According to the Vatican, a valid Communion host must contain sufficient gluten to ensure the proper confection of the bread, and its processing must not alter its fundamental nature.

Wheat, a cereal that contains gluten, is the only substance authorised by the Roman Catholic Church for making Eucharistic bread. Additives such as gums or pectin are not permitted. Special ‘low-gluten’ hosts, made from wheat starch, have been approved for individuals with coeliac disease and fall within the gluten-free labelling threshold. While the Coeliac Society classifies these hosts as gluten-free for differentiation from ordinary hosts, the Church refers to them as low-gluten hosts. To avoid confusion, it may be preferable to describe them to your Parish Priest using this term. Other churches permit fully gluten-free communion wafers, made without wheat, and suppliers often provide both varieties to accommodate different needs.

Another option is to take the wine. For those who cannot tolerate even trace amounts of gluten, the Church permits receiving Communion under the species of wine alone. This practice is recognised as fully valid, as the Church teaches that Christ is wholly present under either species, but you must ensure that a separate chalice is used, to avoid cross-contamination, and that the ordinary host is not dipped or has a piece broken off into the wine. It may be useful to talk to your parish priest if you are diagnosed and discuss what option might lead to least difficulty and awkwardness for you.