Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber

Abstract

Enduring misconceptions about the physical effects of fiber in the gut have led to misunderstandings about the health benefits attributable to insoluble and soluble fiber. This review will focus on isolated functional fibers (eg, fiber supplements) whose effects on clinical outcomes have been readily assessed in well-controlled clinical studies. This review will also focus on three health benefits (cholesterol lowering, improved glycemic control, and normalizing stool form [constipation and diarrhea]) for which reproducible evidence of clinical efficacy has been published. In the small bowel, clinically meaningful health benefits (eg, cholesterol lowering and improved glycemic control) are highly correlated with the viscosity of soluble fibers: high viscosity fibers (eg, gel-forming fibers such as b-glucan, psyllium, and raw guar gum) exhibit a significant effect on cholesterol lowering and improved glycemic control, whereas nonviscous soluble fibers (eg, inulin, fructooligosaccharides, and wheat dextrin) and insoluble fibers (eg, wheat bran) do not provide these viscosity-dependent health benefits. In the large bowel, there are only two mechanisms that drive a laxative effect: large/coarse insoluble fiber particles (eg, wheat bran) mechanically irritate the gut mucosa stimulating water and mucous secretion, and the high water-holding capacity of gel-forming soluble fiber (eg, psyllium) resists dehydration. Both mechanisms require that the fiber resist fermentation and remain relatively intact throughout the large bowel (ie, the fiber must be present in stool), and both mechanisms lead to increased stool water content, resulting in bulky/soft/easy-to-pass stools. Soluble fermentable fibers (eg, inulin, fructooligosaccharide, and wheat dextrin) do not provide a laxative effect, and some fibers can be constipating (eg, wheat dextrin and fine/smooth insoluble wheat bran particles). When making recommendations for a fiber supplement, it is essential to recognize which fibers possess the physical characteristics required to provide a beneficial health effect, and which fiber supplements are supported by reproducible, rigorous evidence of one or more clinically meaningful health benefits.

Structural, antioxidant and adsorption properties of dietary fiber from foxtail millet (Setaria italica) bran

“…These results also confirmed that SDF was more viscous and adsorbable. The viscosity of SDF was related to many functional properties, such as lowering serum cholesterol and improving glycemic control and intestinal fermentation …”

Cactus stem (Opuntia ficus‐indica Mill): anatomy, physiology and chemical composition with emphasis on its biofunctional properties

“…Mucilage showed 28% and 8% more water‐ and oil‐holding capacities than pectin, respectively. Comparing mucilage and pectin behavior, it was shown that mucilage consumption causes clinically meaningful health benefits as cholesterol lowering, improving glycemic control, effects highly correlated with the viscosity of soluble fibers; increasing with water‐holding capacity …”

The nutritional and health attributes of kiwifruit: a review

“…In the hind-gut, the physiological benefits of fibre are believed to arise from the products of bacterial fermentation of the soluble fibre, and from the physicochemical properties of any fibre that remains unfermented [59, 60]. Among the most important physicochemical properties of kiwifruit fibres are the hydration properties, which include water retention, capacity and swelling, viscosity (which requires solubility), and properties that depend on the size, shape and porosity of undigested particles.…”

“…Among the most important physicochemical properties of kiwifruit fibres are the hydration properties, which include water retention, capacity and swelling, viscosity (which requires solubility), and properties that depend on the size, shape and porosity of undigested particles. Water retention is physiologically relevant because it influences transit time, faecal bulk, stool consistency and other functional benefits [60]. The high swelling and water retention of kiwifruit fibre in comparison with other forms of dietary fibre such as wheat bran, commercial preparations of sugar beet fibre and apple fibre, accentuate the value of consuming kiwifruit as a natural whole product that has had minimal processing.…”