Sustainability isn’t about absolute energy usage or energy usage per capita, or anything like that. It’s about ensuring that the activity at it’s current level of energy consumption can continue. So if it takes a 1000 acres of land to have a genetically diverse herd of animals that can feed a village of a given size forever, then the fact that 1000 times more calories in the form of anything else could be used on that land instead doesn’t mean the herd of animals are unsustainable.
Diseases are a non-sequitur for sustainability discussions as is “wasted” energy. Efficiency is nice, but not necessary for sustainability.