But I wonder if we cannot and should not go further. Why stop at insects? Why be satisfied with their admittedly glowing report card of optimized efficiency when we could go all the way? The logical conclusion of this train of thought, as far as I can see, is clear: Let us engineer the perfect closed loop. Let us eat, and only eat, our own poop.
This modest pooposal is not as far-fetched as it might first sound. So let me try to nip any doubts in the bud. A common objection often involves nutritional composition. How would feces ever be nutritive enough to sustain the life from which it came? The key lies in processing. As it turns out, feces is quite a complex substance. In addition to the waste of digested foods, there is actually quite a bit of useful material to reclaim from it: undigested or unabsorbed proteins, fats, carbohydrates, micronutrients such as vitamins and minerals, and even some enzymes. There is fiber, water, and dead cells from the body in there—such as red blood cells and cells from the intestinal lining—not to mention many of the microorganisms living in the gastrointestinal tract.
Often around 50 percent of the original energy contained in the ingested food is still left in the fecal matter, so there's quite some recon potential. Using existing sewage treatment and fractionating technologies, it is possible, in principle, to obtain safe, pure, nutritive compounds that could be recombined in new ways to meet all of our nutritional needs. And these technologies will only become more sophisticated with time.
A common rebuttal then addresses recovery rate. Let's say that fecal matter does become our entire food source: Even if we assume that 50 percent of the energy of ingested food is not absorbed by the body, then each cycle of ingestion and reclamation will reduce the total feces-derived food supply by half—and that's not even taking into account whether there are still the right amounts of proteins and fats and other things.
There are at least a few strategies we can use to address this issue. The first and most viable solution involves microbes. As I mentioned before, our feces is already rich with gut bacteria and fungi, and the most nutritive of these are what we will be able to mass-culture. With further research, we will likely be able to tweak their yield and even induce their production of different nutritive compounds as necessary so that we will be able to generate the remaining 50 percent of our energy and nutrition from the feces itself.
Another possible solution involves existing animal livestock. Once there is no longer any reason to eat all these cows and their kin, we can instead release them and let them roam free, harvesting some of their feces as necessary for similar processing.
