I've been wondering if priests throughout the globe, due to a screwed-up understanding of how agriculture works (due to distancing themselves from menial labor), may have led their faithful subjects to sacrificing humans to enrich barren soil, and disguised it as the will of gods or whatnot. Could this have partially explained the correlation between the long lasting droughts and increase in human sacrifices in Aztec-controlled central America (besides the sociopolitical morale games started with the Flower Wars)?
As a point of interest, the idea that Aztec sacrifice was fueled by nutritional deficiencies has long since been debunked. While human sacrifice did have a disproportionate status in regards to past, and even contemporary, Mesoamerican societies, it was not based on nutritional needs. Captive taking was not nearly so common as popular misconceptions have it, with a single warrior taking 4 captives a remarkable event worthy of promotion to the highest levels.
The idea that Aztec cannibalism was based in protein deficiency was proposed by Harner (1977) following in the cultural materialism footsteps of Harris. The hypothesis, however, was almost immediately refuted by Ortiz de Montellano (1978), who rightly noted that the numbers of individuals sacrificed was far too low contribute a substantial level of nutrients to the Aztec diet. He also -- quite correctly -- noted that Harner's analysis of nutritional sources was predicated on a Eurocentric concepty, excluding foodstuffs known to be commonly consumed by the Aztecs, such as salamanders, Spirulina spp., and a variety of insects.
The other point to make is that humans, given their long growth period and nutritional needs, are highly inefficient sources of nutrients, whether indirectly or directly. This is, in fact, a central tenet of Harris' theory as to why cattle became sacred in India; they were more valuable for their labor and dung then for the meat. If you really want to stretch the analogy, you should know that "night soil" (i.e., human excrement) was widely used as a fertilizer, with the Aztecs having organized collections of the material.