Put a lion and a human on a diet of pure lean meat, and watch what happens.
The lion thrives. An obligate carnivore can pull around 70 percent of its energy straight from protein and feel magnificent doing it. Its liver runs the nitrogen-clearing machinery permanently flat out, built from the factory to mop up the ammonia that all that protein leaves behind.
Now you. Push much past 35 or 40 percent of your energy from protein and the wheels start to come off. Your liver can only turn so much of that toxic ammonia into urea before the excess backs up into your blood. Headaches first, then nausea, then weakness, and if you carry on, a genuinely grim way to go. The old explorers called it rabbit starvation. Endless lean rabbit, full belly, dead inside three weeks.
So here is what the lean-protein crowd never quite reckon with. We are predators walking around with a built-in protein ceiling. We are not lions. We cannot live on muscle meat alone, and we were never built to.
The way around that ceiling is fat. Fat carries the calories lean protein cannot safely deliver, which is precisely why our ancestors hunted the fattest animals they could find and went for the marrow, the organs and the back fat first. Fat is what kept them alive.
A lion is built to run on protein. You are built to run on fat. Mind the ceiling, and eat accordingly.