Thomas Aquinas, Compendium of Theology

This enables us to answer the objections that some raise against the resurrection. For instance, they say that a cannibal may have eaten human flesh, and later, thus nourished, may beget a son, who eats the same kind of food. If what is eaten is changed into the substance of the eater's flesh, it seems impossible for both to rise in their full integrity, for the flesh of one has been changed into the flesh of the other. The difficulty apparently grows if semen is the product of surplus food, as the philosophers teach; for the semen whereby the son is begotten would then be derived from the flesh of another person. And so it seems impossible for a boy begotten from such seed to rise, if the men whose flesh the father and the son himself devoured rise intact.

But this state of affairs is not incompatible with a general resurrection. As was pointed out above, not all the material elements ever present in any man need be resumed when he rises; only so much matter is required as suffices to keep up the amount of quantity he ought to have. We also pointed out that if anyone is lacking in the matter required for perfect quantity, divine power will supply what is needed.

We should note, moreover, that the material elements existing in man's body are found to pertain to true human nature in various degrees. First and foremost, what is received from one's parents, is brought to perfection within the reality of the human species, as its purest element, by the parents' formative causality. Secondly, what is contributed by food, is necessary for the proper quantity of the body's members and lastly, since the introduction of a foreign substance always weakens a thing's energy, growth must eventually cease and the body must become old and decay, just as wine eventually becomes watery if water is mixed in with it.

Furthermore, certain superfluities are engendered in man's body from food. Some of these are required for special purposes, for instance, semen for reproduction and hair for covering and adornment. But other superfluities serve no useful end, and these are expelled through perspiration and other eliminating processes, or else are retained in the body, not without inconvenience to nature.

At the general resurrection all this will be adjusted in accord with divine providence. If the same matter existed in different men, it will rise in that one in whom it fulfilled the higher function. If it existed in two men in exactly the same way, it will rise in him who had it first; in the other, the lack will be made up by divine power. And so we can see that the flesh of a man that was devoured by another, will rise not in the cannibal, but in him to whom it belonged originally. But as regards the nutritive fluid present in it, it will rise in the son begotten of semen formed from that flesh. The rest of it will rise in the first man in this series, and God will supply what is wanting to each of the three.