In Roman Catholicism, stercoranism (Latin, stercorarius, “belonging to dung”, from stercus, “dung”) is the heretical belief, or doctrine, that if the bread and wine of the Eucharist (Holy Communion, Lord's Supper, etc.) become the literal Body and Blood of Christ upon consecration, then they must subsequently be subject to the normal digestive processes after ingestion, eventually passing through the intestines and being excreted through defecation.
Transubstantiation — the teaching of the Catholic Church concerning the change of substance by which the bread and the wine offered in the sacrifice of the sacrament of the Eucharist during the Mass, become, in reality, the body and blood of Jesus Christ — has occasioned a large number of subordinate controversies over the centuries relating to just how this takes place. Early Church theologians, such as Origen (184– 253), were willing to allow that the consecrated elements of Christ’s body were digested and excreted in the manner of typical food, but the 9th century Carolingian theologian Paschasius Radbertus found this wholly unacceptable. He wrote an influential tract around 832 upholding the literal interpretation of Christ in the Eucharist, but repudiating the belief that the consecrated elements of the Eucharist were physically digested and excreted. Rejection of this belief, termed “stercoranism” at this time, became part of a general program of apologetics directed in favor of the general concept of transubstantiation in the time of the Frankish King Charles the Bald (reigned 840-877) and the following two centuries, and advanced by eminent theologians such as Ratramnus of Corbie (died c. 870) and Berengar of Tours (c. 999–1088). Berengar and Ratramnus accused their opponents, who believed in a “real” change of physical substance as opposed to a spiritual one, of stercoranism. There is little evidence, however, that anyone actually advanced the stercoranist concept formally.
The Church was eventually persuaded to explicitly reject stercoranism as a heresy and has held to this rejection up to the present day. The website Unam Sanctam Catholocam lays out the current official stance on how the Body of Christ is spared the indignity of defecation:
We receive the true Body of Christ into our own body. But at some point, we know not when, through the natural process of breaking down the host, the host is no longer discernible to the senses as bread and the presence of Christ is no longer there. Some posit this happens very early, shortly after the host is chewed and swallowed, since we are no longer sense perceptive of any particular piece of something inside our stomach; others say it happens later. Regardless of when we say it happens (and nobody knows precisely when), the fact is that the very process of consuming the sacred host sooner or later destroys the sacramentum tantum, and well before the act of defecation would occur. Therefore, even if it were true that some infinitesimally small particles of the host are ejected from the body during defecation, these are not recognizable to the senses as bread and hence are not a sacramentum tantum, for the sacramental sign was destroyed in the act of consuming.