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.