The unleavened bread used to celebrate the Eucharist during Catholic masses can be made with genetically modified organisms, the Vatican has said, but they cannot be entirely gluten-free.
Low-gluten hosts are allowed as long as the wheat has enough gluten to obtain the bread without the use of additives or other "foreign materials".
Cardinal Robert Sarah of the Vatican's Congregation for Divine Worship and the Discipline of the Sacraments said the guidance was needed now that the Eucharistic bread and wine can be found in supermarkets and "even over the internet".
In a letter issued last month, the cardinal also reminded bishops that hosts should be made by people "distinguished by their integrity".
He said that adding fruit or sugar is a "grave abuse".
"It is altogether forbidden to use wine of doubtful authenticity or provenance," he added.
But for people who cannot tolerate wine, the use of mustum, or must, a thick non-fermented grape juice, is considered "valid matter" for the sacrament, which Catholics believe turns the bread and wine into the body and blood of Jesus Christ.