In yogic teachings, the practice of fasting can be associated with pranayama (energy control) and pratyahara (sensory control). Pranayama is most often connected with breathing since it is such an elemental part of our existence. (I wouldn’t live long if I stopped breathing.) Eating too is a deeply-rooted part of my physical existence, so its pull on my consciousness is strong. As pranayama practice develops, it naturally evolves from the breath toward eating and other “essential” life-functions.
Fasting is also part of pratyahara, the practice of looking at my sensory input and how it affects the way I perceive the world (and myself). My sense of taste is obviously related to food, and our culture has increasingly turned toward foods that stimulate my taste receptors, so much that I often consume food for the way it stimulates my mouth (and brain). When I control my food intake, I quickly come face to face with the powerful connections between my senses (in this case, taste) and my reality.
Controlling the energy and senses is hard, so fasting is hard. Quite literally it challenges my constructed perceptions of who I am. This is why dieting and food control is often a lost cause. The yogis consider fasting a relatively advanced form of control, so it makes sense when so many people struggle with it.
