The Church merely requires the bare minimum when it comes to fasting. Many saints encourage us to go beyond the mere minimum. For example, the following are a couple of short passages from the famous spiritual work, “Introduction to the Devout Life” by St. Francis DeSales:
“If you are able to fast, you will do well to observe some days beyond what are ordered by the Church, for besides the ordinary effect of fasting in raising the mind, subduing the flesh, confirming goodness, and obtaining a heavenly reward, it is also a great matter to be able to control greediness, and to keep the sensual appetites and the whole body subject to the law of the Spirit; and although we may be able to do but little, the enemy nevertheless stands more in awe of those whom he knows can fast. The early Christians selected Wednesday, Friday and Saturday as days of abstinence. Do you follow therein according as your own devotion and your director’s discretion may appoint.” (Part III.23)
On the other hand, in the passage below he also cautions us against being immoderate in our fasting. It is better to be slow and steady than excessive because being excessive can lead to break downs that leave a person worse off than in the beginning. It is important to note here that he speaks of immoderate fasting as when people grow too thin. This has never been a great concern of mine because like most Americans, I am much fatter than I need to be. A doctor once told me that Americans, on average, tend to be about 20 pounds overweight, but they don’t think of themselves as overweight because most everyone around them is overweight too. Nevertheless, it is always important to keep moderation in mind when fasting.
“I am prepared to say with S. Jerome (to the pious Leta) that I disapprove of long and immoderate fasting, especially for the young. I have learnt by experience that when the colt grows weary it turns aside, and so when young people become delicate by excessive fasting, they readily take to self-indulgence. The stag does not run with due speed either when too fat or too thin, and we are in peril of temptation both when the body is overfed or underfed; in the one case it grows indolent, in the other it sinks through depression, and if we cannot bear with it in the first case, neither can it bear with us in the last.” (Part III.23)
I might add here another reason to avoid immoderate fasting: pridefulness. There is a fine line between summoning the effort and courage it takes to fast appropriately and falling into a sinful state of pride that can seep into your soul by turning your fasting into something that makes you feel superior. Always remember, fasting is supposed to make you more humble, not less!
In summary: St. Francis DeSales says here that we want to conquer our own greediness and we certainly want to protect ourselves from the enemy. On the other hand, we must be prudent and moderate in our fasting if we are to be faithful and effective disciples of Christ for the long run. A good lesson from a spiritual giant.