The short answer is “yes.” The appearances of bread and wine left after the Consecration have all of the physical characteristics (“accidents”) of bread and wine, including the ability to be incorporated into the body.
(Obviously, the quantities are insufficient for someone to live off them, but they are metabolized by the body in the normal way. If we are to interpret the passage from Isaiah given by the O.P. as applying to the Eucharist, then we would have to interpret it more metaphorically. Food, drink, and nourishment could easily be metaphors for the grace that the faithful obtain by receiving the Eucharist, which is actually more important—in and of itself—than physical nourishment.)
What changes is what theologians—using Aristotle’s terminology—call the substance (what a thing is simply because it exists). In this case, the substance of the bread (or wine) is converted into the very substance of Jesus Christ.
However, all of the physical properties and characteristics, or accidents, remain the same. There is no physical or chemical test that could distinguish the Corpus or Sanguis from bread or wine.
The Eucharist is one of those few phenomena in which faith alone can tell us the difference (in the words of the Eucharistic hymn attributed to St. Thomas Aquinas, Ad firmandum cor sincerum / Sola fides sufficit, “In order to strengthen a sincere heart [to believe this mystery], faith alone suffices.”)
Note that the substantial presence of the Lord remains only for as long as the appearances of bread and wine remain. Hence, the Real Presence stops once the Eucharist has dissolved in the body.
The Catholic doctrine on the Eucharist (which is essentially the same as that of the Eastern Orthodox and the other apostolic churches) is summed up in numbers 1373-1381 of the Catechism of the Catholic Church.
