Faith, hope, charity and gluten are to be regarded as the Church's most important virtues, according to a new memo from the Vatican.

A letter, circulated to bishops from Cardinal Robert Sarah, prefect of the Congregation for Divine Worship and the Sacraments, has advised gluten-free bread is not to be taken for Holy Communion in the Roman Catholic Church.

Like a cafe that doesn't allow for any menu alterations, only low-in-gluten options are approved for the celebration of the Lord's Supper.

"Hosts that are completely gluten-free are invalid matter for the celebration of the Eucharist. Low-gluten hosts [partially gluten-free] are valid matter, provided they contain a sufficient amount of gluten to obtain the confection of bread without the addition of foreign materials and without the use of procedures that would alter the nature of bread," the statement, sent on the request of Pope Francis, said.

Canon law states that bread used for Holy Communion must be made from wheat and water in order for transubstantiation to occur. A process which, according to Catholic teachings, is when bread and wine is transformed into the body and blood of Jesus.

However over the years the church has been forced to issue a number of reminders to the 1.2 billion strong Roman Catholic community that going GF is not OK.

The Congregation for the Doctrine of the Faith has now released five official updates since 1980 regarding the use of low-gluten breads for the Catholic liturgy. All decrees since then have stated that bread used for communion must contain enough gluten to attain the "confection of bread".

For about 20 years low-gluten wafers have been a best seller in Sydney's Church Stores.

"It's really an essential part of what we do now," manager Tim Wallace told Fairfax Media. "We spend a lot of time explaining to priests and parents, on the eve of their children's first Holy Communion, why low-gluten options are available and necessary."

In his eight-year tenure at the supply store, Mr Wallace said gluten-free options had grown in popularity. "But they have to be made by wheat. Those made of rice, corn or tapioca starch, like some were a few years ago, are no longer available as they were deemed 'invalid matter' by the Church," Mr Wallace said.

Carmelite nuns in Perth have been making low-gluten altar breads out of pure wheaten starch since 2013.

Sister Joanne told The Record her mixture contained less than five parts per million of gluten. The Australia New Zealand Food Standards Code defines "low gluten" as no more than 20 milligrams of gluten per 100 gram of the food.

Father Robert Cross, a diagnosed coeliac sufferer who is also the executive assistant to the Archbishop of Perth, is thankful for the offerings.

"I think a lot of priests think that being a coeliac is all in the mind, but it's not," he said. "It's basically an allergic reaction that happens in your intestines. A lot of priests don't consider it serious, but coeliacs do because they're the ones suffering."

Coeliac disease is an auto-immune disorder where gluten causes irreparable damage to the small bowel.

Qualified dietitian and nutritionist Dr Joanna McMillan said the Catholic Church needs to change in order to cater to parishioners who have been diagnosed.

"This isn't some fad diet, it's a very serious condition. The church needs to follow the times, back when those scriptures were written coeliac didn't exist, the science we have now didn't exist but they have a duty for suffers who wish to safely practice their religion," Dr McMillan said. "It's 2017, I don't understand why the body of Christ can't be gluten free."