system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is in the foods that make you constipated? And what happens to the body when you’re constipated?
gpt
It's not so much what's in them but missing something that allows the stool to take water in. People talk about fiber making you "regular" and bulking up a stool but what it means in practicality is that the fiber absorbs water from the colon. Without the water the stools become harder, drier, but also stickier since the water the stool absorbs also lubricates it as it passes through the colon.

So if you're eating foods that contain nothing that's indigestible that can also absorb water you're liable to have harder stools that move slower through the colon and then you get constipation.