15 Give us this day our substantial bread. This common bread is not substantial bread, but this Holy Bread is substantial, that is, appointed for the substance of the soul. For this Bread goes not into the belly and is cast out into the draught, but is distributed into your whole system for the benefit of body and soul. But by this day, he means, “each day,” as also Paul said, While it is called today.
Source: Catechetical Lectures (New Advent)