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There are several types of dietary fibers.8 Nutritionists classify the types of dietary fibers that can be converted into butyrate as either soluble or insoluble. Examples of soluble dietary fiber include the beta-glucans (including arabinogalactans and lactoferrin), gums, mucilages, oligosaccharides and pectins. Soluble dietary fiber comprises 10% to 20% of the total dietary fiber content of such foods as fruit, okra, beans, turnips, oats, parsnips, sea weeds, and prunes. Examples of insoluble dietary fiber include the celluloses and lignins. They are found in more fibrous foods. Because it exhibits variable characteristics, a type of dietary fiber called hemicellulose is classified as both soluble and insoluble.
The soluble dietary fibers are the most readily “fermentable” in the human colon, meaning that it is easier for the normal bacterial flora to convert them into other nutrients, including butyrate. Fermentable dietary fiber serves as a “pre-biotic” in that it promotes the growth and viability of beneficial species of gut bacteria. However, dietary fiber has benefits for humans beyond the fermentability by colonic bacteria. The results of a study published recently in the Journal of Nutrition show that different kinds of dietary fiber can bind to glucose and fats in the digestive tract, preventing their absorption into the blood, regardless of their “solubility.”9 Fiber thus can influence the maintenance of healthy blood sugar levels and cholesterol.
Another type of dietary fiber, non-fermentable dietary fiber, is found in such foods as oat hulls, methylcellulose and wood pulp cellulose. These food components are not processed by microbes to any appreciable extent. Instead, they function in the human colon to carry the fermenting bacteria along through the colon and rectum; decrease the absorption of glucose in the small intestine and increase the glucose content of stool; dilute pathogens and toxins in the digesta and stool; distend the colonic mucosa, stimulating peristaltic contractions and increasing the rate of movement of the digesta through the digestive tract (an increased “rate of passage” decreases the amount of time that colon cells are exposed to any toxins in the digesta); and inhibit the induction of inflammation in the colon by unhealthy organisms. By promoting stool bulk, this type of fiber maintains gut ecology and supports healthy colonic function.
Next Best Kept Secrets to Healthy Aging topic:
How Much Dietary Fiber Does Your Colon Need Us to Eat?
8. Kay RM. Dietary fiber. J Lipid Res 1982;23:221-242.
9. Dikeman CL, Murphy MR, Fahey GC Jr. Dietary fibers affect viscosity of solutions and simulated human gastric and small intestinal digesta. J Nutr 2006;136:913-919.