A spiritual fast is one in which a person says no to something or someone less important, so they can say yes to something and someone more important. A popular Christian food fast is the Daniel Fast. If the Lord leads someone into a food fast, then the Lord will empower that person to live not on bread alone, but on the Word of God. It is a powerful experience to experience the sustaining power of the Word. It has been six years since I have felt so led, but I am encouraged to read and hear of many who are being called into food fasts. For now, for me I sense a fast of those things that really corrupt a soul (mine). It is not the things that go into our mouths (food) that corrupt us, but the things (words, thoughts, attitudes), that come out of our hearts that corrupt us. So, whatever, whoever, and wherever, you feel so led to fast, or say no to, make sure to spend some time saying no to the things that really hurt you, so you can say yes to the things that can help you.
For anyone who feels that if they engage in a food fast, it somehow moves the hand and heart of God because of your sacrifice, you are missing the point and are living under the old covenant. Jesus' disciples didn't fast food, like John's disciples, and it is a point worth considering. The first time I was told this, I got mad at the messenger, until I eventually understood. So… fast, or say no to anything, anyone, anyplace that is dissipates your energies away from the most important people, things, and places, and rest in the finished work of Calvary.