And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
–Genesis 2:16-17, 3:22
Notice, a fast encompasses both the affirmations and prohibitions of the Lord–what we should and shouldn’t do–even as we are to refuse carnal knowledge and partake of righteous instruction (Genesis 3:22; 1 John 2:16). Consequently, the most common fast is not simply abstaining from certain foods, but also partaking of certain foods, according to the direction of God. This is a partial fast, after the manner of the prophet Daniel:
And the king appointed them a daily provision of the king’s meat, and of the wine which he drank … But Daniel purposed in his heart that he would not defile himself with the portion of the king’s meat, nor with the wine which he drank: therefore he requested of the prince of the eunuchs that he might not defile himself. Now God had brought Daniel into favour and tender love with the prince of the eunuchs. And the prince of the eunuchs said unto Daniel, I fear my lord the king, who hath appointed your meat and your drink: for why should he see your faces worse liking than the children which are of your sort? then shall ye make me endanger my head to the king. Then said Daniel to Melzar, whom the prince of the eunuchs had set over Daniel, Hananiah, Mishael, and Azariah, Prove thy servants, I beseech thee, ten days; and let them give us pulse [vegetables] to eat, and water to drink. Then let our countenances be looked upon before thee, and the countenance of the children that eat of the portion of the king’s meat: and as thou seest, deal with thy servants. So he consented to them in this matter, and proved them ten days. And at the end of ten days their countenances appeared fairer and fatter in flesh than all the children which did eat the portion of the king’s meat. Thus Melzar took away the portion of their meat, and the wine that they should drink; and gave them pulse. As for these four children, God gave them knowledge and skill in all learning and wisdom: and Daniel had understanding in all visions and dreams.
–Daniel 1:5, 8-17
Those who fast to gain favor with God do not understand fasting.
We fast to acknowledge what God has already done–to show faith in His character–and contentment with His provision.
That said, there are secondary benefits to fasting, as the above account concerning Daniel details:
Fasting improves our physical health.
Fasting insulates us from the deceptions and temptations of this world.
Fasting enables us to receive the vision of the Lord. [This translates to increased fruit-bearing: for fasting emphasizes the believer’s reliance upon God, removing the leaven from our lives and drawing us apart from the rudimentary principles of this world.] We see this in another account in Daniel:
I ate no pleasant food, no meat or wine came into my mouth, nor did I anoint myself at all, till three whole weeks were fulfilled. Then I lifted up mine eyes, and looked, and behold a certain man clothed in linen, whose loins were girded with fine gold of Uphaz: His body also was like the beryl, and his face as the appearance of lightning, and his eyes as lamps of fire, and his arms and his feet like in colour to polished brass, and the voice of his words like the voice of a multitude. And I Daniel alone saw the vision: for the men that were with me saw not the vision; but a great quaking fell upon them, so that they fled to hide themselves.
Important Note: The fact that we were created to fast should bolster our faith that we are fully able to fast.