Back when the greenielefties were last predicting Global Cooling (which is indeed going to happen now, not then), they were also predicting overpopulation and global famine (instead population grew and global nutrition improved drastically). That was the time when they inspired that movie, Soylent Green, starring Edward G. Robinson. If you haven't seen it, here's the spoiler: Soylent Green is People! And that's how they feed the whole population every Tuesday.
Of course, as with all the greenieleftie scares, it does not compute. How can they possibly achieve a 1/14 energetic yield with this racket? Half a day's ration once every week -- you can't sustain that recycling unless you get 1/14th of the total energy you put in a man out of digesting him. Bloody unlikely. Considering a human can have an average of say 50kg of edible meat (order of magnitude), at 4 Cal/g, that's 2e5 Cal usable by eating a person. With a 2000 Cal/day diet (order of magnitude), that's just 100 day of feeding. Which is one hundredth of the lifespan of a 28-year old adult. While there's room for plenty of profit in such an operation, a simple calculation shows that anthropophagy can never sustainably be a statistically meaningful part of your average man's ordinary diet. Once again, he who refuses to do arithmetic is doomed to talk nonsense. (Exercise for the reader: look at the arithmetics of another greenieleftie theme of the movie, the disappearance of nature and forest.)
Of course, as the Toltecs and Aztecs demonstrated, in an age where physical strength matters, a minority can use anthropophagy on a large scale to gain a nutritional advantage over the surrounding underfed majority. Anthropophagy can certainly be fun, and I've read of old cannibals regretting the taste of roasted baby hands. Indeed if your army is too backwards to establish slavery, to what better use to put your vanquished enemies (at least the males)? Anthropophagy can also be a factor in political bonding; it was notably used by Mao's commissars as a test to distinguish between those ready to literally "eat the rich" and those to be eaten next or sent to the laogai. And anthropophagy has been known as an emergency technique to save the lives of quite a few people stranded in a group without access to other food. Or to damn the soul of an enemy. Finally I admit I cannot dismiss the retirement plan once offered to old women by Tierra del Fuego indians, as reported by Darwin. Goddess bless the greenieleftie mandated return to the simple natural life! But beware: done ritually and systematically, anthropophagy can be a vector of diseases. In any case, there has never been and can never be such a thing as anthropophagy as a common source of nutrients for the common man. The myth of anthropophagy as such only demonstrates the fears and obsessions of those who propagate it, and their first thought in times of direness. Speak of people with twisted minds. I know who I don't want to live next to when the shit hits the fan.
When people in the average bring more to civilization than they take away, there cannot be problem with overpopulation, or with funding education and retirement — people are indeed an asset. When people are all in all more consumptive than productive, then they are a liability indeed, surviving by spending away the capital accumulated by better men; their society is going down the drain and none of the proposed statist remedies can help, whether fostering reproduction or non-reproduction, immigration or emigration, early retirement or euthanasia. Actually, State intervention is usually the one cause that puts society on such a bad slope. And when society collapses, anthropophagy is a poor way to slow down this collapse if it does at all; actually, inasmuch as it would imply murder and/or deception (which as Heinlein noted it needn't necessarily, but which it does in the greenieleftie boogeymen) anthropophagy is rather a sign of the collapse accelerating than slowing down. As Julian Simon wrote, The Ultimate Resource is the creativity of free humans. And the Ultimate Waste is precisely the destruction of this freedom and creativity by ecologists and other Statists.
