21 “And bread strengthens man's heart.” What is this, brethren? As it were, he has forced us to understand what bread he was speaking of. For while that visible bread strengthens the stomach, feeds the body, there is another bread which strengthens the heart, in that it is the bread of the heart....There is therefore a wine that truly makes glad the heart, and knows not to do anything else than to gladden the heart. But that you may not imagine that this indeed should be taken of the spiritual wine, but not of that spiritual bread; He has shown this very point, that it is also spiritual: “and bread,” he says, “strengthens man's heart.” So understand it therefore of the bread as thou dost understand it of the wine; hunger inwardly, thirst inwardly: “Blessed are they,” says our Lord, “who hunger and thirst after righteousness; for they shall be filled.” That bread is righteousness, that wine is righteousness: it is truth, Christ is truth. “I am,” He said, “the living bread, who came down from heaven;” and, “I am the Vine, and you are the branches.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)