Fasting isn't the same as dieting. The purpose of dieting is to improve the health and beauty of our bodies. The purpose of fasting is to turn our attention to God, other people, and ourselves. Dieting means the regulation of food intake as a health measure - as in low calorie, low fat, or low sodium foods voluntarily for a time and to eat sparingly as a spiritual exercise. Though both dieting and fasting can have physical benefits, only fasting gives spiritual ones.
Like mortification in general, the practice of limiting the amounts and kinds of our food and drink are forms of penance common to all great religions. People fast partly in order to overcome an indulgent spirit which seeks comfort and pleasure above all else. Thomas Merton wrote that the desires for food, drink, sex and pleasure are like little children - the kind which truly honours God - is the kind of self-denial which, in the prophet Isaiah's words, results in deeds of justice and compassion, in freeing the oppressed, in sharing one's bread with the hungry.
[Source: Supplement, St. Paul Roman Catholic Cathedral, Saskatoon, SK, Canada; March, 2010]