Mechanisms of Dietary Fiber
Bulking - Some fibers (i.e., wheat bran, cellulose and psyllium) may help reduce constipation and reduce the risk of colon disease because they absorb water, which increases bulking and promotes regularity. Different types of dietary fiber contribute a wide range of bulking effects, with wheat bran (cellulose) and psyllium contributing 4-5 grams of bulking effects per gram of dietary fiber in the food.
Viscosity - Viscous fibers (i.e., beta-glucan and psyllium) reduce the absorption of cholesterol and other nutrients because they thicken the contents of the intestinal tract and slow down the migration of nutrients to the intestinal walls. Health benefits related to viscosity include lower blood cholesterol levels and reduced glycemic response (slowing down how quickly blood sugar levels rise after eating). The viscosity of the fiber determines its effectiveness in reducing absorption.
Fermentation - The colon is heavily colonized with bacteria, and some types of dietary fibers play an important role in ‘feeding’ the microflora. The colonic bacteria produce, among other things, short chain fatty acids (SCFA), which can play an important role in maintaining the integrity of the colonic cells as well as triggering a cascade of additional beneficial effects. Some fibers stimulate the growth of specific beneficial bacteria (microbes) within the large intestines – i.e., bifidobacteria and lactobacillus have been recognized as a beneficial organisms which are increased with dietary consumption of inulin, fructooligosaccharides and galactooligosaccharides. The bacteria consume the fiber in a process known as fermentation, and produce a range of short-chain fatty acids (SCFAs), primarily acetate, propionate and butyrate. Fermentable fibers contribute moderate bulking benefits through increasing the microbial mass; inulin, resistant starch, pectin and legumes each contribute 1-2 grams of bulking effect per gram of dietary fiber.
The SCFAs effectively lower the pH of the intestinal tract, which makes minerals such as calcium and magnesium more soluble and thus more absorbable. The SCFAs also contribute benefits because they are used as energy by the colon cells as well as absorbed into bloodstream and used as energy within the body. Finally, emerging research is demonstrating that specific SCFAs also trigger important biochemical pathways linked to a wide range of health benefits. They stimulate peristalsis, and help to promote regularity. They increase the production of hormones beneficial to satiety and insulin sensitivity (GLP-1, PYY, adiponectin, etc.), and they assist in maintaining a healthy immune system.
Different dietary fibers can be fermented to different degrees. For instance, oligosaccharides and resistant starch are fully fermented while polydextrose and resistant maltodextrin are partially fermented. Different dietary fibers also produce different ratios of SCFAs, which also leads to different health effects.