Fasting is a complicated topic, which should be approached in the context of obedience to a spiritual director.
Fasting, like all external works, can be taken to excess for many reasons, including (but not limited to): vainglory, pride, excessive concern about one's figure, etc.
One thing that should be kept in mind regarding fasting: Food today has almost no nutrition in it, compared to 200 years ago.
So that bread crust YOU eat is NOTHING AT ALL LIKE the crust of bread that you read about a saint eating. The wheat for that bread was not grown in over-farmed soil, sprayed with pesticides and chemical fertilizer to make it grow. Modern agriculture treats the soil like a sponge, to be filled with chlorinated water, chemical fertilizers, etc. Trace elements are long gone -- along with most vitamins.
Then when you microwave food, even MORE vitamins are destroyed. It's a testament to how well-designed the human body is that most of us aren't dead right now.
Long story short: It's providential, and clearly God's will, that the fasting regulations aren't unreasonable for the times we live in. Fasting was never intended to render the majority of men useless throughout Lent.
So we need to keep all this in mind when discussing the topic of fasting.
That having been said, there is room for mortification in each of our lives. If nothing else, we can forego seasonings, or include seasonings that don't belong (cinnamon on your hamburger) and stay healthy WHILE being mortified.
Matthew