The Gospel of John has no “Institution Narrative” directly parallel to the scene at the Last Supper in the Synoptics, where Jesus establishes a commemorative meal at which his followers will eat his flesh and drink his blood. Instead, John develops an institution narrative out of one of the few stories he shares with the Synoptics, namely, the Feeding of the Five Thousand.
More specifically, John likely alters the Synoptic account of this story to associate it with a Eucharistic liturgy: he exchanges the Synoptic eulogeso for eucharisteo (so moving it toward the “institution narratives,” cf. Jn 6:11, Lk. 22:17, or 1 Cor. 11:24) and airo for sunago (a term that, at least by the early 2nd century, had acquired a technical relation to eucharistic gatherings, cf. Jn 6:12, Didache 9:4, Ignatius Ep.Poly. 4:2, 1 Clement 34:7). Further, John sets the miracle “near Passover” (6:4), with its immediate association to the Last Supper and Jesus’ Passion. Finally, John makes this act the occasion for a debate about eating Jesus’ flesh and his blood: Ignatius, in the Ep.Phil. 4 employs the paired “sarx/haima” to describe the Eucharist (see also Ignatius’s Ep.Smyr. 6:2); and the near parallel of soma and haima of course occurs in the Institution narratives. I would suggest that these parallels account for the paired appearance of “flesh and blood” more fully than the does the manna tradition alone (which would explain Jesus “body/bread” but not his “flesh/blood”), which is also clearly be alluded to in this story. These textual indicators strongly suggest that we should read this passage as a commentary of sorts on Eucharistic practice, one that, in its use of the manna traditions, anamnetically recalls the imperfect hints of the Church’s practice (and contemporary failures) in the wilderness wanderings, and that, in its use of Isaiah 55, proleptically points to the eschatological feast of heaven.
Clement of Alexandria (as quoted by Eusebius of Caesarea, Hist. Eccl. 6.14.5-7) remarked about the Gospel of John: “Last of all John, perceiving that the bodily facts had been made plain in the gospel, being urged by his friends, and inspired by the Holy Spirit, composed a spiritual gospel.” That strikes me as an excellent explanation of John’s use of the Institution narrative to interpret the Feeding of the Five Thousand.