one of the best fiber you can get from nature is from the corn or sweet corn. Not only it is sweet and tasty to eat but it also make your bowel run smooth - it works as good as sweet potato and pomelo
You just need to boil the corn in hot water for about 30 to 45 minutes then you are ready to eat them. There is nothing to loose trying them it make your stomach full anyway - and if you have a processed vitamin stomach cleaner Super Colon Cleanse. Try it and see yourself the difference.
If you have constipation just drink a lot of water and eat sweet corn it helps for me and i used to keep some stocks of them at home -- and here "How to make your stomach absorb food better" is the reason why you should boil them instead of cooking it in any other way