Effects of Dietary Fiber on Digestive Enzyme Activity and Bile Acids in the Small Intestine

In humans and in a number of animal species consumption of high fiber diets has been associated with reductions in the digestibility and availability of protein, fats, and other nutrients such as minerals, vitamins, and carbohydrates. The ability of fiber to alter the rate of digestion and absorption in the gastrointestinal tract appears to be important in understanding its effects on metabolism, such as reducing plasma lipids and altering the glycemic response to a meal. The assimilation of nutrients from the diet requires the movement of digesta through the gastrointestinal tract, the enzymatic hydrolysis of complex compounds into simpler compounds which can be absorbed, the uptake of these compounds into the intestinal cell, and movement of nutrients from the intestinal cells to the portal circulation or lymph ducts. Dietary fibers influence these processes through several mechanisms which include altering the availability of bile acids and digestive enzyme activity, changing the characteristics of the intestinal contents where digestion occurs, altering the morphology of the small intestine so that structural changes are associated with functional changes in the gut, and causing adaptation in the synthesis of enzymes or compounds needed for nutrient absorption from the intestine. The objective of this paper is to consider the potential effects of dietary fiber on the rate of digestion in the small intestine. The effects of fiber on absorption from the intestine and subsequent metabolism of nutrients will be considered in other papers in this series. One of the first questions to consider is the effect of various sources of dietary fiber on the activity of digestive enzymes in vitro. Several studies have reported changes in the activity of pancreatic enzymes following incubation with purified and nonpurified fiber sources (1-9).