in secret, shall reward thee openly."
Isaiah 58... the whole chapter gives wisdom on fasting.... but these scriptures in particular....
6 “Is not this the kind of fasting I have chosen: to loose the chains of injustice and untie the cords of the yoke, to set the oppressed free, and break every yoke?
7 Is it not to share your food with the hungry and to provide the poor wanderer with shelter— when you see the naked, to clothe them, and not to turn away from your own flesh and blood?
8 Then your light will break forth like the dawn, and your healing will quickly appear; then your righteousness[a] will go before you, and the glory of the LORD will be your rear guard.
9 Then you will call, and the LORD will answer; you will cry for help, and he will say: Here am I.