It is not denying yourself of earthly food.
The fast that God has chosen is not one of physical abstinence from food, but of spiritual abstinence from falsehoods. In Isaiah 58, Yahweh defines His kind of fast, emphasizing the rejection of false doctrines and the corrupt influences of the world.
By turning away from false teachings, He has promised us power to “loose the bands of wickedness” and “undo the heavy burdens” (v. 6). This spiritual fast grants us divine strength as we align our thoughts and actions with the mind of Christ. He is an invisible Spirit. “And they that worship Him must worship in spirit and in truth” (John 4:24). This includes fasting.
Precious Sheep Of Renown 