A review of gastrointestinal physiology and the mechanisms underlying the health benefits of dietary fiber: Matching an effective fiber with specific patient needs

Abstract

A primary function of the gastrointestinal tract is to provide the body with water, electrolytes and nutrients. It accomplishes this by ingestion, mechanical shearing, mixing and movement of the food through the gastrointestinal tract (motor events), secretion of digestive juices (≈ 6 liters per day) for degradation of complex nutrients into simple nutrients (e.g. complex carbohydrate into simple sugars), absorption of simple nutrients, water and electrolytes, and bacterial degradation of residue in the large bowel / elimination of waste. The small bowel is ≈ 7 meters long with a mucosa that is studded with millions of villi (each with microvilli), giving the small bowel a surface area approximately the size of a tennis court. Most digestion/absorption normally occurs early in the proximal small bowel. Non-digested/absorbed chyme arrives as a liquid in the cecum, and, under normal conditions, is gradually dehydrated as it slowly passes through the large bowel until it is eliminated as formed stool. Different types of dietary fiber can exert specific mechanical effects on each of these gastrointestinal functions, which can lead to local benefits (e.g. relief of constipation) as well as systemic benefits (e.g. cholesterol lowering, improved glycemic control). Three characteristics of fiber (solubility, viscosity and fermentation) are predictive of the health benefits of different fiber types. Based on these three characteristics, dietary fiber can be divided into four clinically meaningful designations that support recommendations to address the specific healthcare needs of each patient. This review will describe the four clinically meaningful designations for dietary fiber, and provide a summary of the clinical studies that support the health benefits for each.

Dietary fiber and prebiotics and the gastrointestinal microbiota

“…With regard to origin, plant-based fibers can be separated into fibers derived from cereals and grains, fruits, vegetables, nuts, and legumes. However, it is important to note that the fibers present in different types of plants will also have variable chemical compositions, as well as physicochemical properties 26-28 . For example, bananas contain resistant starch and inulin-type fructans, while apples are a source of pectin.…”

“…Fibers that are highly fermentable while also possessing high solubility and viscosity include β-glucan and pectins 27 . These fibers are naturally found in the diet in whole grains such as oats and barley (β-glucan) and fruits such as apples (pectin) 26,28 . Slowed glucose absorption and binding of bile acids—the mechanisms underlying the physiological benefits of psyllium, β-glucans, and pectin —are also purported to impact the gastrointestinal microbiota.…”

Evidence-Based Approach to Fiber Supplements and Clinically Meaningful Health Benefits, Part 1

“… 3 , 11 For soluble fibers, viscosity refers to the ability of some fibers to “thicken” when hydrated, in a concentration-dependent manner (Figure 1 ). 3 , 11 – 15 Gel formation refers to the ability of a subset of soluble viscous fibers to form cross-links, resulting in a viscoelastic gel when hydrated (Figure 2 ). 3 , 11 – 13 …”

“… 3 , 11 – 15 Gel formation refers to the ability of a subset of soluble viscous fibers to form cross-links, resulting in a viscoelastic gel when hydrated (Figure 2 ). 3 , 11 – 13 …”

“…Fermentation refers to the rate and degree to which a dietary fiber, after resisting enzymatic digestion in the small bowel, can be degraded by gut bacteria, producing fermentation byproducts such as short chain fatty acids and gas. 3 Most fibers are not exclusively soluble or insoluble, so for the purposes of this review, the predominant characteristic will be discussed (eg, a fiber that is 70% soluble will be considered a soluble fiber). Using the 4 fiber characteristics described above, fiber supplements can be divided into 4 clinically meaningful categories:…”