Fasting before receiving communion is one of those topics that everyone should know about, but there are always a few people who don't.
The first thing to note is that this rule is not a joke or a simple recommendation. It is stated in Canon Law, “One who is to receive the Most Holy Eucharist is to abstain from any food or drink, with the exception only of water and medicine, for at least the period of one hour before Holy Communion.” This translates to 20-30 minutes before Sunday Mass and 40 minutes before daily Mass. There are exceptions to this rule. For example, persons over the age of 60, those who are infirm, people who care for the infirm, and those who cannot, within reason, conveniently observe the full fast need only fast 15 minutes (See Immensae Caritatis.) Also, a priest must fast only before his first Mass of the day. So if you see me chowing down a doughnut and chugging a coffee before noon Mass, don’t Judge.
So, those are the basic rules of the practice. I want to mention at this point that we should not overly stress ourselves about the time. If you fast 30 minutes before Sunday Mass and then, for whatever reason, Mass ends up being extra short for whatever reason, you don’t need to count the minutes or seconds. If you made the effort in good conscience, that is enough. But the question of ‘why’ probably lingers for many. Is this just a random and arbitrary act of Church enforcement?
We should note first that this practice did not pop up out of thin air. It has roots in Scripture itself. Fasting in preparation for significant spiritual undertakings and before special encounters with God occur throughout the Bible. As a widespread practice and rule, fasting before receiving the Eucharist goes back to the earliest times of legalized, organized Christianity, though the rule was a 12 hour fast until it was lessened in the 20th century.
The reasons for this practice are twofold: Preparation and Penance. You have probably heard me say how we, as creatures of body and spirit, can help prepare and orient our spirits by bodily actions and practices. This is one of those situations. Though certainly less obvious and noticeable with a single-hour fast, we encourage ourselves to ‘hunger’ for the Eucharist when we fast from all other food leading up to Communion. We let our stomachs settle, in a sense. Instead of being another little morsel on top of the heap, the Eucharist comes to us as food to a hungry person. By encouraging this bodily hunger for the Eucharist, we encourage our souls to experience a spiritual hunger.
Another aspect of preparation is the idea of ‘clearing the way.’ When we have a special guest coming to our house, our instinct is to spend some time cleaning up, making the space appropriate. And so, we, though we are unworthy that he should ‘enter under our roof’, should do our best to prepare for Jesus’ coming by ‘cleaning up the house’ of our bodies. He is, after all, the most special of guests.
There is also an element of penance to the act of fasting before Holy Communion. Penance is not a word we are used to hearing these days outside of the small acts or prayers the priest asks you to do after the Sacrament of Reconciliation. But Penance should be a normal and consistent part of life for every Catholic. St. Paul reminds us, "Continually we carry about in our bodies the dying of Jesus, so that in our bodies the life of Jesus may be revealed" (2 Cor 4:10). We too are charged to convert our whole lives—body and soul—to the Lord. This conversion process involves doing penance—including bodily mortification like fasting—for our sins and weaknesses, which in turn strengthens and heals us. Pope Paul VI exhorted the faithful in his apostolic constitution "Paenitemini" (1966), "Mortification aims at the liberation of man, who often finds himself, because of concupiscence, almost chained by his own senses. Through 'corporal fasting' man regains strength, and the wound inflicted on the dignity of our nature by intemperance is cured by the medicine of a salutary abstinence." The act of fasting an hour before receiving Jesus is a very small sacrifice. Yet, it is a small act of penance, an act of inviting the Grace of God to change us in body and spirit to be more like Christ. It is also a reminder for us to make acts of penance a normal part of our lives.
Because of the significant effects of preparation and penance on our spiritual lives, we are required to fast an hour before receiving Holy Communion. But that is the minimum. Voluntarily fasting for a longer period, within the bounds of prudence, only intensifies the effect. I would highly encourage you, if not frequently then at least once, to fast for 12 hours or more before you receive Holy Communion. The experience of receiving the bread of life as the first taste of food after a long period of hunger, expectation, and preparation is a beautiful thing.