Evidence-Based Approach to Fiber Supplements and Clinically Meaningful Health Benefits, Part 1: What to Look for and How to Recommend an Effective Fiber Therapy

Abstract

Dietary fiber that is intrinsic and intact in fiber-rich foods (eg, fruits, vegetables, legumes, whole grains) is widely recognized to have beneficial effects on health when consumed at recommended levels (25 g/d for adult women, 38 g/d for adult men). Most (90%) of the US population does not consume this level of dietary fiber, averaging only 15 g/d. In an attempt to bridge this "fiber gap," many consumers are turning to fiber supplements, which are typically isolated from a single source. Fiber supplements cannot be presumed to provide the health benefits that are associated with dietary fiber from whole foods. Of the fiber supplements on the market today, only a minority possess the physical characteristics that underlie the mechanisms driving clinically meaningful health benefits. The first part (current issue) of this 2-part series will focus on the 4 main characteristics of fiber supplements that drive clinical efficacy (solubility, degree/rate of fermentation, viscosity, and gel formation), the 4 clinically meaningful designations that identify which health benefits are associated with specific fibers, and the gel-dependent mechanisms in the small bowel that drive specific health benefits (eg, cholesterol lowering, improved glycemic control). The second part (next issue) of this 2-part series will focus on the effects of fiber supplements in the large bowel, including the 2 mechanisms by which fiber prevents/relieves constipation (insoluble mechanical irritant and soluble gel-dependent water-holding capacity), the gel-dependent mechanism for attenuating diarrhea and normalizing stool form in irritable bowel syndrome, and the combined large bowel/small bowel fiber effects for weight loss/maintenance. The second part will also discuss how processing for marketed products can attenuate efficacy, why fiber supplements can cause gastrointestinal symptoms, and how to avoid symptoms for better long-term compliance.

FIGURE 1.

Linear versus branched polymers. Drawings representing linear and branched polysaccharides. The volume “swept out” by a fully extended linear fiber is much greater than a fiber with an equal number of sugar units (same molecular weight) but with a “bush-like,” highly branched configuration. Because the volume occupied by a polymer molecule is a function of the radius cubed, even small increases in effective hydrodynamic size can translate into a large increase in viscosity for linear fibers.

FIGURE 2.

Viscous and gel-forming linear polymers. Drawings representing viscous linear polymers (top) and gel-forming linear polymers (bottom). Long-chain linear polymers orient parallel to adjacent fibers and increase viscosity in a concentration-dependent manner. Some long-chain linear polymers also can form cross-links that create a gel in a concentration-dependent manner (behave as a viscoelastic solid). Gel formation is an important driver of several metabolic health benefits for dietary fiber supplements, including cholesterol lowering, improved glycemic control, satiety, weight control, and stool normalization (soften hard stool in constipation and firm loose/liquid stool in diarrhea). (Drawings recreated with permission from John D. Keller, Jr, Keller Konsulting LLC, Freehold, NJ.)

FIGURE 3.

Absorption of nutrients in the small bowel is delayed by viscous fiber. Diagrams of nutrient absorption in the small bowel. Nutrients normally absorb very early in the proximal small bowel (A). Introduction of a viscous, gel-forming fiber (eg, guar gum, psyllium, high-molecular-weight β-glucan) can delay nutrient absorption to more distal regions of the small bowel (B). Rapid nutrient absorption (C: grey line, corresponds with A) is reflected by the higher peak concentration of blood glucose followed by a transient hypoglycemic trough below baseline. With the introduction of a viscous, gel-forming soluble fiber, the delay in nutrient absorption (C: black line, corresponds with B) results in an attenuation of glucose excursions: lower peak concentration of blood glucose, and attenuated hypoglycemic trough. (Drawings recreated with permission from Thomas Wolever, PhD, University of Toronto.)