From the book Living Theology

The fast is a means of acquiring the Holy Spirit. In brief, negligence and heedlessness with regards to fasting, are the first weapons of the enemy for the secularizing of the human soul. In other words, the neglect of fasting enables the devil to make the human soul wordly. And this leads to people viewing fasting in purely worldly terms, as a form of physical dieting instead of a means of becoming spiritual. For such people, fasting is not important, for they cannot see its spiritual significance. It is better, they say, to avoid anger, irritability, not to speak untruths, not to allow evil to issue from our mouth. Of course it is good not to do these things. Yet there is no comparison here, for fasting is the means by which we are able to avoid such spiritual sins. 

from the book Living Theology