According to chapter 4; page 95-100 of Whitney|Rofles “Understanding Nutrition” 13 edition
All starchy foods come from plants. Grains are the richest food source of starch, providing much of the food energy for people all over the world—rice in Asia; wheat in Canada, the United States, and Europe; corn in much of Central and South America; and millet, rye, barley, and oats elsewhere. Legumes and tubers are also important sources of starch.
Dietary fivers are the structural parts of plants and thus are found in all plant-derived foods– vegetables, fruits, whole grains, and legumes. Most dietary fibers are polysaccharides. As mentioned earlier, starches are also polysaccharides, but dietary fivers differ from starches in that the bonds between their monosaccharides cannot be broken down by digestive enzymes in the body. For this reason, dietary fibers are often described as nonstarch polysaccharides.
Even though most foods contain a variety of fibers, researchers often sort dietary fibers into two groups according to their solubility. Such distinctions help to explain their actions in the body.
Some dietary fibers dissolve in water (soluble fibers), form gels (viscous), and are easily digested by bacteria in the colon (fermentable).** Commonly found in oats, barley, legumes, and citrus fruits, soluble fibers are most often associated with protecting against heart disease and diabetes by lowering blood cholesterol and glucose levels, respectively.
Other fibers do not dissolve in water (insoluble fibers), do not form gels (non-viscous), and are less readily fermented. Found mostly in whole grains (bran) and vegetables, insoluble fibers promote bowel movements, alleviate constipation, and prevent diverticular disease.
As mentioned, dietary fibers occur naturally in plants. When these fibers have been extracted from plants or are manufactured and then added to foods or used in supplements, they are called functional fibers–if they have beneficial health effects. Cellulose in cereals, for example, is a dietary fiber, but when consumed as a supplement to alleviate constipation, cellulose is considered a functional fiber.
A few starches are classified as dietary fibers. Known as resistant starches, these starches escape digestion and absorption in the small intestine. Starch may resist digestion for several reasons, including the body’s efficiency in digesting starches and the foods physical properties. Resistant starch is common in whole or partially milled grains, legumes, and just ripened bananas. Cooked potatoes, pasta, and rice that have been chilled also contain resistant starch. Similar to insoluble fibers, resistant starch may support a healthy colon.