The answer to your question is while there is a limit to genius, there is no limit to stupidity. All plant foods (like watermelon) have plenty of fiber unless man took it out. No animal foods have it. You have soluble and insoluble fiber. The former slows down transit time and absorbs bad things. The latter speeds up transit time. Then there are differnt types that work differently like cellulose, hemi-cellulose, pectins, gums etc.
The fiber in wheat bran is like the worst fiber and can irritate your intestines. For more on fats that help you lose weight according to recent studies and more about the healthiest foods to eat and why see this site on Losing Weight. Carrots are considered the best food (not counting herbs like slippery elm bark and cats claw) for the colon. It is very good at healing colon problems. Cats claw is a very powerful herb for helping colon problems. Fruits and vegetables are healthier than whole grains. The heaviest person alive weighs over 1,200 pounds. The largest tumor removed was over 300 pounds.