After prayer comes fasting, that is to say, not only fasting properly so called, which consists in abstaining from food, but also all penitential works of what nature soever they may be. It must here be remarked that this is a question not only of the great austerities practiced by the saints, but… also of the least mortifications, the smallest sacrifices which we impose upon ourselves or accept for the love of God, and which we offer to His Divine Mercy for the relief of the holy souls.
A glass of water, which we refuse ourselves when thirsty, is a trifling thing, and if we consider this act in itself, we can scarcely see the efficacy it possesses to alleviate the sufferings of Purgatory. But such is the Divine Goodness that deigns to accept this as a sacrifice of great value.
—from Purgatory: Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J., Part Second: Purgatory, The Mystery of God’s Mercy – Ch. 23
The day you leave the table without having done some small mortification you have eaten like a pagan.
—from The Way by Saint Josemaria Escriva, 681