Objection 1. It seems that the sacramental species cannot nourish, because, as Ambrose says (De Sacram. v), "it is not this bread that enters into our body, but the bread of everlasting life, which supports the substance of our soul." But whatever nourishes enters into the body. Therefore this bread does not nourish: and the same reason holds good of the wine.
Objection 2. Further, as is said in De Gener. ii, "We are nourished by the very things of which we are made." But the sacramental species are accidents, whereas man is not made of accidents, because accident is not a part of substance. Therefore it seems that the sacramental species cannot nourish.
Objection 3. Further, the Philosopher says (De Anima ii) that "food nourishes according as it is a substance, but it gives increase by reason of its quantity." But the sacramental species are not a substance. Consequently they cannot nourish.
On the contrary, The Apostle speaking of this sacrament says (1 Corinthians 11:21): "One, indeed, is hungry, and another is drunk": upon which the gloss observes that "he alludes to those who after the celebration of the sacred mystery, and after the consecration of the bread and wine, claimed their oblations, and not sharing them with others, took the whole, so as even to become intoxicated thereby." But this could not happen if the sacramental species did not nourish. Therefore the sacramental species do nourish.
I answer that, This question presents no difficulty, now that we have solved the preceding question. Because, as stated in De Anima ii, food nourishes by being converted into the substance of the individual nourished. Now it has been stated (5) that the sacramental species can be converted into a substance generated from them. And they can be converted into the human body for the same reason as they can into ashes or worms. Consequently, it is evident that they nourish.
But the senses witness to the untruth of what some maintain; viz. that the species do not nourish as though they were changed into the human body, but merely refresh and hearten by acting upon the senses (as a man is heartened by the odor of meat, and intoxicated by the fumes of wine). Because such refreshment does not suffice long for a man, whose body needs repair owing to constant waste: and yet a man could be supported for long if he were to take hosts and consecrated wine in great quantity.
In like manner the statement advanced by others cannot stand, who hold that the sacramental species nourish owing to the remaining substantial form of the bread and wine: both because the form does not remain, as stated above (Question 75, Article 6): and because to nourish is the act not of a form but rather of matter, which takes the form of the one nourished, while the form of the nourishment passes away: hence it is said in De Anima ii that nourishment is at first unlike, but at the end is like.
Reply to Objection 1. After the consecration bread can be said to be in this sacrament in two ways. First, as to the species, which retain the name of the previous substance, as Gregory says in an Easter Homily (Lanfranc, De Corp. et Sang. Dom. xx). Secondly, Christ's very body can be called bread, since it is the mystical bread "coming down from heaven." Consequently, Ambrose uses the word "bread" in this second meaning, when he says that "this bread does not pass into the body," because, to wit, Christ's body is not changed into man's body, but nourishes his soul. But he is not speaking of bread taken in the first acceptation.
Reply to Objection 2. Although the sacramental species are not those things out of which the human body is made, yet they are changed into those things stated above.
Reply to Objection 3. Although the sacramental species are not a substance, still they have the virtue of a substance, as stated above.
Source: Summa Theologica (New Advent)