Those who faithfully fast have been promised great blessings. "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 rereward. 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; 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: 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. And they that shall be of thee shall build the old waste places: thou shalt raise up the foundations of many generations; and thou shalt be called, The repairer of the breach, The restorer of paths to dwell in." (Isa. 58:8-12).
There are so many blessings in there that it is important to understand each one separately. First, we are promised that if we faithfully fast and share our substance with the needy, our light will break forth as the morning. The light of the Lord will shine in our countenances. We will be included in the great spiritual army of Zion, which army "looketh forth as the morning, fair as the moon, clear as the sun, and terrible as an army with banners" (Song of Solomon 6:10; see also D&C 5:14). The radiance of the Lord will shine round about us.
Second, we are promised that our health will spring forth speedily. There are known health benefits from fasting regularly. Additionally, the Lord here promises us that as we are faithful in fasting, we can be healthier than we were - spiritually and physically. This promise reminds one of the promise in the Word of Wisdom, which is also related to eating: "And all saints who remember to keep and do these sayings, walking in obedience to the commandments, shall receive health in their navel and marrow to their bones.... And shall run and not be weary, and shall walk and not faint" (D&C 89:19,21).