The book of Isaiah contains many plain and precious truths of the gospel that have rarely been expressed as succinctly and beautifully by other prophets. There is a reason that Isaiah is the most quoted prophet in the scriptures. There is a reason the Savior specifically told the Nephites to read his words. In Isaiah chapter 58 we learn much about the law of the fast, about fasting. Isaiah criticizes those who "fast for strife and debate," (Isa. 58:4) who fast for the wrong reasons and are irritable and short-tempered. If we do not fast with sincere purpose, we are just starving ourselves for little benefit. We are more likely to "exact all [our] labours" (Isa. 58:3), or in other words, make sure others know we are suffering. This is what the Savior taught about on His sermon on the mount. "Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward." (Matt. 6:16). Then the Lord continues with how we should fast. "But thou, when thou fastest, anoint thine head, and wash thy face; That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly" (Matt. 6:17-18).
Returning to Isaiah, we learn more about fasting. When we fast we should bow our heads down and express sorrow for sins, becoming penitent before the Lord. "Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?" (Isa. 58:6). Through fasting we can free ourselves from the bondage of sin. We can be free from oppression and lightened of burdens. The yoke we use to pull our heavy cart of sin can be replaced by the Lord's, which is easy and light [see 2 Ne. 15:18; Matt. 11:30].
Isaiah continued with things that we should do in conjunction with fasting. "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?" (Isa. 58:7). When we fast, we should also help feed the hungry and care for the poor and clothe the naked. That is what our Fast Offerings do today. They go to those in need in our ward, then stake, then the wider church. Through our offerings, we literally can bless the lives of our neighbors. Fasting gives us opportunity to stop focusing on ourselves so that we can focus on those around us.