Bread used to celebrate the Eucharist during Roman Catholic Mass must not be gluten-free - although it may be made from genetically modified organisms, the Vatican has reminded its bishops.
In a letter, Cardinal Robert Sarah said the bread could be low-gluten.
But he said there must be enough protein in the wheat to make it without additives.
The cardinal said the reminder was needed because the bread was now sold in supermarkets and on the internet.
Roman Catholics believe bread and wine served at the Eucharist are converted into the body and blood of Christ through a process known as transubstantiation.
The letter reiterated advice first given in 2004.
The wine used must also be "natural, from the fruit of the grape, pure and incorrupt, not mixed with other substances", said Cardinal Robert Sarah of the Vatican's Congregation for Divine Worship and the Discipline of the Sacraments.
The ruling was issued at the request of Pope Francis, the letter said.
There are about 1.2 billion Roman Catholics around the world.
Correction 24 July 2017: This story has been revised to make clear that the letter reiterates advice previously given in 2004.