Intestinal Health Through Dietary Fiber, Prebiotics, and Probiotics

The intestine, the organ through which all nutraceutical (bioactive) compounds pass and are absorbed, can be an organ of disease if not properly nourished. Dietary fiber, prebiotics, and probiotics are nutrients and/or nutraceuticals that can promote a healthier intestine. To what degree this statement can be justified requires additional research and is the focus of my research. 

Dietary fiber was declared a nutrient by the Nutrition Labeling and Education Act of 1993. This designation was necessary to place it on the Nutrition Facts panel of packaged foods. A Daily Value (DV) of 25 g is used as the denominator in calculating the percent a serving provides. There is a “Dietary Fiber Hypothesis” that suggests that this nutrient helps prevent many diseases prevalent in modern and affluent societies. The converse of this hypothesis is that consumption of processed foods that remove dietary fiber is a contributing factor to these diseases. 

One of the primary physiological values of dietary fiber in our diets is as a bulking agent, which contributes to normal bowel function. A few viscous sources of dietary fiber will help lower blood cholesterol levels, e.g., beta-glucan and psyllium. The DV for dietary fiber used today is based on producing a normal amount of solid waste at adequate intervals. At present, there are no recommended intakes for dietary fiber by age group, as there are for other nutrients. However, the Food and Nutrition Board (FNB) is considering levels to recommend, and these values might be forthcoming this year. 

Dietary fiber can be described as a nutrient because most sources are fermented to varying degrees in the large intestine by our bacteria; it provides energy, but mainly for bacterial growth. A definition of dietary fiber must be established before appropriate intake levels can be recommended. Basically, we still have the question of which ingredients in foods and our diets (including supplements) constitute dietary fiber. Discussions on this topic were conducted in 1999–2000 by the American Association of Cereal Chemists and are being continued by FNB. 

FNB’s current suggested definitions of dietary fiber, although not final, are significant: “Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants” and “Added Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans.” 

Aside from its laxative effects, possibly the most dynamic function of dietary fiber is its fermentation, which causes a myriad of biochemical, physiological, and microbiological changes in the large intestine. The potential impact of fermentation in the intestine is helping us better understand that the entire intestine can contribute to better health if properly nourished. This belief and a growing understanding of dietary fiber fermentation helped coin the term “prebiotic.” The irony of prebiotics is that many of these nondigestible carbohydrates may be defined not as “dietary fiber” but as “added fiber” because they are not recovered by existing AOAC methods for total dietary fiber analysis. 

Prebiotics promote the growth and redistribution of intestinal bacteria. The results of these changes are generally described as creating a healthy intestinal environment and thus reducing the incidence of intestinal diseases and disorders. This is a topic worthy of more detailed information. Increased intestinal fermentation will notably cause a reduction in pH brought about by higher levels of acids, most notably short-chain fatty acids. Small changes in intestinal pH can significantly influence the distribution of gut microflora and reduce the levels of harmful metabolites, e.g., ammonia and hydrogen sulfide. Collectively, these changes brought about by the increased fermentation of dietary fiber and prebiotics are thought to justify the statement “promotes a healthy intestine.” 

That a change to more-acid-tolerant bacteria, e.g., Lactobacillus and Bifidobacteria, in the colon can promote a healthy intestine is not a new idea. The use of bacteria originally isolated from humans and properly processed for stability and safety has been studied for at least the past two decades. The concept of using “probiotics” appears to show great promise in treating various intestinal disorders and diarrhea. 

Dietary fiber, prebiotics, and probiotics are sterling examples of the “functional foods” concept and are the theme of the IFT Basic Symposium, “Dietary Fiber, Prebiotics, Probiotics and their Role in Intestinal Health” (see www.am-fe.ift.org/t1p5.php) to be held on June 14–15 in Anaheim, Calif., prior to the IFT Annual Meeting. Many questions remain about the applications of these functional food ingredients for normal intestinal and bodily functioning and health, and I encourage you to attend and submit questions to me ([email protected]) prior to the symposium.

by DENNIS T. GORDON
Professor
Dept. of Cereal Science
North Dakota State University