John Calvin, in the 16th century, said,
Let us say something about fasting, because many, for want of knowing its usefulness, undervalue its necessity, and some reject it as almost superfluous; while, on the other hand where the use of it is not well understood, it easily degenerates into superstition. Holy and legitimate fasting is directed to three ends; for we practice it either as a restraint on the flesh, to preserve it from licentiousness, or as a preparation for prayers and pious meditations, or as a testimony of our humiliation in the presence of God when we are desirous of confessing our guilt before him. (Institutes, IV.12, 14, 15)
Martin Luther wrote,
Of fasting I say this: It is right to fast frequently in order to subdue and control the body. For when the stomach is full, the body does not serve for preaching, for praying, or studying, or for doing anything else that is good. Under such circumstances God’s Word cannot remain. But one should not fast with a view to meriting something by it as by a good work.