Recently, the Congregation for Divine Worship and the Discipline of the Sacraments reiterated the Church’s teachings regarding the matter of the Eucharist – notably that Communion hosts must be wheat bread, and therefore contain some amount of gluten.
This has led some people to think that celiacs (or simply those allergic to wheat / gluten) have effectively been “banned” from receiving the Eucharist. Others, though, have stated that celiacs need not worry about receiving the host, because after it is consecrated the host is really Jesus instead of wheat. Both of these positions are theologically mistaken.
Philosophically speaking, “substance” refers to what a thing truly is, and “accidents” refer to how a substance can be modified. If a skinny dog changes into a fat dog, that’s “accidental change” because it’s still a dog. If a cow dies, that’s “substantial change” because it turns into meat. The Eucharist is the result of transubstantiation – a miraculous type of change where the host’s accidental properties (bread) remain exactly what they were even though its substance changes (from bread into the body, blood, soul, and divinity of Jesus Christ). The material properties of the bread remain, though. Therefore, saying that celiacs don’t have to worry about receiving the host because it is really Jesus instead of wheat is like saying someone who is allergic to beef can safely eat a hamburger because it isn’t a cow. Just as one can get drunk from the wine before and after the consecration (1 Cor. 11:21), a celiac can have adverse reactions to the host whether consecrated or not.
Now, wheat bread and grape wine are the proper matter of the Eucharist because Christ instituted it this using those materials (Mt. 26:26-29 – see also Jesus comparison of Himself to wheat in Jn. 12 and the grapevine & Jn. 15). So, a gluten-free host makes for an invalid sacrament. Celiacs are not excluded from Communion, however. They may either request low-gluten hosts, or receive communion under the “species” (sensible qualities) of wine alone. This is because “both under the species of the bread and under the species of the wine the living Jesus Christ is all present, with His Body, His Blood, His Soul and His Divinity” (The Catechism of St. Pius X, Q&A 17).