18 Again the Lord Himself revealing this mystery of His birth, speaks thus: I am the living bread Who have descended from Heaven: if any one shall eat of My bread he shall live for ever : calling Himself the Bread since He is the origin of His own body. Further, that it may not be thought the Word left His own virtue and nature for the flesh, He says again that it is His bread; since He is the bread which descends from heaven, His body cannot be regarded as sprung from human conception, because it is shown to be from heaven. And His language concerning His bread is an assertion that the Word took a body, for He adds, Unless you eat the flesh of the Son of Man and drink His blood, you have not life in you. Hence, inasmuch as the Being Who is Son of Man descended also as bread from heaven, by the 'Bread descending from heaven' and by the 'Flesh and Blood of the Son of Man' must be understood His assumption of the flesh, conceived by the Holy Ghost, and born of the Virgin.
Source: On the Trinity (New Advent)