Objection 1. It seems that food or drink taken beforehand does not hinder the receiving of this sacrament. For this sacrament was instituted by our Lord at the supper. But when the supper was ended our Lord gave the sacrament to His disciples, as is evident from Luke 22:20, and from 1 Corinthians 11:25. Therefore it seems that we ought to take this sacrament after receiving other food.
Objection 2. Further, it is written (1 Corinthians 11:33): "When you come together to eat," namely, the Lord's body, "wait for one another; if any man be hungry, let him eat at home": and thus it seems that after eating at home a man may eat Christ's body in the Church.
Objection 3. Further, we read in the (3rd) Council of Carthage (Can. xxix): "Let the sacraments of the altar be celebrated only by men who are fasting, with the exception of the anniversary day on which the Lord's Supper is celebrated." Therefore, at least on that day, one may receive the body of Christ after partaking of other food.
Objection 4. Further, the taking of water or medicine, or of any other food or drink in very slight quantity, or of the remains of food continuing in the mouth, neither breaks the Church's fast, nor takes away the sobriety required for reverently receiving this sacrament. Consequently, one is not prevented by the above things from receiving this sacrament.
Objection 5. Further, some eat and drink late at night, and possibly after passing a sleepless night receive the sacred mysteries in the morning when the food it not digested. But it would savor more of moderation if a man were to eat a little in the morning and afterwards receive this sacrament about the ninth hour, since also there is occasionally a longer interval of time. Consequently, it seems that such taking of food beforehand does not keep one from this sacrament.
Objection 6. Further, there is no less reverence due to this sacrament after receiving it, than before. But one may take food and drink after receiving the sacrament. Therefore one may do so before receiving it.
On the contrary, Augustine says (Resp. ad Januar., Ep. liv): "It has pleased the Holy Ghost that, out of honor for this great sacrament, the Lord's body should enter the mouth of a Christian before other foods."
I answer that, A thing may prevent the receiving of this sacrament in two ways: first of all in itself, like mortal sin, which is repugnant to what is signified by this sacrament, as stated above (Article 4): secondly, on account of the Church's prohibition; and thus a man is prevented from taking this sacrament after receiving food or drink, for three reasons. First, as Augustine says (Resp. ad Januar., Ep. liv), "out of respect for this sacrament," so that it may enter into a mouth not yet contaminated by any food or drink. Secondly, because of its signification. i.e. to give us to understand that Christ, Who is the reality of this sacrament, and His charity, ought to be first of all established in our hearts, according to Matthew 6:33: "Seek first the kingdom of God." Thirdly, on account of the danger of vomiting and intemperance, which sometimes arise from over-indulging in food, as the Apostle says (1 Corinthians 11:21): "One, indeed, is hungry, and another is drunk."
Nevertheless the sick are exempted from this general rule, for they should be given Communion at once, even after food, should there be any doubt as to their danger, lest they die without Communion, because necessity has no law. Hence it is said in the Canon de Consecratione: "Let the priest at once take Communion to the sick person, lest he die without Communion."
Reply to Objection 1. As Augustine says in the same book, "the fact that our Lord gave this sacrament after taking food is no reason why the brethren should assemble after dinner or supper in order to partake of it, or receive it at meal-time, as did those whom the Apostle reproves and corrects. For our Saviour, in order the more strongly to commend the depth of this mystery, wished to fix it closely in the hearts and memories of the disciples. and on that account He gave no command for it to be received in that order, leaving this to the apostles, to whom He was about to entrust the government of the churches."
Reply to Objection 2. The text quoted is thus paraphrased by the gloss: "If any man be hungry and loath to await the rest, let him partake of his food at home, that is, let him fill himself with earthly bread, without partaking of the Eucharist afterwards."
Reply to Objection 3. The wording of this decree is in accordance with the former custom observed by some of receiving the body of Christ on that day after breaking their fast, so as to represent the Lord's supper. But this is now abrogated, because as Augustine says (Resp. ad Januar., Ep. liv), it is customary throughout the whole world for Christ's body to be received before breaking the fast.
Reply to Objection 4. As stated in the II-II, 147, 6, ad 2, there are two kinds of fast. First, there is the natural fast, which implies privation of everything taken before-hand by way of food or drink: and such fast is required for this sacrament for the reasons given above. And therefore it is never lawful to take this sacrament after taking water, or other food or drink, or even medicine, no matter how small the quantity be. Nor does it matter whether it nourishes or not, whether it be taken by itself or with other things, provided it be taken by way of food or drink. But the remains of food left in the mouth, if swallowed accidentally, do not hinder receiving this sacrament, because they are swallowed not by way of food but by way of saliva. The same holds good of the unavoidable remains of the water or wine wherewith the mouth is rinsed, provided they be not swallowed in great quantity, but mixed with saliva.
Secondly, there is the fast of the Church, instituted for afflicting the body: and this fast is not hindered by the things mentioned (in the objection), because they do not give much nourishment, but are taken rather as an alterative.
Reply to Objection 5. That this sacrament ought to enter into the mouth of a Christian before any other food must not be understood absolutely of all time, otherwise he who had once eaten or drunk could never afterwards take this sacrament: but it must be understood of the same day; and although the beginning of the day varies according to different systems of reckoning (for some begin their day at noon, some at sunset, others at midnight, and others at sunrise), the Roman Church begins it at midnight. Consequently, if any person takes anything by way of food or drink after midnight, he may not receive this sacrament on that day; but he can do so if the food was taken before midnight. Nor does it matter, so far as the precept is concerned, whether he has slept after taking food or drink, or whether he has digested it; but it does matter as to the mental disturbance which one suffers from want of sleep or from indigestion, for, if the mind be much disturbed, one becomes unfit for receiving this sacrament.
Reply to Objection 6. The greatest devotion is called for at the moment of receiving this sacrament, because it is then that the effect of the sacrament is bestowed, and such devotion is hindered more by what goes before it than by what comes after it. And therefore it was ordained that men should fast before receiving the sacrament rather than after. Nevertheless there ought to be some interval between receiving this sacrament and taking other food. Consequently, both the Postcommunion prayer of thanksgiving is said in the Mass, and the communicants say their own private prayers.
However, according to the ancient Canons, the following ordination was made by Pope Clement I, (Ep. ii), "If the Lord's portion be eaten in the morning, the ministers who have taken it shall fast until the sixth hour, and if they take it at the third or fourth hour, they shall fast until evening." For in olden times, the priest celebrated Mass less frequently, and with greater preparation: but now, because the sacred mysteries have to be celebrated oftener, the same could not be easily observed, and so it has been abrogated by contrary custom.
Source: Summa Theologica (New Advent)