When we look at the three Gospel accounts (Matthew 4:1-11, Mark 1:12-13, and Luke 4:1-13) of Jesus’ 40-day fast nothing is said regarding whether Jesus drank water or not. Luke 4:2 specifically says, “He ate nothing during those days and at the end of them he was hungry.” There is no reference to the drinking of water or another liquid. We may assume that Jesus did, therefore, drink water.
Medical literature says that a human cannot live long without water (two to ten days, depending on the physical health of the individual and the climatic conditions), but can live a long time (four to six weeks) without food.
However, an acceptable religious fast is much more than solely abstaining from food and water. God told the nation of Israel what true fasting was in Isaiah 58.
God rebuked Israel for their unacceptable fasting. “Is it only for bowing one’s head like a reed and for lying on sackcloth and ashes? Is that what you call a fast, a day acceptable to the LORD? Is not this the kind of fasting I have chosen: to loose the chains of injustice and untie the chords of the yoke, to set the oppressed free and break every yoke? Is it not to share your food with the hungry and to provide the poor wanderer with shelter—when you see the naked, to clothe him, and not to turn away from your own flesh and blood? . . . Then you will call, and the LORD will answer; you will cry for help, and he will say; Here am I.” (Isaiah 58:5-7,9) This passage makes it very clear that an acceptable fast is not merely abstinence from food or water, but a decision to fully obey God’s commands to care for the poor and oppressed. Zechariah 7:5-10 also repeats this message.
In Matthew 6:16-18 Jesus warns his followers against fasting only to make others admire them. He provides practical steps on how to fast in private.
“When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show men they are fasting. I tell you the truth, they have received their reward in full. But, when you fast, put oil on your head and wash your face, so that it will not be obvious to men that your are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret will reward you.”