What is the purpose of fasting? Physical fasting from physical food and drink is to prepare  and strengthen us for the spiritual fasting from sin,  both sins of commission and sins of omission. Fasting helps us to become more attentive to the areas of sin in our life with the determination of avoiding and uprooting these sins with the help of God's grace. These are sins of commission. At the same time fasting gives us a clearer insight in the sins of omission, primarily where we are blinded in the areas of the corporal and spiritual works of mercy, namely the needs of others. Mere physical fast without the spiritual fast will only affect us physically and externally, but not spiritually and interiorly.