Fasting is a medicine.
But like all medicines, though it be very profitable to the person who knows how to use it, it frequently becomes useless (and even harmful) in the hands of him who is unskillful in its use.
For the honor of fasting consists not in abstinence from food, but in withdrawing from sinful practices, since he who limits his fasting only to abstinence from meats is one who especially disparages fasting.
Do you fast?
Give me proof of it by your works.
If you see a poor man, take pity on him.
If you see an enemy, be reconciled with him.
If you see a friend gaining honor, do not be jealous of him. And let not only the mouth fast, but also the eye and the ear and the feet and the hands and all members of your bodies.
St. John Chrysostom