So Should You Fiber is mostly complex carbohydrates. The building blocks are sugar molecules linked in a way that our digestive enzymes don't break them apart.
Fiber increases dietary bulk, making chewing more work, and giving a feeling of fullness.
A classic experiment in the fiber field was done more than 20 years ago and is still one of the best examples of its benefit. If you squeeze two apples, you get a glass of juice that you can drink in 30 seconds and you may feel hungry half an hour later. If you cook the two apples to make apple sauce, you can eat this in a few minutes and you will feel hungry one or two hours later. But if you eat two apples, it will take you 20-30 minutes and you won't be hungry for several hours. The bulking action of fiber improves intestinal function by feeding the bacteria which live in your colon, holding water like a sponge, and speeding up the passage of intestinal contents.