The life of a Christian is a strange mix of living in two worlds, the earthly and the heavenly.
John Piper said, “If we are full of what the world offers, then perhaps a fast might express, or even increase, our soul's appetite for God. Between the dangers of self-denial and self-indulgence is the path of pleasant pain called fasting.” 
What John Piper calls ‘pleasant pain’ is the act of denying a comfort in order to achieve a greater good. Through much of church history there were devout believers who practiced asceticism. An ascetic is ‘a person who dedicates his or her life to a pursuit of contemplative ideals and practices, extreme self-denial or self-mortification for religious reasons.’
While many of them went to extremes, it may be that they took a good idea too far. There is a difference between choosing to be uncomfortable and bringing damage to your body.
The World War II German pastor Dietrich Bonhoeffer said, “If there is no element of asceticism in our lives, if we give free rein to the desires of the flesh (taking care of course to keep within the limits of what seems permissible to the world), we shall find it hard to train for the service of Christ. When the flesh is satisfied it is hard to pray with cheerfulness or to devote oneself to a life of service which calls for much self-renunciation.” 
So what should be a balanced relationship with our physical body? Should we become obsessed with diet, exercise and health? Paul makes an important assessment on these things.
What are godless myths and old wives’ tales? If we look at the variety of strong opinions about diet, exercise and staying healthy we discover much that is contradictory, circumstantial or accusatory toward how others live. Taking care of your body and exercising has some definite merit, but there is something more important than spending countless dollars and effort to better your body.
When we fast for the right reasons, we free ourselves from compulsively eating and drinking. The same is true when learn to practice moderation in all things. By learning to control the natural habits, we gain strength for the spiritual habits.
Food is not your master. If you have given yourself to dieting, an exercise regime or specific restrictions for the sake of your health, does it not make sense to practice self-discipline that will result in an eternal benefit?