Why only wheat bread for the Eucharist?

I have previously read why the host can only be made of wheat, but I have completely forgotten the reason. Please refresh my memory.

Dear Scoonie,

The reason is quite simple. When Jesus gave us the Eucharist, He used grape wine and wheat bread. So this is what we use. We have the Eucharist by His initiative. It is not for anyone but Him to make any changes.

Fr. Vincent Serpa, O.P.

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