|St. Paul the Hermit, who was dedicated to fasting and abstinence|
It is well known that, according to popular Church usage, the difference between fasting and abstinence is that a fast-day indicates a restriction of the quantity of food (currently, one large meal and two small meals) while an abstinence-day indicates a restriction of the quality of food (today, no meat). This is summed up well by the Catholic Encyclopedia.
“Laws relating to fasting are principally intended to define what pertains to the quantity of food allowed on days of fasting, while those regulating abstinence, what refers to the quality of viands.” (Catholic Encyclopedia, Abstinence)
Nevertheless, it will be good for us to consider that there is a deeper distinction between fasting and abstinence – one rooted in moral theology. A consideration of this ontological and moral difference will help us to enter more profoundly into the mystery of Lent.
Abstinence is a virtue, while fasting is an act
We turn first to the Angelic Doctor: “Abstinence by its very name denotes retrenchment of food. Hence the term abstinence may be taken in two ways. First, as denoting retrenchment of food absolutely, and in this way it signifies neither a virtue nor a virtuous act, but something indifferent. Secondly, it may be taken as regulated by reason, and then it signifies either a virtuous habit or a virtuous act.” (ST II-II, q.146, a.1)
About fasting, St. Thomas writes: “Habit and act have the same matter. Wherefore every virtuous act about some particular matter belongs to the virtue that appoints the mean in that matter. Now fasting is concerned with food, wherein the mean is appointed by abstinence. Wherefore it is evident that fasting is an act of abstinence.” (ST II-II, q.145, a.2)
Therefore, we may conclude that the essential difference between fasting and abstinence is that abstinence is a stable disposition in the soul, it is a virtue. In fact, abstinence is "a special virtue" which falls under the cardinal virtue of temperance. Meanwhile, fasting is an act of the virtue of abstinence.
If we look to a solid modern source for Catholic Doctrine, we find the same: Abstinence is defined as, “The moral virtue that inclines a person to the moderate use of food or drink as dictated by right reason or by faith for his own moral and spiritual welfare. As commonly understood, abstinence refers to refraining from certain kinds of food or drink and may be undertaken by a person spontaneously or it may be prescribed by ecclesiastical law, whether for the universal Church or for certain territories.” (Fr. John Hardon’s Modern Catholic Dictionary, Abstinence)
Fr. Hardon defines fasting as, “A form of penance that imposes limits on the kind or quantity of food or drink. From the first century Christians have observed fasting days of precept, notably during the season of Lent in commemoration of Christ's passion and death.” (Fr. John Hardon’s Modern Catholic Dictionary, Fasting)
The practices of Lent: Prayer, Fasting, and Almsgiving
This is why the practices of Lent are listed as prayer, fasting and almsgiving, without any mention of abstinence. Abstinence is primarily a virtue, but the “practices” of Lent are acts. Thus, we speak of fasting and not of abstinence when we consider the Lenten discipline.