Dietary fibers are long-chain carbohydrates that are indigestible by the human digestive tract. The value of dietary fiber is that it provides bulk to the bolus moving through the digestive tract. There are two great advantages to this: by bulking up the bolus, eventually the stool, it's easier for the digestive system to move it through, and the bulkier stool also tends to retain moisture to make it easier to eliminate with less straining and abrasion.
There are two principal types of dietary fiber: soluble and insoluble. Insoluble fiber is simply bulk that changes little as it passes through the body. Soluble fiber, on the other hand, forms a soft gel in solution with water. Soluble fiber has been shown to be able to reduce blood cholesterol levels and slows the absorption of glucose from the intestine.
However, massive amounts of soluble fiber can cause diarrhea and irritable bowel syndrome.
Soluble fiber is found in some fruits (particularly orangess, also appless and bananas), oats, legumes (peas, soybeans, and other beans), other vegetables, such as broccoli and carrots, and a grain called psyllium. Legumes also typically contain shorter-chain carbohydrates that are indigestible by the human digestive tract but which are digested by bacteria in the small intestine, which is a cause of flatulence.