As we’ve visited before, the precepts, which are binding upon everyone in the Church, are so important that they are presented as obligations. St. Paul reminds us in his first letter to the Corinthians (9:25) that “athletes deny themselves all sorts of things...to gain a perishable crown.” We shouldn’t think then that we mustn’t endeavor in our Christian lives. After all, even doctors put persons under certain restrictions or obligations that they might increase in health and have a more full life.
Let’s define some terms here. When the Church says fasting, she means: for ages 18-59, persons are permitted to eat a single full meal and two smaller meals that do not equal a full meal; abstinence, when referring to food, is refraining from all meats, which is defined as the product from an animal which has been slaughtered or blood spilled. As such, fish is excluded from the requirement. Abstinence from meat applies to everyone 14 and older. While most Catholics realize that Ash Wednesday and Good Friday and every Lenten Friday are penitential days, many Catholics are shocked to hear that every Friday is a day of penance, which is still traditionally held to abstain from meat. After Vatican II the USCCB (United States Conference of Catholic Bishops), with proper permission from Rome, brought forth an option for non-Lenten Fridays: while it is still laudable to abstain from meat as the penance of every Friday, a person may substitute another penance, provided it is more penitential. This is particularly useful if you have been invited over to someone’s house and are unsure about the menu; instead of refusing the meal, you could polite eat what was put before you and then make another penance.
“But, Father, what the use?” Perfect question, and I’m glad you asked. First, fasting/abstinence/penance is part of divine law; God wills that we do. Second, through these acts we are making justice our sacrifice to God (cf. Ps 4:5). Third, we are sloughing off dead parts from our souls. Fourth, we can cultivate virtues which can only be done so through some sort of suffering: humility, compassion, kindness, long suffering, and gentless.