Isaiah 58

 Fasting is only physical pain and worthless unless we - 

...loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?

7 Is it not to deal thy bread to the hungry, and that thou bring the poor that are cast out to thy house? when thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh?  (Isaiah 58:6–7)

If we fast with a purpose and provide an offering for the oppressed - 

"Then shall thy light break forth as the morning, and thine health shall spring forth speedily: and thy righteousness shall go before thee; the glory of the Lord shall be thy rearward.

9 Then shalt thou call, and the Lord shall answer; thou shalt cry, and he shall say, Here I am. If thou take away from the midst of thee the yoke, the putting forth of the finger, and speaking vanity;

10 And if thou draw out thy soul to the hungry, and satisfy the afflicted soul; then shall thy light rise in obscurity, and thy darkness be as the noonday:

11 And the Lord shall guide thee continually, and satisfy thy soul in drought, and make fat thy bones: and thou shalt be like a watered garden, and like a spring of water, whose waters fail not."  (Isaiah 58:8–11)