Chapter 52 Dietary Fiber
With the possible exception of ice age hunters, most primitive cultures consumed very large quantities of dietary fiber. Early humans obtained fiber principally in the form of soluble fiber from fruits, vegetables, and root vegetables. With the advent of grain cultivation, the human diet shifted more towards insoluble fiber, which has less beneficial metabolic effects, particularly in the context of a higher carbohydrate intake. Although the original Inuit diet of the far north was, calorically speaking, very high in animal fat and, to a lesser extent, animal protein, they supplemented their diet extensively with seasonal berries, roots, various plants, and seaweeds; thus, they consumed relatively large amounts of fiber. Further south, aboriginal peoples made extensive use of numerous plant foods that contributed to a large intake of dietary fiber.1 Around the world, there are few examples of indigenous peoples whose traditional diet did not provide very high quantities of fiber. Only with the advent of commercial food processing and refining has the human diet been so characteristically meager in dietary fiber.
Although we are experiencing a contemporary revival in medical interest surrounding fiber, references to dietary fiber have long existed in the medical literature in the writings of such historical figures as Hippocrates, the ninth century Persian physician Hakim, and the Kellogg brothers of the United States.2 However, our contemporary interest in fiber as physicians as well as the widespread understanding of fiber’s importance among the general public began with a zealous campaign instituted by the missionary surgeon, Denis Burkitt (also the discoverer of Burkitt’s lymphoma), beginning in the early 1970s. Over his 20 year career in Africa, Dr. Burkitt observed a dramatic difference in the incidence of many diseases, such as coronary heart disease, diabetes, diverticulosis, and gallstones among the local tribesmen. He noted that the largely agrarian Africans consumed very large amounts of dietary fiber, often more 100 g/day. He determined that as a result of this diet, their stool transit time was far more rapid than that of Westerners, and their stools were more frequent and bulky. On the basis of his research, he launched a worldwide campaign to raise awareness of the importance of dietary fiber among medical professionals and the general public. Without a doubt, Dr. Burkitt’s zeal was an important factor that catalyzed a veritable revolution in research surrounding dietary fiber.3 During Burkitt’s campaign, contemporaries such as Jenkins et al4 published the first studies on fiber-related concepts such as the glycemic index. Today, we have begun to recognize the complex mechanisms by which fiber exerts its many health effects beyond just its impact on laxation.
In the simplest definition, dietary fiber is a nonstarch polysaccharide in (mostly) plant food that is poorly digested by humans. Based on a recent government consensus report, fiber can exist as either dietary fiber (naturally occurring in food), or functional fiber (added during the processing or preparation of food).5 Fiber can be either insoluble or soluble in water. Insoluble fibers include cellulose, hemicellulose, and lignins, whereas soluble fibers include various gums, pectins, β-glucans, oligosaccharides, resistant dextrans, and resistant starches. Chitin and chitosan are indigestible amino-polysaccharides that are found in or are derived from the exoskeletons of arthropods such as crabs and lobster, as well as the cell walls of most fungi, and could functionally be regarded as fiber, although they are not recognized as fiber by most regulatory authorities. The distinction between soluble and insoluble fibers is due to the chemical properties of the fiber, resulting in its tendency to absorb water. Various physicochemical properties of fiber (viscosity, water holding capacity, cation exchange capacity, adsorption of organic materials, and fermentability) are now thought to be fundamental to its beneficial physiologic effects. The Institute of Medicine has proposed a new definition of dietary fiber that will encompass both its physical characteristics and its physiologic effects in humans.6 A fiber’s viscosity, its water holding capacity, and its fermentability are the chief determinates of fiber’s physiologic effects.
The regulatory classification of fiber varies considerably in different countries. In the United States, dietary fiber is defined based upon analytic methods rather than its physical or physiologic characteristics.
The typical Western diet is largely devoid of dietary fiber, being composed principally of refined grains and other highly digestible sources of starch, sugar, various fats, and animal products. Children in particular are commonly fiber deficient, with daily intakes often under 5 g and with nearly no soluble fiber. Likewise, many adults in Western society consume 5 to 10 g of fiber daily as opposed to the 35 to 50 g that is considered desirable or at least minimal for optimal health. Moreover, because most fiber in the Western diet is derived from cereal grains, the intake of viscous soluble fiber is typically highly inadequate.
A whole foods based diet focusing on a large intake of vegetables and fruits as well as unrefined whole grains and legumes should be the foundation of a healthy lifestyle. With a whole foods based diet, it is certain that dietary fiber intake will substantially increase.7 Unfortunately, only a minority of the population, particularly children, are likely to adopt a largely whole foods diet anytime in the near future. Because of this, efforts are underway to establish effective means to fortify the Western diet with dietary fiber through the use of various fibers as food additives or ingredients as well as the use of readily accepted fiber supplements.
There is compelling epidemiologic and experimental data associating numerous disorders, at least in part, to a lack of dietary fiber. Ischemic heart disease, stroke, atherosclerosis, type 2 diabetes, overweightness and obesity, insulin resistance, hypertension, dyslipidemia, as well as gastrointestinal disorders such as diverticulosis, irritable bowel disease, colon cancer, and cholelithiasis, are just a few of the many conditions that seem to be influenced by the adequacy of dietary fiber intake.8
Numerous studies demonstrated that certain fibers decrease the glycemic response to food, promote satiety, lower serum cholesterol, promote bowel regularity, positively influence colonic microflora, provide nutritional substrates for colonic mucosal cells, improve mucosal barrier function, as well as sequester and eliminate toxic and carcinogenic dietary and environmental compounds. These and other effects constantly interplay to increase or decrease the development of a wide range of health conditions.
Viscous dietary fibers are now clearly correlated with moderation in blood glucose and cholesterol concentrations, prolonged gastric emptying, and slower transit time through the small intestine.9 Among viscous fibers, fermentability is mostly associated with large bowel function. Rapidly fermented fiber sources provide substrates for short-chain fatty acid (SCFA) production by microflora in the large bowel, whereas slowly or incompletely fermented fiber sources improve bowel health by promoting laxation, reducing colonic transit time, and increasing stool weight.10
Dietary fiber exerts its effects through an interaction between the physical properties it imparts to foods accompanied by a complex array of microbiological, biochemical, and neurohormonal influences. Dietary fiber has a strong influence on the palatability of food, thus influencing ingestive behavior. In the stomach, fiber affects the volume and viscosity of food, which has a highly significant effect on satiety. This “volumetric” effect on food promotes a sense of fullness and a delay in gastric emptying, which tends to naturally result in a decrease in caloric intake. Various fibers differ dramatically in their ability to impart volume and viscosity to foods. Becoming familiar with the use of highly viscous soluble fibers enables health care providers to teach patients to choose an eating strategy that promotes and maintains satiety while consuming a diet that is lower in caloric density.
Viscosity as related to dietary fiber refers to the ability of some polysaccharides to thicken or form gels when mixed with fluids, resulting from physical entanglements and hydrophilic interactions among the polysaccharide constituents within the fluid or solution.11 Gums, pectins, and β-glucans make up the majority of viscous dietary fibers. The viscosity that a fiber imparts to the gastric and small intestinal contents is directly correlated with the ability of the fiber to reduce postprandial glycemic response, promote satiety, decrease serum cholesterol, and decrease serum uric acid.12 The viscosity of fiber is also thought to play an important role in the augmentation of gut mucosal protection through the stimulation of enteral mucus production and goblet cell hypertrophy and replication.13 Additionally, those viscous fibers that are largely fermented by colonic microflora exert a wide array of physiologic effects through the production of SCFAs (the principle energy substrates of colonocytes), the promotion of beneficial colonic microbial populations, and the augmentation of important gut-derived peptide hormones.
The viscosity of fiber is best measured by methods that quantify a hydrated fiber’s internal friction and its ability to resist flow. Viscosity is usually expressed in units of millipascal seconds or centipoise.14 Other factors, such as shear stress (e.g. mastication, peristalsis), acid pH, dilution, and chemical components of food determine the real viscosity that a fiber will impart to food rather than just its in vitro viscosity.
The concept of the glycemic index came about through the work of Jenkins et al as they examined the impact of viscous fiber ingestion on glucose tolerance.15 It is now generally accepted that the glycemic index of a carbohydrate containing food is directly correlated to the viscosity of that food after ingestion.
More recently, one research group was involved in several studies looking at the effects of adding a novel, highly viscous functional fiber (Polyglycoplex [PGX]) to various foods on glycemic index, serum cholesterol, hunger, and satiety in healthy humans. The novelty of this fiber relates to its viscosity, which is higher than that of any other fiber thus studied. Unlike some viscous fibers, such as psyllium, which are not fermented, PGX is highly fermentable and prebiotic.16 This composition, made from glucomannan, sodium alginate, and xanthan gum, is taken through several steps of processing, and the resultant modified fiber is approximately three times the viscosity of glucomannan, a naturally occurring fiber from the Konjac root that may possess the highest viscosity of any naturally occurring fiber. In addition, it was demonstrated that PGX is a novel and stable molecular entity with viscosity that is several-fold higher than its constituent ingredients, through a wide range of pH and sheer stress conditions, and when added to a variety of foods and beverages.17,18 In addition, this fiber is tasteless, and disperses readily when added to food or mixed with beverages. Its viscosity evolves several minutes after initial hydration, making it very easy to consume the small amounts needed to create a highly viscous gastric milieu, leading to its resultant physiologic effects.
This study demonstrated that substantial reductions in postprandial glycemia along with reduced hunger and increased satiety can be achieved with a very modest quantity of this highly viscous fiber. These findings support the contention that the viscosity that a fiber imparts to the gastric contents is more important than simply the grams of the fiber consumed.19–23 In a study of Zucker Diabetic Fatty (ZDF) rats, the effects of diets supplemented with nonviscous, insoluble fiber (cellulose); nonviscous, prebiotic fiber (inulin, oligofructose); and with a highly viscous, prebiotic fiber (PGX) were compared. In this study, only the diet supplemented with highly viscous fiber substantially decreased postprandial blood glucose and insulin secretion, decreased hepatic fatty infiltration, and preserved pancreatic β cell mass. These effects were accompanied by an increase in the production of the important glucoregulatory incretin hormone, glucagon-like peptide-1 (GLP-1) and, in a human clinical trial, the appetite reducing hormone peptide YY (PYY).24 Human clinical trials are currently underway to see if this novel, highly viscous fiber has utility in the management of obesity and diabetes, but the substantial glycemic index modifying effects have been well confirmed in human studies.18-20