Objection 1. It would seem that it is not requisite for fasting that one eat but once. For, as stated above (Article 2), fasting is an act of the virtue of abstinence, which observes due quantity of food not less than the number of meals. Now the quantity of food is not limited for those who fast. Therefore neither should the number of meals be limited.
Objection 2. Further, Just as man is nourished by meat, so is he by drink: wherefore drink breaks the fast, and for this reason we cannot receive the Eucharist after drinking. Now we are not forbidden to drink at various hours of the day. Therefore those who fast should not be forbidden to eat several times.
Objection 3. Further, digestives are a kind of food: and yet many take them on fasting days after eating. Therefore it is not essential to fasting to take only one meal.
On the contrary, stands the common custom of the Christian people.
I answer that, Fasting is instituted by the Church in order to bridle concupiscence, yet so as to safeguard nature. Now only one meal is seemingly sufficient for this purpose, since thereby man is able to satisfy nature; and yet he withdraws something from concupiscence by minimizing the number of meals. Therefore it is appointed by the Church, in her moderation, that those who fast should take one meal in the day.
Reply to Objection 1. It was not possible to fix the same quantity of food for all, on account of the various bodily temperaments, the result being that one person needs more, and another less food: whereas, for the most part, all are able to satisfy nature by only one meal.
Reply to Objection 2. Fasting is of two kinds [Cf. 1, ad 3]. One is the natural fast, which is requisite for receiving the Eucharist. This is broken by any kind of drink, even of water, after which it is not lawful to receive the Eucharist. The fast of the Church is another kind and is called the "fasting of the faster," and this is not broken save by such things as the Church intended to forbid in instituting the fast. Now the Church does not intend to command abstinence from drink, for this is taken more for bodily refreshment, and digestion of the food consumed, although it nourishes somewhat. It is, however, possible to sin and lose the merit of fasting, by partaking of too much drink: as also by eating immoderately at one meal.
Reply to Objection 3. Although digestives nourish somewhat they are not taken chiefly for nourishment, but for digestion. Hence one does not break one's fast by taking them or any other medicines, unless one were to take digestives, with a fraudulent intention, in great quantity and by way of food.
Source: Summa Theologica (New Advent)