Satan is depicted in the Scripture as a serpent and a roaring lion. He is described as a liar and a deceiver. He is sly and cunning and stupid. Whoops! Maybe that last adjective is not Scriptural, but it is appropriate, nonetheless. The basis for the observation is found in the story of the Temptation of Jesus. It is one well remembered. It is told by the first three gospel writers. After His baptism the Spirit leads or drives Jesus into the wilderness where He does two things. He is tempted by Satan. And, He fasted. It is surely a moment when Satan miscalculated. If he figured Jesus would be easier prey because He was famished, he was wrong.
A person who is fasting is not in a weakened spiritual condition, but a stronger one. A fast brings the one fasting to a place of having an intense desire for God. Such a person longs not for food, but for a deeper awarness of the presence of God. While I am no expert on fasting, it is a discipline I have entered into a few times. My fasts were not of the forty days and forty nights variety, but more like between Maundy Thursday Communion and Easter Sunrise. They never were as long as they might have been, but long enough for me to be aware that the end result was a hunger and thirst for God.
Whenever we are hungry and thirsty for God, we are not in a weakened spiritual position. We are actually more likely to be able to rebuke the tempter than we are in the ordinary moments when it seems that we have everything under control. Remember, too, that Jesus said, "Blessed are those who hunger and thirst for righteousness, for they will be filled." (Matthew 5:6) Satan was stupid to think a physically hungry Jesus was easy prey. He was instead in a position of greater strength as are we when we come to a place of deep desire for God.