Fiber, soluble and insoluble: Fiber (the portion of plants that cannot be digested by the human digestive tract) is classified as soluble and insoluble.Oats, beans, dried peas, and legumes are major sources of soluble fiber whereas wheat bran, whole grain products, and vegetables are major sources of insoluble fiber.or example, soluble fiber delays the time of transit through the intestine whereas insoluble fiber speeds up intestinal transit. For another example, soluble fiber and may decrease the level of cholesterol in the blood whereas insoluble fiber has no effect on serum cholesterol.