What does insoluble fiber do in digestive system?
It prevents constipation and aids excretion.
Fiber is important in the digestive system for moving the food along the digestive tract. This is as it cannot be broken down, and so can push the waste along. It also absorbs a lot of water and so helps to keep stools soft, while simultaneously adding bulk to it.
A lack of fiber can lead to digestive issues and constipation - however, excess fiber can cause flatulence and bloating as bacteria start to break it down instead (and subsequently produce a lot of gas)!
The main sources of insoluble fiber are seeds, whole grains, fruit and vegetables (specifically their skins).
Hope this helps, let me know if I can do anything else:)