Dietary fiber is a form of carbohydrate that humans consume through plant foods like fruit, vegetables, beans, and whole grains. Unlike other carbohydrates that are made up of chains of sugars, however, fiber is largely indigestible and therefore contributes almost no calories or nutrients to the diet. Instead, the effects of fiber on the digestive system are that it eases the passage of partially digested food through the gastrointestinal tract, improves the health and regularity of bowel movements, and improves the health of the colon itself. Fiber also helps to lower LDL or “bad” cholesterol levels by decreasing the amount of bile reabsorbed into the bloodstream through the intestinal wall, and slows the absorption of glucose through the walls of the digestive tract, which stabilizes blood sugar levels.
Of these effects of fiber on the digestive system, the most well known is that it contributes to colon health by moving partially digested food more rapidly through the intestines. Dietary fiber has two forms: soluble and insoluble fiber. Soluble fiber is the kind available in legumes, vegetables, fruits, and some whole grains. It dissolves in water and affects the absorption of certain nutrients through the intestinal wall. Insoluble fiber is the kind found in whole-wheat products, nuts, and many vegetables. This kind of fiber contributes bulk to the food that passes through the gastrointestinal tract and subsequently leaves the body as stool.
The effect of insoluble fiber on the digestive system is that it therefore improves its function. It contributes the weight and bulk to stool that encourages quicker passage through the colon, and it also eases bowel movements by softening the stool. Furthermore, the consumption of insoluble fiber has been linked to a lowered risk of such ailments of the colon as diverticulitis, hemorrhoids, and colon cancer, and it is said to diminish symptoms of irritable bowel syndrome.
There is a similarly beneficial effect of soluble fiber on the digestive system. This fiber type when mixed with water creates a viscous substance that delays the absorption of certain nutrients through the walls of the intestine and into the bloodstream. Glucose, for instance, the most basic form of carbohydrate that the body uses for energy, enters the bloodstream in this manner, elevating blood sugar levels. Consuming soluble fiber helps to slow this process so that glucose enters the blood more gradually and blood sugar levels do not spike, so it is recommended that diabetics consume plenty of soluble fiber-dense foods.
Additional effects of soluble fiber on the digestive system include its role in lowering blood cholesterol levels. It does so by slowing the reabsorption of bile from the liver through the walls of the colon, so that much of this bile leaves the body in stool. In order to manufacture more of the bile the body requires, the liver uses cholesterol, thereby decreasing the amount in the bloodstream, particularly of LDL or “bad” cholesterol.