system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does your body discard excess protein, but will cling to every last bit of fat that it consumes?

If you eat more than a certain amount of protein per hour, your body rejects whatever it cannot absorb. Why is there no "threshold" for the amount of fat you can consume in an hour? You seem to just hold on to every bit of fat you eat. Can someone set me straight on this?
gpt
You don't get fat from eating fat.  You get fat from eating more calories than you expend (plus a dozen other factors).  Fat is pretty calorie dense, so it does help power the body and, of course, add fat to it.  But it isn't that you eat fat and it passes unchanged into your hips.

The body adds fat every chance it gets and only begrudgingly sheds it because we *used* to live in a world where you could never be sure of eating regularly, so you packed on fat in times of plenty so you'd have an extra source of energy when times were bad.