Fasting, like any other wholesome practice, was, and is, abused. Here is Luther’s recollection of what it was like for him in the monastery, from the Tabletalk.

On March 20 there was talk about the most sumptuous fasts of the papists—which were nothing less than fasts when the meals of bread and wine were without moderation. “Only truly afflicted consciences fasted in earnest,” Martin Luther said. “I almost fasted myself to death, for again and again I went for three clays without taking a drop of water or a morsel of food. I was very serious about it. I really crucified the Lord Christ. I wasn’t simply an observer but helped to carry him and pierce [his hands and feet]. God forgive me for it, for I have confessed it openly! This is the truth: the most pious monk is the worst scoundrel. He denies that Christ is the mediator and high priest and turns him into a judge.

“I chose twenty-one saints and prayed to three every day when I celebrated mass; thus I completed the number every week. I prayed especially to the Blessed Virgin, who with her womanly heart would compassionately appease her Son. Ah, if the article on justification hadn’t fallen, the brotherhoods, pilgrimages, masses, invocation of saints, etc., would have found no place in the church. If it falls again (which may God prevent!) these idols will return.”

Martin Luther, vol. 54, Luther’s Works, Vol. 54  : Table Talk, ed. Jaroslav Jan Pelikan, Hilton C. Oswald and Helmut T. Lehmann, Luther’s Works, 54:339 (Philadelphia: Fortress Press, 1999, c1967).