To fast with evil intention, wicked mind, quarrelling and hatred in the mind is unacceptable (Isa. 58:2-5). Also to fast like hypocrites, to be noticed and praised by men is unacceptable to God (cp. Matt. 6:16-18). Before our fasts are acceptable to Him, we must loose all bands of wickedness, undo every heavy burden and consider the poor and needy (Isa. 58:6-7). To be beneficial, our fasting must be exclusively unto God, in all sincerity and devoid of all ego. Note, a fast is void when it is devoid of true heart contrition and self-effacement, which is the life and soul of the exercise. The showmanship of the hypocrites’ fasts makes it void (Matt. 6:16). And it is sad that one who has in some measure, mastered his pleasure, should be ruined by his pride.
Are you so weak that you cannot fast? Or do you make a show of your fast? Note that great reward accrues to those who fast unto God appropriately (Mtt. 6:16-18).