system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If humans have to eat a balanced diet to be fully functional, how do animals like lions eat nothing but one type of meat and stay healthy?
gpt
Part of it is that lions get lots of exercise and don't eat very often. Have you ever seen a fat lion? Well, not counting ones in zoos.

Meat has a complete set of nutrients so they're all set except for one little thing: sugar. The brain needs glucose, a kind of simple sugar, to run. Most body tissues can be powered by anything but nerve cells absolutely need sugar. Don't worry, your body can store sugar for a long time and if you are an American this probably isn't a top concern.

Carnivores have a way to convert fat to sugar called gluconeogenesis and use it all the time; they have larger and more powerful livers, and better kidneys to handle all the nasty waste products (ketones etc). Some humans are better at this conversion than others. The reason why cat pee smells so awful is because cats are true carnivores and a lot of the protein they eat comes out as ammonia. In fact, if you let a cat eat a lot of sugar its body wouldn't know what to do with it all and the cat would get very sick.

There's also no fiber in meat, which is part of why cat poop is different from human poop.