True fasting is giving up food for the purpose of feeding the hungry, or to have more time to pray and meditate on God’s word for guidance and help (Ex. 34:28; Dt. 9:9; 2 Sa. 12:16-23; 2 Ch. 20:3-4; Ezr. 8:21-23). Fasting could be vicarious (Ezr. 10:6; Est. 4:15-17). Some thought fasting would automatically guarantee an answer to prayer of (Is. 58:3-4) or protect them against the demonic by granting them some supernatural power (Mt. 17:21 KJV). Against this the prophets taught that without proper understanding and conduct fasting was useless (Is. 58:5-12; Je. 14:11-12; Zec. 7; Ho 4:6).

Isaiah 58:3-9 ‘Why have we fasted,’ they say, ‘and you have not seen it? Why have we humbled ourselves, and you have not noticed?’ "Yet on the day of your fasting, you do as you please and exploit all your workers. ⁴ Your fasting ends in quarreling and strife, and in striking each other with wicked fists. You cannot fast as you do today and expect your voice to be heard on high. ⁵ Is this the kind of fast I have chosen, only a day for a man to humble himself? 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? ⁶ "Is not this the kind of fasting I have chosen: to loose the chains of injustice and untie the cords of the yoke, to set the oppressed free and break every yoke? ⁷ 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? ⁸ Then your light will break forth like the dawn, and your healing will quickly appear; then your righteousness will go before you, and the glory of the LORD will be your rear guard. ⁹ Then you will call, and the LORD will answer;

Zechariah 7:4-10 Then the word of the LORD Almighty came to me: ⁵ "Ask all the people of the land and the priests, ‘When you fasted and mourned in the fifth and seventh months for the past seventy years, was it really for me that you fasted?... ⁹ "This is what the LORD Almighty says: ‘Administer true justice; show mercy and compassion to one another. ¹⁰ Do not oppress the widow or the fatherless, the alien or the poor. In your hearts do not think evil of each other.’

Jeremiah 14:12 Then the LORD said... ¹² Although they fast, I will not listen to their cry;
Luke 18:10-14 Jesus told this parable: ¹⁰ "Two men went up to the temple to pray, one a Pharisee and the other a tax collector. ¹¹ The Pharisee stood up and prayed about himself: ‘God, I thank you that I am not like other men—robbers, evildoers, adulterers—or even like this tax collector. ¹² I fast twice a week and give a tenth of all I get.’ ¹³ "But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, ‘God, have mercy on me, a sinner.’ ¹⁴ "I tell you that this man, rather than the other, went home justified before God.
Luke 5:33-35 They said to him, "John’s disciples often fast and pray, and so do the disciples of the Pharisees, but yours go on eating and drinking." ³⁴ Jesus answered, "Can you make the guests of the bridegroom fast while he is with them? ³⁵ But the time will come when the bridegroom will be taken from them; in those days they will fast."

The following references to fasting are not found in the earliest manuscripts. Mt. 17:21 and Mk. 9:29 this kind goeth not out but by prayer and fasting (KJV); Acts 10:30 And Cornelius said, Four days ago I was fasting (Praying NIV) until this hour; (KJV); 1 Cor. 7:5 Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer (KJV) (fasting not in NIV).
The Day of Atonement was a time the Jews were to fast and repent of their sins. References to fasting on the Day of Atonement are: Lev. 23:26-32; Dan. 9&10.
In the following passages the Greek word for fasting, peinwmen, is used to denote that they went hungry: 2 Cor. 6:5; 1 Cor 4:11; 2 Cor. 11:23; Rom. 8:35.

In conclusion we can state there are no scriptures that talk about fasting as a means of power over the devil, sin, or even keeping you own body healthy. Fasting is only taught in the scriptures as a means of making sure the poor have enough to eat.

All quotations taken from the NIV unless other wise noted.