"Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?" ~Isaiah 58.6
We fast to do something for ourselves. But, Jesus instructs that the fast He chose is not about us at all. If we are going to give up food, we need to give up food to buy someone in need a meal or clothing or help in other way that they need, not that we think they need.
The true fast is what we give of our own lives, in and through Christ, by the power of Holy Spirit, so that someone else can have life.