The spiritual principle of fasting is simply that:  spiritual.  Granted you can fast in a secular sense.  But, that is more like dieting.  The spiritual fast puts God in the center of the event.  It has specific consequences and purposes to it.  A spiritual fast denies for the sake of unity:  Entering into the mystery of suffering accompanied with prayer.

In recent decades the Church has dumbed down fasting and reduced it to more of an abstinence, not a genuine fast.  To be prudent, one with health issues (like diabetes) or in advanced age, fasting should be a careful endeavor.  For most people, fasting should increase our hunger and deny particular passions so as to strengthen us for current or future spiritual battles.  It is not a diet plan! 

So, fasting can be used for multiple situations:

1) For spiritual unity with the suffering Christ—by denying our body certain foods or pleasures, we enter into the mystery of suffering with Christ on the cross.

2)    Fasting can help others who are in spiritual difficulty—by  fasting and prayer, we can assist others by uniting  our action with their intention.

3)   Fasting and prayer can overcome evil in us or around us.  Fasting is to be applied as a remedy for our own sinful habits, or for problems in our families, friends, or the world in general.

4)   Fasting can temper our personal passions—perhaps there is a habit or sin that needs healing.  Fasting with prayer can help to overcome a personal sin or desire that has been difficult to master.

5)   A side effect of prayerful fasting can be weight loss—A Christian, however, doesn’t use fasting in this way.  Fasting is not a diet, but, God can grace us with some loss of weight only as a by-product.

So, when fasting, keep it spiritual and wrapped in prayer.  If you want a diet, then exercise and eat Lean Cuisine or Nutrisystem.