(Florentine Codex 1570)
Anthropologists have offered up many theories as to why the Aztecs engaged in cannibalism and sacrifice. Some of these theories are a bit extreme and others are more logical.
Ritual
The Aztecs believed that the world was very unstable and that their sacrifice to the gods every day was required to keep the universe stable. In relation to this Cannibalism, the Aztecs believed that the bodies of the victims became divine. It was said that those who consumed the flesh of a sacred person were consuming a sacred entity. "the flesh of all those who died in sacrifice was held truly to be consecrated and blessed. It was eaten with reverence, ritual and fastidiousness - as if it were something from heaven" (Duran 1971). This means that cannibalism isn't motivated by starvation but by a belief that this is a way to commune with the gods.
Along with this there was also the theory that the sacrificial victim offered to the gods by a warrior was a substitute for himself as a way to extinguish guilt. And because of the victim being his substitute sinner he would not partake in the eating of the divine sacrificed captive.
Protein Deficiency
The next theory offered up comes from Michael Harner, he believed that the Aztecs had a protein deficiency and consumed the human sacrifices as a way to get their daily dose of protein. This however is a bit of a radical idea because the combination of maize beans, squash and Aztec staple crops alone have the 9 essential amino acids you need in your diet. Another argument against this is in
Another explanation to disprove the protein deficiency theory, is the fact that the torsos of the sacrificed were fed to the carnivores in the royal zoo. And they were also given domesticated dogs thus competing with the people for a common source of protein.( Diaz del Castillo 1956)
Harner is very criticized in this hypothesis because of the ways he arrives at his conclusions. For example the way he arrived at his guess to the numbers of people sacrificed. He is less than a perfectionist when it comes to statistical methods. He arrived at his figure by multiplying the "thousands of temples" by "the estimated one to three thousand" sacrificed each year. (Arens 1979). Harner is less than qualified to offer such theories but he does at least try to explain the reasons offered for cannibalism.
Demographic
The Aztecs' population size is considered a big reason why they partook in cannibalism. If population in Tenochtitlan and the Aztec empire was already large and they could not support the people, why then did they continue to expand and acquired more people and land? If population was too large then why did the upper-class nobles still practice polygyny? And if the Aztecs were in fact starving then why did the greatest amounts of sacrifices and cannibalistic acts coincide with times of harvest and not periods of scarcity?
Harner also offers up the information that the lack of domesticated animals in relation to the population size implied a protein deficiency which led to "large scale cannibalism disguised as sacrifice" (Arens 1977). But this also hasn't much relevance to population because the Aztecs didn't have many domesticated animals besides the dog and turkey.
Population doesn't seem to be much of a reliable excuse for cannibalism, the only logical theory is the one offered by Duran. His reasoning of the Aztecs eating the sacrificed to get closer to the divine Gods makes the most sense.